FACULTY OF METEOROLOGY

ADVANCED PROFESSIONAL KNOWLEDGE COURSE(METEOROLOGY)

BOOK – 01/12NUMERICAL WEATHER PREDICTION

PREPARED BY VS SRINIVAS &

CLIMATOLOGYPREPARED BY WG CDR SHREYA PANDIT

NOTE: - READ THE STARTER KIT WHICH HAS BEEN SENT TO YOU. IN CASE OF DOUBTS YOU MAY CONTACT TRAINING COORDINATION OFFICER (TCO) ON TELE: 2367000/7314

NOTE: - It has been the endeavour of Faculty of Meteorology to update courseware with the latest Policies/ Orders/ Instructions. However the contents of this book are to be used as course material, and not to be quoted as authority at any stage. Officers may refer to the references quoted in the text for official use.

INDEX: BOOK – 5/12

Subject Chapter Page No.

NWP

1. NWP MODEL CHARACTERISTICS

2. DOMAIN AND BOUNDARY CONDITIONS

3. HORIZONTAL RESOLUTION

1-26

CLIMATOLOGY 1. CLIMATOLOGY OF SE ASIA 27-168

AMENDMENT RECORDS

Date Amendment Page No. Authority

NUMERICAL WEATHER PREDICTION

CHAPTER -1

NWP BASICS

Chapter objectives

After reading this chapter, you should be able to:-

Understand the NWP model structure.

Know the significance of Domain and Boundary conditions.

Understand horizontal resolution.

Structure

(a) Introduction to Model structure

(b) Historical overview

(c) NWP components

(d) Introduction to Domain and Boundary conditions

168

(e) One way, two way interactions

(f) Various Boundary conditions

(g) Introduction to Horizontal resolution

(h) Grid Space area

(j) Calculating Eta Surfacs

(k) Grid point Equivalency

(l) Limitations of the Eta Vertical Coordinate

(m) Conclusion

NWP MODEL STRUCTURE

Introduction

1. One of the ultimate goals of theoretical meteorology is the development of means of computing the future state of the atmosphere from the basic theoretical equations which govern that state. This goal has been pursued by many of the greatest hydrodynamicists and meteorologists of the past, including von Helmhotz, V Bjerknes, and Richardson. The formulation of the problem was proposed by V Bjerknes as early as 1904. The first attempt at producing a forecast by this method was described by Richardson in 1922. In principle, Richardson used the equations of horizontal motion and the equation of continuity, replaced the derivatives by finite differences, and computed the time increments of the wind components (u, v) and pressure (p). By feeding the increment back into the equations and repeating the operation, a numerical; integration was obtained. However, the first indications of real success have come only since 1948.

Historical Overview

2. About 1910 Richardson began to explore the possibility of utilizing the complete set of hydrodynamic equations for this purpose. These equations may be written, for a non-viscous adiabatic atmosphere, in the form

)(21

wCosvSinxp

zuw

yuv

xuu

tu

uSinyp

zvw

yvv

xvu

tv

21

guCoszp

zww

ywv

xwu

tw

21

zw

yv

xu

zw

yv

xu

t

zw

yv

xu

zpw

ypv

xpu

tp

where the first three are the equations of motion, and the other two are the equation of continuity and the law of conservation of energy for an adiabatic process. It will be noted that this set is complete; there are five independent equations in five dependent variables: u, v, w, p, p. These equations have been written with all time derivatives on the left and all space derivatives on the right. Richardson1 realized that the possibility of writing these equations in this manner meant that the instantaneous local rates of change of each of the dependent variables could be computed from a knowledge of the distribution in space of these variables at a given moment. Since it is theoretically possible to measure the spatial distribution of the variables at any time, it should be possible to determine their rates of change from Eq. (1). This constitutes a prediction of the values of these variables at an infinitesimally small increment of time later.

Example of a finite-difference grid

3. Richardson realized further that these equations could be solved by the method of finite differences, consisting of four steps. First, in the region under consideration, a grid of points is laid out. These points are usually equally spaced in one or more of the coordinate directions. At each grid point, at a given moment the values of the several variables are determined, either by direct measurement or by interpolation from analyzed fields.

4. Second, whenever a first derivative of a variable is needed, it is approximated at every grid point by taking the finite difference in the variable between surrounding grid points and dividing by the distance separating these two points. Thus in the grid of Fig. 1, an approximation to the value of ∂p/∂y at the point 0 is (p2 - P4)/2d, where d is the distance separating grid points. In fact we may write

dpp

yp

d 2lim 42

0

This equation states that ∂p/∂y becomes equal to this ratio of differences only when the distance separating points approaches zero. Nevertheless, the derivative will he approximated sufficiently well even if d remains finite, so long as d is small compared to the distance which separates maxima and minima of the variable. With these restrictions

dpp

yp

242

Similar expressions can be written for all other derivatives. Thus )/()(/ 0101 ttpptp , where subscripts 0 and 1 respectively refer to the initial

moment and some future moment a finite time later.

5. Third, all derivatives in the set of governing equations are replaced by such ratios of finite differences. The resultant finite-difference equations are then solved for the values of the variables, at the future moment. The great strength of this procedure is that it reduces a set of differential equations, which may not be solvable directly, to a set of algebraic equations, whose solution can always be obtained. This solution may be at the expense of large amounts of numerical computation, but involved computation has become a far smaller problem with the advent of high-speed electronic computing machines.

6. Finally, one takes advantage of the fact that all this computation results in the values of the variables at each grid point at a future time. Since this is the kind of information that was needed to begin the process, it is apparent that one may now repeat the procedure so as to extend the results farther in time. As a matter of fact, in theory the process can be repeated as many times as desired until a forecast for a predetermined, large, future time is reached.

7. Richardson used essentially the equations and procedure outlined above to make the first real attempt at numerical weather prediction. After a great deal of laborious hand computation he obtained results that were grossly disappointing. He predicted changes in the pressure, for example, which were from ten to a hundred times too large. As a result of this initial failure and the extensive labour required for such

forecasts little further work was done for nearly a quarter of a century.

Components of NWP

8. Producing an accurate forecast is the result of a complex process that involves careful consideration of many meteorological data sources and principles, including numerical weather prediction (NWP) models. The flowchart shows how the components of NWP fit into the forecast process. The process starts at the bottom and proceeds upwards.

9. Data. Data are collected to describe the initial state of the atmosphere. Data sources include observations from satellites, profilers, surface stations, aircraft, upper-air soundings, and radar.

10. Quality Control and Analysis. Through a series of checks and tests, data are quality controlled to ensure the viability of the information input into the forecast model. This helps to ensure that inaccurate data are adjusted or removed before going into the analysis. The judgments of trained meteorologists are a critical part of the process.

11. Computer Resources. The capacity and speed of the computing resources available to run a forecast model govern the amount and complexity of the data and forecast model components used. Thus, computer resources can be a significant limitation to NWP.

12. Numerics. Model numerics refers to the following:-

(a) Model characteristics such as the mathematical formulation used to solve the model forecast equations.

(b) How data is represented.

(c) Model resolution.

(d) Computational domain.

(e) Coordinate system.

These all affect the handling of dynamics and how consistently the initial conditions and physical processes are represented.

13. Dynamics.In NWP, dynamic processes refer to atmospheric processes that most often involve the forcing or movement of air, such as advection, pressure gradient forces, and adiabatic heating and cooling. These processes are described by a set of horizontal and vertical momentum, mass conservation, and thermodynamic equations within the forecast model.

14. Physics. In NWP, physical processes refer to three types of processes:-

(a) Those operating on scales smaller than the model resolution but which exert a cumulative effect felt at resolvable scales

(b) Those involving exchanges of energy, water, and momentum between the atmosphere and external sources (for example, radiation and land and sea surface processes)

(c) Cloud and precipitation microphysics

15. Assimilation. An assimilation system is a complex procedure in which observed meteorological parameters are converted to forecast variables and blended with short-range forecasts from an earlier model run to produce the initial conditions used to start a new forecast. The assimilation system tries to find the initial fields of the forecast variables that will optimize the accuracy of the forecast based on the available data.

16. Post-processing. In post-processing, computations are made to the raw model output to transform it to a format readily usable by forecasters. Diagnostics and meteorological parameters are derived from the forecast variables. In addition, model variables are interpolated vertically to surfaces used by forecasters (isobaric, isentropic, and constant altitude) and interpolated horizontally to forecast locations or output grids. Contour plots are also made. Additional post-processing, such as using AWIPS algorithms, may be done later. The resulting products are collectively referred to as "numerical guidance."

17. Numerical Guidance. Numerical guidance products are produced through post-processing of the model output. They are in a form that can be readily used by forecasters and are usually displayed on a grid with a different resolution than the original model. Examples include geopotential height charts, MSL pressure, and surface temperature. Aircraft turbulence and icing charts are examples of fields calculated from numerical model output using physically based empirical relationships.

18. Statistical Guidance. Some sensible weather elements, such as visibility and thunderstorms, are not predicted by the model and cannot be derived directly from the model forecast variables. Other parameters, such as surface maximum temperature, are sensitive to model weaknesses and vary locally. Statistical techniques, such as Model Output Statistics (MOS), have been developed to predict weather elements at particular point locations from direct and post-processed model fields and other pertinent data, including climatology.

19. Direct Model Output. Direct model output typically refers to gridded forecast data provided at each model grid point and vertical level. These data are not interpolated for locations between model gridpoints and levels. The output data are used by forecasters to develop a wide variety of local forecast and diagnostic products and provide a look inside the model.

20. Model Output. Model output products include all products that use model fields. The model forecast variables can be looked at directly, post-processed into grids,

plots, station predictions, etc., and used in combination with climatology and other data sources in statistical forecasts. Collectively, they are an important part of the forecast process.

21. Centralized Guidance. Using all of the forecasting tools at their disposal, NCEP meteorologists produce subjective, centralized guidance products, such as hurricane track predictions, severe weather outlooks and discussions, and quantitative precipitation forecasts. These products are added to the mix of tools and resources used by forecasters.

22. Forecast Process. In the forecast process, model output and current observations are combined with the forecaster's understanding of meteorological principles to develop a forecast for the area of responsibility. Centralized subjective guidance is used to help with specialized aspects of the forecast. The meteorological variables required in the forecast and the customer needs drive the types of guidance products and observations used in the forecast development process.

23. Observations. Observations of all types are needed to ascertain current atmospheric conditions and to evaluate the accuracy of a model's analysis or forecast. Observations provide the ground-truth data and are used to help assess the reliability of model output and to make necessary adjustments.

24. Understanding Meteorological Principles. A thorough understanding of basic meteorological principles and relationships is necessary to intelligently use model guidance so one can, for example, identify when model output is not meteorologically sound or consistent. As models become more complex and predict more detailed and realistic-looking features, there is a greater need to understand meteorological principles in order to intelligently take advantage of NWP and avoid being misled. Knowledge of local climatology, terrain influences, and model performance in the local area is also important to developing the best possible forecast.

25. Forecast. This represents the final product for which NWP was ultimately developed. The format, meteorological variables, forecast period, and frequency are driven by customer needs

26. Verification. Forecasters use model verification data to identify specific limitations and statistical biases of model guidance and to compensate for them. Modelers use verification data to help identify deficiencies so they can improve forecast model components. Model verification is an integral part of the NWP development process.

DOMAIN & BOUNDARY CONDITIONS

Introduction

27. Model domain refers to a model's area of coverage. Limited-area models (LAMs) have horizontal (lateral) and top and bottom (vertical) boundaries, whereas global models, which by nature cover the entire earth, have only vertical boundaries. For

limited-area models, larger-domain models supply the data for the lateral boundary conditions.

28. For all models, accurate information must be provided for all forecast variables and along each model boundary (lateral, top, and bottom) in order to solve the forecast equations. Boundary values can be obtained from a variety of sources, including:-

(a) Data assimilation systems.

(b) Forecast values from a current or previous cycle of a large-scale model (as is the case for lateral boundary conditions used in LAMs).

(c) Some type of climatological or fixed value (for specifying certain surface characteristics, such as soil moisture, sea surface temperature, and vegetation type).

Model Domain and Boundary Conditions

29. The domain of an NWP model can be viewed as a three-dimensional array of cubes similar to that illustrated here. Each cube encompasses a volume of the atmosphere corresponding to a model grid point. Forecast values for the meteorological variables in each cube are derived from the current values within the cube plus those from the surrounding cubes. Because the cubes on the boundaries are not surrounded by other cubes on all sides, the information needed to provide forecast values for the meteorological parameters cannot be determined using only the data contained in the model. The information for the outside boundaries must be supplied from another source.

30. The remainder of the domain & boundary conditions section discusses these and other issues related to vertical and lateral boundary conditions, including the methods used to incorporate meteorological variables from outside the model domain and the errors introduced into the model domain by boundary conditions.

Impact of Horizontal (Lateral) Boundary Conditions

31. The methods discussed here apply only to limited-area models and are used to provide needed information about changes to the meteorological forecast variables occurring outside of the model domain. Ideally, boundary conditions should be based on observed data (as they are in research case studies). However, the best that can be

done in weather prediction is to use boundary conditions based on another forecast model.

32. The quality of limited-area or local model predictions is greatly affected by the quality of predictions produced by the model supplying the lateral boundary conditions. Errors in forecasts from larger-domain models will move into the LAM's forecast domain and can, in some instances, amplify. For example, the NCEP Eta Model is run with boundary conditions from an earlier run of the AVN, so that in westerly flow, systematic errors in the AVN over the Pacific are also present in the western part of Eta forecasts. Likewise, if a LAM is run at a forecast office using boundary conditions from the Eta Model, errors similar to those in the Eta can spread rapidly across the limited domain of the local model forecast.

33. The lateral boundary conditions largely control the position and evolution of features that cover the entire forecast domain. For example, for a domain covering the 48 states, long-wave patterns are almost entirely determined by the boundary conditions. Weaker impacts are noted on jet streaks and fronts, especially in regions far downwind from the upstream boundary. Similarly, in a high-resolution mesoscale model running over a small section of the country, the placement and timing of synoptic-scale features are determined almost completely by the synoptic-scale model supplying the boundary conditions.

34. The figure illustrates how far an air parcel can travel into a limited-area model domain from the model boundary during a 48-hour forecast period. Since the influence of boundary conditions spreads away from (and particularly downstream of) the boundaries and, in some cases, the effects amplify downstream, the area of primary forecast concern should be located as far from the boundaries as possible, especially the upstream boundary. Because some of the boundary influence is carried by the wind, the speed and direction of greatest forecast impact will vary from one flow regime to another.

Forecasters should pay attention to how long it takes a trajectory to move from the model boundary to the vicinity of their forecast area.

35. The graphic shows a favourable configuration of model boundaries relative to the forecast area of interest. Note that the boundaries (especially along the upstream portion of the model domain) are located far from the area of interest. By placing the upstream boundary well outside the forecast area, the influence of the boundary errors within the area of interest can be reduced. It should also be noted that in this idealized case, the total model forecast domain is nearly four times larger than the relatively small area of primary forecast interest. As the size of this area increases, the relative size of the portion of the overall forecast domain "wasted" to prevent boundary-condition errors decreases.

One-Way Interaction

36. Lateral boundary conditions are usually obtained from a previous run of a larger-domain model. For example, a 6- to 54- hour forecast from a global model run at 06 UTC could supply the lateral boundary conditions for a 0- to 48-hour forecast from a regional model starting at 12 UTC. Information flows in one direction, from the previously integrated forecast over the larger domain to the smaller-domain model. Therefore, this is called one-way interaction.

Two-Way Interaction

37. Some limited-area models, including the UW-NMS, ARPS, and MM5, are run with small-area, finer-resolution grids nested inside of coarser-resolution grids within the same model. This nesting is necessary because computer memory and speed limitations prohibit fine-resolution grids from covering the entire model domain.

38. The information for the outermost boundaries of these nested-grid models is still supplied from an outside source, using one-way interaction. However, the interfaces between the grids inside the nested grid model are determined from the forecasts within the model itself.

39. Where the fine grid covers the coarse grid, the forecast variables for the coarse grid are updated based on the fine-grid prediction. The coarse-grid prediction, in turn, affects the fine-grid prediction by supplying boundary conditions on the mesh interface. Since information flows both ways, this is called two-way interaction.

40. Two-way interaction is highly advantageous for several reasons:-

(a) It allows fine-scale processes resolved on the fine mesh to affect the larger-scale flow on the coarse mesh. For instance, a large convective system resolved on the fine mesh can amplify an upper ridge, slowing the progression of waves in the westerlies in the larger-domain coarse resolution grid.

(b) Because predictions on coarse-resolution grids take relatively little computer time and memory resources, the outermost boundary of the model can be moved far from the region of forecast interest, while the fine-resolution domain remains small enough to run in real-time.

(c) The use of these "real-time" boundary condition procedures within a model with the same basic forecast dynamics and physics greatly reduces the influence of errors associated with using boundary conditions from a model with older data and a different vertical coordinate, topography, and physics.

Additional Source of Error

41. At the interface between grid meshes of differing resolution, atmospheric waves in a numerical model behave much the same way as light waves at the interface between air and water.

42. The speed of wave propagation in a model varies with the number of points used to represent the wave. A well-resolved wave will be forecast to move at the correct speed, while a poorly-resolved wave will be forecast to move slower than its true speed. This means that a wave passing through a mesh interface can bend, changing its orientation. Additionally, boundary conditions can force the slower wave motion upon the finer-resolution mesh, disrupting the better solution near the boundary. In the worst case, waves can even be reflected at a model or mesh boundary. However, improved numerical methods now reduce or eliminate this behaviour.

43. The following weather features can be affected by refraction or redirection of atmospheric waves at a model's lateral boundaries:-

(a) Precipitation fields.

(b) Temperature fields.

(c) Jet stream pattern.

(d) Vertical motion field.

(e) Intensity and placement of surface lows and fronts.

Note that this type of error does not affect waves that are well-resolved on the coarser mesh supplying the boundary conditions (either within the LAM or the larger-domain model).

44. This type of error will become less significant as global model resolution improves.

Upper Boundary Conditions

45. All forecast models, including global models, require that boundary conditions be specified at the top and bottom of the model domain.

46. Model tops are placed well above the tropopause. Assumptions must be made as to how the forecast variables will change above the assigned top throughout the forecast period.

Boundary Conditions

47. Most models employ a rigid upper boundary condition, which means that no vertical motion is allowed through the top of the model. Problems occur when gravity waves (such as those generated by convection in the model or flow over the model topography) reflect off the top of the model. If untreated, these gravity waves can "bounce" around the entire model depth and severely affect vertical motion and precipitation forecasts. Fortunately, special numerical treatments, such as the addition of an "absorbing" or "damping" layer near the model top, have been developed to avoid this problem. These treatments can only be applied when the model top is much higher than any weather features to be forecast, since the forecast for the highest model layers will not be realistic.

Lower Boundary Conditions

48. The lower boundary is defined by the interface of the model's lowest atmospheric level with the model topography or model sea surface. The accuracy with which this boundary condition represents conditions at the earth's surface depends upon the specific surface physics and parameterizations used in the model as well as its source of information for snow cover, soil temperature and moisture, soil type, and vegetation cover.

49. Vertical motion at the ground is set to zero, except for an upslope or downslope component due to flow along the model topography. Horizontal winds are predicted as an average for the lowest layer rather than at the ground or anemometer level. Near-surface winds are then empirically determined.

50. Since most models predict near-surface conditions using energy balance principles, errors will be introduced due to inadequate handling of terrain, albedo, the amount of rainwater available for evaporation from the surface, lake and sea temperature, vegetation cover, the method of simulating soil-vegetation-atmosphere interaction, and many other details relating to model representation of physical processes.

HORIZONTAL RESOLUTION

Introduction

51. The horizontal resolution of an NWP model is related to the spacing between grid points for grid point models or the number of waves that can be resolved for spectral models. 'Resolution' is defined here in terms of the grid spacing or wave number and represents the average area depicted by each grid point in a grid point model or the number of waves used in a spectral model. Note that the smallest features that can be accurately represented by a model are many times larger than the grid 'resolution.' In fact, phenomena with dimensions on the same scale as the grid spacing are unlikely to be depicted or predicted within a model.

52. Although horizontal resolution plays a key role in determining a model's ability to resolve features, other factors, such as the vertical resolution, vertical coordinate, and physics package also have a significant impact.

Grid Spacing Introduction

53. A grid point model's horizontal resolution is defined as the average distance between adjacent grid points with the same variables. For example, if all of a model's forecast variables (u-wind, v-wind, temperature, and moisture, etc.) are predicted at each of its grid points (as shown in the figure), the model is considered to have a resolution equal to the minimum spacing between adjacent grid points at a specific latitude and longitude. In the example, all of the variables are computed at each grid point so the resolution is 50 km. A similar model with 10 km between adjacent grid points is considered to have 10-km horizontal resolution.

54. Whether a model is considered high or low resolution depends upon the size of the domain and the scale of weather phenomenon that the model is trying to predict. A resolution on the order of 20 to 50 km is considered high for a global model, while for a storm-scale model, a resolution of 100 to 500 m is considered high and necessary to resolve the internal processes of convection.

55. Categorizing models as to their resolution (high or low) is a poor method for describing models since the term is relative and changes as new models come on-line. Today's high-resolution model may be tomorrow's low-resolution model, independent of its domain.

Grid Box Area

56. It is important to know the amount of area between grid points, since atmospheric processes and events occurring over areas near to or smaller than this size will not be included in the model. For a 50-km model, each grid box covers 2500 square kilometres, with the grid points located at the centre of the boxes. Hence, the central "grid point" represents the mean value of the data within the 2500 square kilometre area of the grid box surrounding the "grid point."

57. In the example, the grid point is assigned a value of 26, which approximates the average of the observed values within the grid box. This representation may be adequate when the area is under a rather homogeneous, large-scale feature, such as a large high-pressure system. However, if the scale of the phenomena being forecast is less than the area represented by the grid box, the phenomena will not be correctly represented and can, if not treated properly, actually degrade the quality of forecasts of large-scale features.

58. Grid spacing also has direct and indirect effects on the following:-

(a) Model terrain representation.

(b) Truncation errors in equation computations.

(c) Computer resources.

(d) A model's ability to resolve different scales of features.

These effects are discussed more throughout the rest of the Horizontal Resolution chapter.

Spectral to Grid Point Resolution Equivalency

59. In spectral models, the horizontal resolution is designated by a "T" number (for example, T80), which indicates the number of waves used to represent the data. The "T" stands for triangular truncation, which indicates the particular set of waves used by a spectral model.

60. Spectral models represent data precisely out to a maximum number of waves, but omit all, more detailed information contained in smaller waves. The wavelength of the smallest wave in a spectral model is represented as

Minimum Wavelength = 360 degrees / N

where N is the total number of waves (the "T" number).

61. Complications arise because non-linear dynamics and physics are calculated on a grid and then converted to spectral form to incorporate their effects in a spectral model. This introduces errors, which make the final result less exact than one might expect from calculations done strictly in spectral space.

Grid Point Equivalency

62. Grid point models can incorporate data at all resolutions, but can introduce errors by doing so. It takes about five to seven grid points to get reasonable approximations of most weather features. Still more points per wave feature are often necessary to get a good forecast.

63. Because spectral and grid point models preserve information in different ways, no precise equivalent grid spacing can be given for a spectral model resolution. However, we can approximate the grid spacing to obtain equivalent accuracy to a spectral model with a fixed number of waves using a very simple approach. First, we assume that three grid points are sufficient to capture the information contained in each of a series of continuous waves. The approximate grid spacing with the same accuracy as a spectral model can then be represented as

ΔX ≈ 360° / 3 x N

64. For a T80 model, this results in a maximum grid spacing for equivalent accuracy of about

ΔX ≈ 360° / 3 x 80 ≈ 1.5° ≈ 160 km

65. In fact, the dynamics of spectral models retain far better wave representation than grid point models with this grid spacing. However, the spectral model physics is calculated on a grid, with about three times as many grid lengths as number of waves used to represent the data. Since it takes five to seven grid points to represent 'wavy' data well and even more for features that include discontinuities, the resolution of the physics is poorer than the above formulation indicates and degrades the quality of the spectral model forecast.

66. In summary, spectral models do a fine job with 'dry' waves in the free atmosphere, but have coarser representation of the physics, including surface properties. The resulting overall forecast quality is somewhere between these two extremes and varies on a case-by-case basis. The more physics that is involved in the evolution of the forecast, the less the advantage in spectral model forecasts compared to comparable resolution grid point forecasts.

Conclusion

67. This chapter provides an understanding of the basics of NWP components, Domain & Boundary conditions and Horizontal resolution. The second and third parts are part of Dynamics of the NWP.

Check Assimilation

Q.1. There is no grid point equivalency (T/F)

Q.2. Spectral model resolution is the distance between the grid points (T/F)

Q.3. Direct model output is understandable product (T/F)

Q.4. Three boundaries are available for a spectral model (T/F)

Q.5. For Two-way interaction minimum one domain is required (T/F)

Answers

1. False. It is available.

2. False. Number of waves

3. False. It needs post processing for understandable products.

4. False. Top and bottom only

5. False. Two

Bibliography

ECMWF Tutorial notes downloaded through Internet

Haltiner, G and R. Williams (1979) Numerical Prediction and Dynamic Meteorology, John and Wiley Sons, Inc.

Houghton, D.D., R.A. Petersen, and R. Wobus, 1993: Spatial resolution impacts on National Meteorological Center Nested Grid Model simulations.

Training notes at AFAC.

CLIMATOLOGY

CHAPTER – 1

CLIMATOLOGY OF PAKISTAN

Chapter objectives

After reading this chapter, you should be able to:-

Assimilate the Physical Features, Climatic Elements and different

Climatic Regions of Pakistan.

Understand how the Pressure, Temperature, Rainfall and Humidity

Vary during different Seasons in different regions.

Understand the different seasons of Pakistan and Aviation weather

Hazards.

Structure

1 Introduction

2. Physical Features

3. Climatic Regions

4. Climatic Elements.

5. Seasons.

6. Aviation weather Hazards

7. Conclusion.

Introduction

1. Climatology has become a central focus of numerous disciplines, concerned with the present and future environment of this planet. No longer seen as been concerned solely with average weathers, it serves as an important reference point in terms of NWP in the current scenario.

2. Pakistan has a typical climatic personality by virtue of its peculiar location with reference to latitudes, mountains, deserts, rivers and sea. Climatically, a large part of Pakistan is arid to semi arid, with large spatial and temporal variability in many climatic parameters. The orography features of Pakistan have a great influence on its climate, and there are peculiarities resulting from its geographical location. It lies on the western fringes of monsoon depressions, on the northern edge of SW monsoon current over the subcontinent, and form the peripheral region of Western Disturbances, travelling from far west.

Physical features

3. Pakistan is situated in the extreme northwestern parts of Indian subcontinent, with latitudinal extension from 24.00N to 37.00N and between longitudes of 61.00E and 75.00E. The western and northern regions of the country are bounded by hill ranges. To the SE and East is the Indus basin, bordering on great Thar Desert. To the west of plains in the south are Kartiyar ranges, separating the plains from Baluchistan. The Kartiyar ranges extend northwards as a series of ridges till the Sulaman ranges are reached. From here the terrain rises and in the NW are Hindukush ranges, ultimately reaching the Pamir Knot. Radiating eastward from Pamir Knot are the Karakoram ranges. To the south of these, stretching eastward are Great Himalayas. There are relatively lower mountain ranges scattered in the west-southwest along the coast. The hilly terrain of the country has an average height of 600m above msl.

4. The country occupies a total geographical area of 8,03,943 Km2 which is divided in to four administrative provinces as

(a) Punjab

(b) Sindh

(c) Baluchistan

(d) North West Frontier Provinces

Climatic Regions

5. Based on Koppel and Thornthwaite classifications which are based on quantitative analysis of climatic parameters, most of the Pakistan falls in arid or semi arid type. But these classifications are global in outlook and do not bring out the detailed climatic picture of Pakistan. Kazi divided Pakistan into four climatic regions based on the variation of plant life, soil and human activity over varied topography (Kazi 1952) and they are:

(a) Tropical coast land. Moderate temperature, low rainfall, coastal areas dominated by steady in flow of sea breeze during summer with high humidity and low diurnal and annual ranges of temperature. Annual rainfall slightly above 180 mm. May and Jun are the hottest months with a secondary maximum in October. Jan is the coldest and the mean annual temperature is 32°C.

(b) Sub tropical continental low lands. Includes the whole of Indus plain (except the coast lands) characterized by aridity and continentality, with high summer temperatures and late summer rains. Excluding NE parts, rainfall is less than 250 mm. Jul and Aug being the rainiest, while Oct and Nov driest. May and Jun are the hottest with mean maximum over 41°C. Jan is the coldest with mean minimum of about 4°C.

(c) Sub tropical continental high lands. Comprise of broad belt of mountains in north and west of Indus plains. Principal characteristic is cold snowy winters. Rains are generally in winter and spring with monsoon rains in mountains facing south and east. Temperature and rainfall vary with elevation.

(d) Very arid plateau. Baluchistan plateau, excluding coast lands in south and high lands in north east, is hottest and driest. Summer is extremely hot and dusty with mean maximum temperature in Jul 40°C and mean minimum in Jan 4°C. Jan and Feb account for meagre rainfall.

Climatic Elements

6. Pressure Pressure at mean level does not directly influence the Human activity. However, it manifests itself through wind and moving pressure system like Monsoon Depression and Western Disturbances. The sea level pressure in the morning 0300 UTC is studied through low sun month (January) and high sun month (July).

(a) January Isobars . The east to west orientation of Himalayas in north and almost north to south lay out of Hindukush and Sulaman ranges along the western border does not allow the cold air, From dominant Siberian High of winter, to spread directly in to Pakistan. The cold heavy Siberian air moving through Turkistan and eastern Iran enters northwest Baluchistan and Sindh. It then spreads eastward into NWFP and Punjab. The pattern of Isobars at msl in Jan shows a general ENE to WSW orientation with pressure gradient directed southward.

(b) July Isobars. The high temperature belt extends from Lut desert (Iran) through Baluchistan Plateau to upper Sindh and adjoining south Punjab. The lower pressure are recorded in northwest Baluchistan in July with its trough extending into upper Sindh adjoining Punjab and thence to Indian Punjab. The isobaric pattern shows a west to east orientation along coast, with pressure gradient directed northward in to Baluchistan plateau and Sindh. In Punjab, NWFP and mountainous northeast Baluchistan the isobars have an approximate south to north orientation with a perceptible east to west pressure gradient.

7. Temperature: The several factors which cause variation of surface temperature in Pakistan are:

(a) Latitudinal extent of Pakistan.

(b) Variation of mountain terrain in the northern and western parts of Pakistan.

(c) Ameliorating effects of sea in the south and irrigation network in the plains.

8. The description of mean daily temperature month by month in Pakistan will be cumbersome therefore these temperature have been condensed further for the summer and winter season.

(a) Winter mean daily temperature : The isotherm of 150 C almost demarcates the mountainous areas of Punjab, NWFP and Baluchistan. These temperatures progressively increase with decrease in height. The temperature increases:

(i) From north to south due to latitudinal effect and

(ii) From west to east in Baluchistan and Sindh due to repeated cold air incursion from higher latitudes in to northwest Baluchistan with its subsequent progress eastward. The highest mean daily temperature >210C occurs in south along the coast. Temperature gradient is steep in mountainous area due to variable height and slack in the Indus plain (probable due to irrigation effect).

(b) Summer mean daily temperature . The isotherm of 29 0 C almost demarcates the mountainous areas of Punjab, NWFP and Baluchistan. The temperature increases:

(i) From north to south due to latitudinal effect and

(ii) Inland from coast because the temperature on coast are tempered down by sea breeze.

9. Rainfall . By virtue to its peculiar location Pakistan experiences both summer and winter rainfall. The pattern and character of summer and winter rain gets so intermixed that in the transition zone, in the northern parts of Pakistan it is difficult to determine which one is dominant. However, it can be generalised that whole of Punjab and Sindh get more summer rains, while Baluchistan and almost the whole of NWFP gets more winter rains. Elevation plays an important role in increasing precipitation on the windward slope of mountains. The rainfall pattern shows two distinct rainfall regimes separated by meager rainfall period i.e. winter rainfall (from Dec to Apr), summer rainfall (from May to Sep) and Meagre rainfall (Oct to Nov) period.

(a) Winter Rainfall . The winter rains in Pakistan are a result of Western Disturbance moving in lat south of 37.0°N. Rainfall pattern shows that winter rains are 400-500 mm in northern mountain and upper NWFP and decrease to as low as 9 mm in southeastern corner of Sindh. Winter rains also decrease from west to east 100-200 mm in lower NWFP and northeast Baluchistan and 80 mm in Punjab. Winter rains are above 100 mm in coastal areas and 40-80 mm in rest of Baluchistan eastward in Sindh it drops to 9 mm in Chhor. Thus mountainous areas of NE Baluchistan, NWFP and Punjab gets major share of winter precipitation, part of which is in the form of snow over the mountain.

(b) Summer rainfall . Summer rains are caused in several different ways:

(i) The southern slopes of the northern mountains and adjoining submontane districts receive orography rainfall from southeasterly-deflected monsoon stream moving along foothills of Himalayas.

(ii) Punjab, Sindh, NWFP and adjoining NE Baluchistan generally receive cyclonic rainfall due to convergent lifting of monsoon air in east west moving MD through northern India in to Pakistan.

(iii) The mountainous area in the north receive rainfall from lifting of moist air or weak monsoon by convergence associated with occasional WD moving north of 35 0 N lat

(iv) Any part of Pakistan may get showers due to local heat convection if moisture becomes available. Rainfall pattern shows that summer rains are little over 1000 mm (Murree) in the mountainous Punjab and decrease westward to around 300 mm in the upper NWFP, and thence decreasing southward to a little over 10 mm on west Baluchistan coast. These rains also decrease southward from northern to around 60 mm in northern Sindh, thence increasing again to 220 mm along Sindh coast, because of moisture from Arabian Sea.

(c) Meagre rainfall In Oct and Nov the withdrawal of monsoon, the fall of temperature as the sun goes south of Equator and WD still moving mostly at higher latitudes results in meager rainfall (0-10 mm) over most Pakistan.

10. Humidity.

(a) Average Humidity in winters:

(i) Humidity generally ranges between 40% - 70% in plains of Pakistan.

(ii) Humidity exceeds 70% in north Punjab.

(iii) It is less than 40% in mountainous region of NWFP and N Baluchistan.

(b) Average Humidity in summer:

(i) During summer humidity ranges from 40%-70% in NWFP.

(ii) In N Punjab it is less than 40% from Apr to Jun and 40%-70% during Jul to Oct.

(iii) Mekran coast has more than 70% humidity.

Seasons

11. The season s of Pakistan is divided in to four parts like India.

(a) Cold Season - From mid November to mid April

(b) Hot Season - From mid April to June

(c) Monsoon Season - From July to mid September

(d) Transition - From mid September to mid November

12. Cold Season . This period is characterised in general by fine weather, low humidity and large diurnal ranges of temperature. WD's moves across Pakistan from west to east. These are more frequent in northern parts of Pakistan but as the season advances, these WD's affect the whole Pakistan down to the coast of Arabian Sea Winter rainfall from these WD's is not large as compared to that received during monsoon period. Rainfall decreases from north to south and from west to east. The cold wave in the wake of some of these disturbances may cause min temp to fall below freezing point even in plains. April is the winter month in the mountain.

13. Hot Season. WD does continue to move along northern lat of Pakistan in this season. These cause thunder storm (with some rain) over mountain and dust storm or dust raising wind over the plains and Baluchistan plateau. Hail storms are frequent in Apr but rare in May. As the hot season progresses the belt of highest day temp moves from south to north. Some of the highest day temps of south Asia have been recorded in Pakistan during May and Jun. RH in May and Jun varies from 50% in morning to 25% or less in afternoon. At several locations it is even less than 20%. While the interior is blazing hot in May and Jun, the coastal belt of Sindh- Mekran enjoys the pleasant sea breeze which keeps the temp down around 35°C.

14. Monsoon Season . The deflected monsoon current moving west ward along the foothills of Himalayas reaches Pakistan toward the beginning of July. And establish there by the middle of the month. The Arabian branch of the monsoon reaches Sindh-Mekran coast by end of June. However this branch is of small vertical extent and generally produces stratus clouds in the coastal belt. The monsoon current remains steady till it begin retreating toward the beginning of Sept. Rainfall decreases from N to S and From E to W.

15. Transition Season . The retreat of monsoon from N Arabian Sea in marked by disappearance of stratus cloud over Sindh- Mekran Coast. There is a gradual rise in day temp in coastal belt in Sep and Oct where a secondary maximum temp occurs in Oct. This the period of min rainfall.

Aviation Weather Hazards

16. Winter Season . Skies are normally clear & weather is Fine. Major weather systems are WD's. Land & Sea operations are generally not affected by weather on most of the days. Heavy Snowfall activity in hills may affects Ops for 2-3 days.

Visibility Rainfall Ts/Hail/Squall Upper winds

Generally Good- Sindh

- NWFP bad in Em & E in association with WD .

- Punjab (Jan & Feb) WS Mist/HZ ( M/E).

3-5 rainy days / month

1-2 inches of rainfall

rainfall increases toward North

over hills Snowfall

TS 2-5 Days / Month, pronounced in Mar over Punjab & NWFP.

Hail/Squall- associated with active WD( Jan- Mar)

Upto 3 Km -NWLy -NLy

10-15 Kts

3-6 Km - NW Ly 30 Kts

6-9 Km - W Ly 50 Kts

9-12 Km - W Ly 80 Kts

WD - Lower levels backs

10 - 20 Kts

-Higher level strengthen

100 Kts

Icing Jet stream CAT F/L & M/L

Clear air- No icing

Active WD- Lt- Mod

Very Active WD- Severe

270 N- 120 Kts

340N - 100 Kts

11 Km- Strong W Ly Jet

Associated with Jet

250 N-27 N

300 N- 32 N

F/L

Sindh- 4 Km,

NWFP & Punjab - 2-3 Km

In WD - 1 Km

M/L - 9-12 Km

17. Hot Season. Major weather systems affecting weathers are WDs and Troughs in Westerly. Temperature increases gradually. Weather is generally dry and Hazy except on coast and near hills. Sea operations are not affected by weather. Land Operations are rarely affected when Dust storm reduces Vis to few meters and wind speed > 50 Kts. On an average 25 days in each month TS/DS affects Air Ops for few hours especially during Night / Evening hrs.

Visibility Areas Rainy Days / Season

TS /Season

DS/ Month Hail Squall

Poor due to Dust

No of days increases from Apr to Jun

Mekran

Sindh

Punjab

Baluchistan

NWFP

03

02

1-2

1-2

6

02

1-2

2-4

5-7

3-4

-

1-2

3-4

2-3

1-2

Rare (Apr)

Rare (Apr)

Rare

Rare

More

Rare

Nil

Nil

Nil

Nil

Upper winds CAT Jet stream F/L & M/L

< 3 Km – W Ly - SW Ly 15 Kts

3-6 Km - W Ly - NW Ly 30 Kts

6-9 Km - NW Ly

40 Kts

9-12Km- W Ly

65 Kts

Very Rare

28-300N 11-13 Km Lt - Mod

Severe Deep UAT

Very Few

290 N -12 Km

270° 60- 70 Kts F/L

Sindh 4.5 Km

Rest - 3-4 Km

M/L

10-13 Km

18. Monsoon Season . This month major system affecting weather are Monsoon Current, Monsoon Depressions and WD’s (North Pak). The clouding is control by monsoon current and is usually of cumuliform with Cb development in a/n hrs.

Visibility Rainfall TS &DSHail/Squall

-Poor due HZ - Coast- $-7 days in Jul & Aug Improves in Sep

-

Poor due DHZ & DS - Punjab Baluchistan &Sindh 11-Jul,10- Aug & 06 - Sep.

- Poor vis days over rest Pak is 2-4 days.

Baluchistan- Hills gets more rainfall than Plateau

-1-2 R/Days- Coast

-2-3 R/Days- Sindh

-Freq Increases toward N & E side of Hills in NWFP

3- Jul, 3- Aug & 2- Sep

- Increases toward NE Punjab 3-4 R/Days

- RF decreases In Sep

-More - Punjab, NWFP & Hills

-Less - Baluchistan & Sindh

-Rawalpindi 13-14

-NWFP-TS 3-6 DS-7

-Peshawar TS 8-9 DS 4-3

-Baluchistan TS 1-2 DS 4-6

-Sindh TS 1-3 DS 2-5

-Mekran- No Hail but Squall 1/ Season

- TS in NWFP, Hills of Punjab & Baluchistan is accompanied with Hail & Squall

Upper Winds CAT & Icing F/L & M/L

UPTO 27°N 27°-34° N CAT - Absent

Icing Few occasion above 5 Km in Rain or thick AS, Large CU/CB.

F/L

Sindh 5 Km

Rest- 6 Km

M/L 12- 15 Km

1.5 Km

1.5- 3.0 Km

3.0- 6.0 Km

6.0- 9.0 Km

9.0-12 Km

WNWLY 5-10 Kts

NE LY 5 Kts

NE LY 5-10 Kts

NE LY 15-20 Kts

ENE LY 25-30 Kts

SE LY 10 Kts

WNWLY 5 Kts

NWLY 5-10 Kts

NWLY 15-20 Kts

NWLY 30 Kts

19. Transition Period This is the best season for air operations with cloudless skies. Sindh coast may be affected by CS or Depression for couple of days on rare occasion.

Visibility Rainfall TS &DS Hail/Squall

Good to Excellent except for M & E it reduces due to Haze

Frequency Very Less

Coast, Sindh, Baluchistan & Punjab- <1 D/Season

Hills of Punjab & NWFP-1-2 D/Season

Coast - Rare( Oct)

Punjab & Baluchistan - 1 DS/TS per season

NWFP- 1-2 DS / TS (Oct) Rare( Nov)

01 in Two Season

Upper winds CAT Jet stream F/L & M/L

<1.5KmNW-W8-10 Kts

1.5-3.0 Km- W/NW 15-Kts

3.0-6.0 Km- NW 20 Kts

6.0-9.0 Km - W/NW 25 Kts

9.0-12 Km - W/NW 25 Kts

CAT near STJSTJ start appearing in North by mid Oct at 12 Km 80-100 Kt

Sindh- 4.5-5 Km

Rest- 4-4.5 Km

M/L - 11-14 Km

20. Seismic Disturbances . Pakistan is subject to frequent seismic disturbances because the tectonic plate under the subcontinent hits the plate under Asia as it continues to move northward and to push the Himalayas ever higher. The region surrounding Quetta is highly prone to earthquakes. A severe quake in 1931 was followed by one of more destructive force in 1935. The small city of Quetta was almost completely destroyed, and the adjacent military cantonment was heavily damaged. At least 20,000 people were killed. Tremors continue in the vicinity of Quetta; the most recent major quake occurred in January 1991. Far fewer people were killed in the 1991 quake than died in 1935, although entire villages in the North-West Frontier Province were destroyed. A major earthquake centered in the North-West Frontier Province's Kohistan District in 1965 also caused heavy damage.

Check Assimilation

State true or false / Fill in the blanks

1. (a) Pakistan experiences both summer and winter rainfall. (True/False)

(b) During Monsoon season the major system affecting weather are Monsoon Current, Monsoon Depressions and WD’s (North Pak). (True/False)

2. (a) Major weather systems affecting weathers during hot season are ________and ___________.

(b) The retreat of monsoon from N Arabian Sea in marked by disappearance of stratus cloud over __________Coast.

.

(c) The country is divided into four administrative provinces _______, ____________, ___________ & ______________.

(d) The best season for air operations with cloudless skies is between ____________& ____________.

21. Conclusion

(a) Pakistan has four provinces which are separated by either Mountain rangers or Indus River.

(b) Like India, Pak also has four seasons; with difference that monsoon season is for lesser duration. (Mid July - First week of Sep)

(c) The sub-Tropical location of Pakistan that tends to keep the temperature high, particularly in summer.

(d) Weather over Pak occurs mainly due to WD's/induced low and trough in westerlies.

(e) Probability of cyclonic storm reaching Pak is very less.

(f) There are two sources of rainfall in Pakistan: the Monsoon and the Western Depression. The former takes place from Jul to Sep and the latter, Dec to Mar.

(g) Pak receives major amount of rainfall in monsoon season.

Check Assimilation: The Key

1. (a) True.

(b) True.

2. (a) WD’s and trough in westerly

(b) Sindh – Mekran Coast

(c) Punjab, Sindh, Baluchistan &North West Frontier Provinces

(d) Mid Sept & Mid November

Note: Climatological details given in the chapter are by no means exhaustive

CHAPTER II

CLIMATOLOGY OF TIBET & HIMALAYAS

Chapter objectives

After reading this chapter, you should be able to:-

Assimilate the Physical Features, Climatic Elements and different Climatic Regions of Tibet & Himalayas.

Understand how the Pressure, Temperature, Rainfall and Humidity vary during different seasons in different regions of Tibet & Himalayas.

Understand the different seasons of Tibet & Himalayas and Aviation weather Hazards.

Structure

1. Introduction

2. Physiography of Tibet

3. Climate

4. Seasons of Tibet

5. Climatology of Himalayas

6. Physiography and Seasons

7. Conclusion

Introduction

1. Politically autonomous Tibet is the land of Buddhist people, as the monastery of Dalai Lama is situated in Tibet. Tibetan Plateau is famous among the meteorologists also as it has a very prominent role in governing the climate of south Asia. The plateau is surrounded by large mountain ranges. The availability of data over the region is very meagre. The primary impact of Tibetan Plateau is on thermodynamics of the atmosphere. In addition to the thermal influence, this plateau exerts major mechanical blocking influence on both the zonal and the meridional flow, which results in strong horizontal and vertical perturbations in the atmospheric flow. The plateau is also the major region of maximum dissipation of atmospheric kinetic energy and momentum. It modifies the intensity and speed of synoptic scale systems moving near the plateau.

Physiography

2. On the northern slope of Central and Eastern Himalayas lies the Tibetan Plateau, the highest and the most extensive plateau of the world covering an area of more than one million square kilometer. Tibet is the highest plateau in the world with average elevation of 5,000 m above sea level. The Chinghai-Tibet Plateau covers Chingha province, including Chamdo region, portions of Western Szechwan and Western Yunnan from the northern part of the Himalayas.The surface of Central Asia consists of two primary elevations separated by a trough like depression. The northern elevation is the Tienshan Plateau with an average elevation of 3,300 m and the southern elevation is the Tibetan Plateau. The intervening depression is the Tarim Basin. The Kunlun Mountain Systems form the northern boundary of the Tibetan Plateau, while the Central and Eastern Himalayas form its southern boundary. Some famous rivers like Brahmputra, Mekong, Yantze and Hwang Ho originate from Tibet. Nair Singh range, Bayan Kara Sangh and Naan Ling Shan are few ranges on the plateau.

Climate

3. It is a region of extreme climate, with large seasonal and diurnal variations of weather elements. Near the ground, Tibet has lowest temperature, lowest pressure, smallest absolute humidity and greatest wind speeds when compared to other regions of the earth’s surface at the same latitude. In free atmosphere at the same height,

Tibet is marked by highest temperature, lowest pressure and maximum absolute humidity in summer, highest pressure in winter and minimum wind speed in annual average compared to the free atmosphere conditions at the same height over the other regions in the same latitudinal belt. Following are some of the parameters of the climate of Tibet:

(a) Rainfall. Tibetan Plateau is generally classified as an arid or semi-arid region. Along the slopes of the plateau, particularly along the southern slopes, rainfall is plentiful. In the western and northern sectors of the plateau, arid and semi-arid climates prevail. In the interior plateau, the annual amounts of rainfall rapidly decrease from 60 cm in the eastern part to 5 cm in the western part. Rainy season extends from April to October but most of the rainfall occurs during the period from June to August. Each year, about two to three heavy rain spells are observed over the plateau.

(b) Planetary Boundary Layer . The PBL of Tibetan Plateau is the highest in the world by virtue of the height of the plateau. The thermal Low of summer and the cold High of winter over the plateau are formed in this layer and these constitute the world’s highest PBL pressure systems with usual thermodynamic characteristics. The summer thermal Low may reach 400 hPa. This may be compared with thermal Low in North Africa reaching only to 850 hPa level and in Iraq reaching 700 hPa level. Over the Indian Plains, the thermal Low reaches to 500 hPa. During winter, the cold High over the Tibetan Plateau may reach to 500 hPa. These boundary layer systems become important sources for transients in the pressure and wind systems of the middle and lower troposphere in the northern hemispheric sub-tropical belt.

(c) Thunderstorms . During the year, there are more thunderstorm days (about 90) over the Tibetan Plateau than elsewhere in this zonal belt, as compared to 5-10 days in Iran & North Africa, 10-20 days over the Atlantic, 40-60 days in China & North America and 5-10 days over the Pacific. Also, there are more hail days, about 55 annually. As many as 64 hail days were observed in the year 1957.

(d) Seasonal Oscillation of the Sub-Tropical Westerly Jet Stream . During the northern summer, the sub-tropical westerly jet stream is relatively weak and lies entirely to the north of the plateau, easterlies prevailing to the south of the plateau in the upper troposphere. During the northern winter, it shifts southwards and intensifies considerably. Before reaching the plateau from the western side, it splits into two branches; the primary stronger branch running south of the plateau and the secondary, weaker branch running north of the plateau and sometimes getting mixed up with the polar front jet stream.

Seasons

4. In a year, Tibet gets following four seasons:

(a) Winter (Dec, Jan, Feb)

(b) Spring (Mar, Apr, May)

(c) Summer (Jun, Jul, Aug, Sep)

(d) Autumn (Oct, Nov)

Winter & summer are the main seasons

(a) Winter

(i) Pressure Pattern. As the Siberian High establishes itself to the north of Tibet, steep pressure gradient is seen over Tibet with pressure values 1020-1026 hPa.

(ii) Upper winds. Upper winds are westerlies with a speed reaching 75 Kts at 200 hPa level (Jet core is to the south of Tibet i.e. over India).

(iii) Weather. This is the coldest season of the year. During this season temperature reaches up to -10˚c. Precipitation occurs in the form of both rain and snow due to passing western disturbances. Visibility deteriorates due to occurrence of fog. Following are some of the parameters:

PARAMETER DEC JAN FEB

Mean Temperature

-01.9 -02.3 0.80

Max Temperature 15.3 06.8 09.2

Min Temperature -09.1 -10.2 -06.9

Relative Humidity 35.0 28.0 27.0

Rainy Days 0.20 0.50 0.50

Snow Days 0.40 0.50 0.50

(b) Spring .This is just a transition season between winter and summer season. Min temp in mar is still -3˚c but in May it touches 12˚c. Rainfall is more in comparison to winters. Thunderstorm & hail occur during this season. As the season progresses, the amount of rainfall in a month increases. Following are some of the parameters observed during this season:

PARAMETER MAR APR MAY

Mean Temperature

04.3 08.3 12.6

Max Temperature 12.0 15.7 19.7

Min Temperature -03.2 0.90 05.1

Relative Humidity 30.0 35.0 40.0

Rainy Days 01.8 03.7 07.7

Snow Days 01.8 02.2 01.3

(c) Summer

(i) Pressure Pattern . During summer, as the heat low forms over central Pakistan and adjoining Rajasthan, over Tibet also low pressures are observed. Pressures are of the order of 1000 – 1002 hPa.

(ii) Upper Winds . Due to northward movement of STR, anticyclone over Tibet is observed. Tibetan High is a large anticyclone that is known to have its largest amplitude near 200 hPa during the summer season.

(iii) Weather. In summer, the max temp reaches up to 23˚c. During this season, due to SE Monsoon over China, Tibet gets maximum rainfall especially over its eastern regions and the slopes. Following are some of the parameters observed during this season:

PARAMETER JUN JUL AUG SEP

Mean Temperature

15.5 14.9 14.1 12.8

Max Temperature 22.5 21.7 20.7 19.6

Min Temperature 09.2 09.9 09.4 07.6

Relative Humidity 53.0 67.0 70.0 65.0

Rainy Days 14.4 20.4 21.2 13.8

Snow Days 0.00 0.00 0.00 0.10

Wind W’ly E’ly E’ly E’ly

(d) Autumn . This season is again a transition between summer and winter season. Amount of rainfall starts reducing. During this season, temperatures start getting lowered reaching up to even – 5˚c sometimes in November. Following are some of the parameters observed during this season:

PARAMETER OCT NOV

Mean Temperature

08.1 01.9

Max Temperature 16.4 11.6

Min Temperature 01.4 -05.0

Relative Humidity 48.0 37.0

Rainy Days 02.9 0.60

Snow Days 0.90 0.70

Wind E’ly E’ly

CLIMATOLOGY OF HIMALAYAS

5. The Himalayan mountain range is the highest at the same time youngest in the world. It evolved in the orogenesis of the tertiary period only about 70 million year ago. During tertiary period there were three main continents with the Tethys Sea spanning the equatorial regions. The disruptions of Gondwanaland give rise to the peninsular India. At the same time the overloaded heavily sediment bed of Tethys Sea got uplifted into the lofty Himalayas.

6. Study of climate of Himalayas is important because Himalayas serve as a great barrier to the cold continental air. In monsoon months Himalayas drive the rain bearing winds towards the Indian plains. Himalayas represent political boundary.

7. Physiography. The geographical limits of the region lay approximately between latitude 27°N and 36°N and longitude 72°E and 100°E. The area lies in the temperate zone. It consists of mountains and plateaus. On the northern side of these mountains lies Tibetan Plateau. The loftiest mountain ranges of the world radiate from the Pamir Knot near the intersection of latitude 38°N and longitude 74°E. It extends over 3000km with a varying length of 250 to 300km

8. To the north of Himalayas and almost parallel to it is the Karakoram range running SE wards from Pamir Knot through the northern parts of Kashmir. Important mountains and peaks are Mt.everest (8848m) K2 (8611) Kanchenjunga (8585m) Daulagiri (8172) Nangaparbat (8126), Nandadevi (7817).

9. The important rivers are the Indus, the Ganges, and the Brahmaputra. The Indus and Brahmaputra start from the Kailash Mountain in Tibet. The Ganges and its tributaries run from the central Himalayas and flow in southerly and easterly direction through the plains of north India.

10. Climatological Divisions. Himalayas are broadly divided into following three divisions.

(a) Western Himalayas

(b) Central Himalayas

(c) Eastern Himalayas

Each region, according to elevation is further divided in to three ranges

(a) Lower range (below 5000ft)

(b) Outer range (5000ft-10000ft)

(c) Inner range (above 10000ft)

11. The study of the climate of Himalayas is hampered due to following reasons; remoteness of areas makes it difficult to have weather stations, nature of mountain terrain restricts the local weather of any station to represent only a very small area, difficulties in making standard observations at mountain stations summit, slope and valley bottom.

12. Climatologically the year can be divided into four seasons as follows:-

(a) Winter season

(b) Spring season

(c) Monsoon season

(d) Autumn season

Winter

13. The period is characterized by the fall in temperature and a corresponding rise in pressure. The winds are W’ly to NW’ly, which bring in cold dry continental air. The

northern portion is extremely cold and dry. The winds above three km are Wly and increase with height. Wly jet stream lies over India at 12 km. severe turbulence occurs over peaks/ ridges in afternoon hours

Western Himalayas: Maximum Temperature

14. Lower Ranges. Men daily max temperature varies between 10 – 22°C. There is a marked fall of 2 -4°C in temperature in Jan. Thereafter the temperature gradually increases with the advance of season. The increase in temperature is quite conspicuous in March and is of the order of 4- 6°C. The seasonal and the monthly variation are between 1- 6° C.

15. Outer Ranges. Mean daily max temperature varies between 5 and 14°C. The arrival of winter is marked by a fall of 1 - 4°C in max temperature. The departure is indicated by a rise in temperature of 5 -7°C in March. Monthly variation is between1 and 2°C.

16. Inner ranges. It varies between 5 to -8°c .The fall in temperature from Dec to Jan is between 3-4°C.The seasonal variation is of the order of 4-8°c

Western Himalayas: Minimum Temperature

17. Mean daily min temperature in the lower ranges varies between 2-11°C. The temperature falls by 1-2°C from Dec to Jan and then increases with the advance of the season. In the outer ranges the mean daily min temp varies between 6 to-4°C. Murree and Srinagar experience sub freezing temperature in Jan and Feb. The seasonal variation is of the order of 4-7°C. The inner ranges experience sub-freezing mean daily temp throughout the season. It varies between -6 to -23°C.

Western Himalayas: Rainfall

18. The distribution of rainfall over the ranges is quite complex because of the complexity of the relief. Over the lower ranges the mean monthly rainfall varies between 2and14cm per month. As the western disturbances move northeasterly direction, they give more rainfall on the ranges facing west. Rainfall on all the ranges shows an increase with the advance of the season from Feb to March .The variation in the mean monthly rainfall over the outer ranges is less than that over the lower ranges; it varies 3-12 cm per month. The rainfall is min in Dec. This is due to lower frequency of western disturbances in Dec. Over the inner ranges the Leah is completely shaded

from the winter rains and gets only 3 cm of rain during this season. The total rainfall for the season is less than 4cm.

Western Himalayas: Thunderstorm

19. Most of the thunderstorms occurring during this season are associated with the western disturbances and occur in the cold front. Thunderstorm does not occur over the lower ranges in Dec and Jan. in Feb and March the frequency of thunderstorms is about 1-2 per month. Outer ranges experience more days of thunderstorms with the exception of Srinagar valley .the frequency increases with the advance of season. Thunderstorm does not occur over the inner ranges during the winter season.

Western Himalayas: Fog

20. Over the outer ranges fog occur more frequently than the lower ranges. The frequency of occurrence of fog is more in Jan and Feb as compared to that in Dec and Mar this is due to the lower temperature that prevail in Jan and Feb.

Central Himalayas: Maximum Temperature

21. Lower ranges. The mean daily max temperature varies between 15-27°C. The fall in temperature from Dec to March is of the order of 1-2°C. The seasonal variation is between 6-8°cand monthly variationbetween1-5°C.

22. Outer ranges. The mean daily max temperature varies between 10-18°C in Dec 8-17° in Jan, 9-19° in Feb and 13-24° in March .the seasonal variation is of the order of 4-7°C.

23. Inner ranges. Over inner ranges the data is scanty. Walachumggola experience max temperature of 14°c in Dec, 6°c in Jan, 7°c in Feb and 9°c in March. The seasonal variation is of the order of 8°c.

Central Himalayas: Minimum Temperature

24. Mean daily min temp over the lower ranges varies between 3-10°C in Dec, 2-9° C in Jan, 4-10°C in Feb and 7-13°C in March. Lowest min is observed in Jan .Seasonal variation is of the order of 4-7°C.In outer ranges it shows wider variation, varies

between -2 to 11°C. Sub-freezing min temperature occurs over the inner ranges. Walchumggola experiences -03°C.

Central Himalayas: Rainfall

25. Rainfall in this region is derived from the passing western disturbances. Over the lower ranges the rainfall varies from 1-7cm per month there is a tendency for mean monthly rainfall to decrease from west to east. Over the western ranges the western half experiences more rainfall than the eastern half. The rainfall over the inner ranges varies between 2and 5 cm per month max rainfall over this region occur in March

Central Himalayas: Thunderstorm

26. Thunderstorm occurring during this season is mostly associated with the western disturbances which move across the area. The frequency of occurrence increases gradually with the advance of the season and reaches the maximum in March. The outer ranges in the western half of the region seem to be more susceptible for the occurrence of thunderstorms. The frequency of occurrence varies from 1-6 per month. Thunderstorms do not occur over the inner ranges.

27. Fog over the lower ranges Tehri gets 6 days of fog in Jan and 2 days each in Feb. and March. The outer ranges get fog more frequently. The frequency of occurrence of fog over these regions varies between 1-4 in Dec 3-8 in Jan 1-6 in Feb and 1-5 in Mar.

Eastern Himalayas: Maximum Temperature

28. The max temperature over sub Himalayan area in this season is about 15-20°C and over outer Himalayas it is 5-20°C and that over the inner Himalayas is 0-5°C. In all the ranges January appears the month when the max is lowest.

Eastern Himalayas: Minimum Temperature

29. It is seen that the zero deg isotherm in all the month passes through the outer Himalayan region and it is least in Jan. Coldest region in all the ranges in this season is the month of Jan.

Eastern Himalayas: Rainfall

30. This is the season of least rainfall over this area. The WDs passing through north or across cause rainfall over the area. It is seen that rainfall amount decreases towards inner Himalayas. Very little rainfall is recorded all along the eastern Himalayas in Dec and Jan. a definite increase in rainfall.

Eastern Himalayas: Thunderstorm

31. From the isolines of number of days of thunderstorm for Dec and Jan it is observed that thunderstorm do not occur over the Himalayas or on its northern side during these months. But the frequency of thunderstorm for Feb is 1 to 2 and that for March is 2-5 over sub-Himalayan and outer Himalayan area. Inner Himalayas are still free from any thunderstorm during these two months also. Thunderstorm of March is of destructive nature and is often associated with heavy fall of hail.

Eastern Himalayas: Fog

32. The fog over the Himalayas is associated with the western disturbances. Frequency of no of days of fog in different months of season is about 1 to 5 days over all the ranges of Himalayas

Eastern Himalayas: Spring

33. This is the period of transition from winter to summer. The pattern of winds and pressure undergoes a remarkable change. The temperature gradually increases during this season and reaches max in May or June. Insolation is intense and sets up convection currents and instability in the atmosphere.

Western Himalayas

34. Maximum temperature. It varies between 22-29 c in April and 30-34 c in May, over the lower ranges. Monthly variation is 6-8 c .over the outer ranges it is about 19 c in April and varies between23-25 c in May the monthly variation is between 4-6 c over the inner ranges it varies between 5-13 c in April and 14-17 c in may. The monthly variation is between 4-9 c.

Southwest Monsoon

35. This period is from June to Sept. monsoon sets over eastern Himalayas in first week of June and extends to western Himalayas by July winds are generally easterly as normal position of AMT is south of Himalayas. During break, AMT shifts to the foothills of Himalayas causing heavy rain fall. Long break causes extensive rainfall over upper catchments area of river originating from Himalayas sometimes lead to flood. Winds move north of 35°N. Recurvature of monsoon depression cause heavy rain in lower altitudes and heavy snowfall over higher latitude.

Western Himalayas

Maximum Temperature

36. Lower Ranges mean daily max temperature lies between 29-37°c. The temperature gradually falls from June to September. The seasonal variation over the entire range is of the order of 4-5°c

37. Outer ranges. Mean daily max temperature lies between17-27°C. Seasonal variation is of the order of 4 -5°C.

38. Inner ranges. Mean daily max temperature varies between 20 -25°C. Unlike the lower ranges the max temperature in Jun and then remains almost uniform during Jul and Aug with a fall in Sep.

Minimum temperature.

39. Lower ranges. Mean daily minimum temperature varies between 18 -26°C. The monthly variation is of the order of 1 -3°C.rhe seasonal variation over the range is about 4°C.

40. Outer ranges. Mean daily minimum temperature varies between6 and 16°C. The seasonal variation is of the order of 3°C.

41. Inner ranges. The minimum temperature over these ranges varies from 5 to 10°C. All the stations indicate remarkable increase of 3°Cfrom Jun to Jul. during Jul and Aug the temperature remains almost constant and then show a fall of 4 to 5°C in Sep. the seasonal variation is of the order of 4 – 5°C.

Rainfall.

42. The rainfall distribution over the ranges shows wider variation. Mean monthly rainfall over the lower ranges varies between 2 -24 Cm and it increases appreciably in Jul with the onset of monsoon and decreases again in Sep with the withdrawal of monsoon. The outer ranges receive comparatively more rainfall than the lower ranges. August is the month of max rainfall over these ranges. The inner ranges experience much less rainfall as compared to the lower and outer ranges mean monthly rainfall varies from 1-5 cm per month.

Thunderstorm .

43. The frequency of thunderstorm over the lower ranges varies from 3-4 per month. Max no of these storms occur in the month of July. Over the outer ranges thunderstorms occur more frequently than the lower ranges and the frequency varies from 2-17 per month. The frequency reduces with the onset of monsoon. Inner ranges do not experience rainfall in June and September, however during July and august the frequency of occurrence is one per month

Fog

44. Over the outer ranges fog occur more frequently than the lower ranges .it is seen that the stations, which are on the windward side of the monsoon current, get fog more frequently than stations on the leeward side. Over inner ranges fog do not occur in July august and September. During June Leh report only one-day fog.

Central Himalayas

Maximum Temperature.

45. Lower Ranges. Mean daily max temperature lies between 24 and30°c.the max temperature falls by 3-4°cwith the onset of monsoon on the western half whereas the corresponding fall on the eastern side is only 1°c.seasonal variation on the western side is about 5°c while on the eastern side is about 1°c.

46. Outer ranges. The mean daily max temperature varies from18-28°c the max temperature fall by 2-3°cin the western half on the onset of the monsoon where as in the eastern half the variation is very less.

47. Inner ranges. The data recorded over this region is very scanty. Walchumgla shows an increase of 1 c in July and subsequently a fall of 1 c in August.

Minimum temperature .

48. Lower ranges. Mean daily minimum temperatures are of the order of 18 c 23 c. the seasonal variation in the western and the eastern half is of the order of 1 – 2 c.

49. Outer ranges. Mean daily min temperature is of the order of 18 to 19 c. seasonal variation is of the order of 1-2 c only.

50. Inner ranges there is not much of the data available from this region Walchumgola shows a min temperature of 8 c in June and increases to 9 c in July.

Rainfall.

51. The period from June to September is marked as the season of rainfall over India. The distribution of rainfall depends mainly on three factors, the movement of the tropical depression, location of the axis of monsoon trough and the tropical features of India.

Thunderstorm.

52. Thunderstorm occurs more frequently over the outer ranges. They are formed due to orography and due to the shift of the axis of monsoon trough. The frequency is maximum in June and reduces gradually with the advance of the season.

Fog

53 Fog does not occur over the lower ranges. Over the outer ranges fog occur quite frequently. The frequency shows an increase in August and a decrease in September, with the withdrawal of monsoon. This closely indicates that the occurrence of fog is closely related with the monsoon current. Fog does not occur over the inner ranges.

Eastern Himalayas

Maximum Temperature

54. During this season the max temp reach its highest value. The mean temperature on the outer ranges is 15-23 c and on the inner ranges is 10-15 c. the rise in temperature is about 2-3 c in all the ranges. The variation of the temperature over the ranges is very small and of the order of 1-2 c.

Minimum Temperature

55. From May to July there is a rise of temperature in all the ranges and it falls from august to September. July and August are the month during which the min temperature are the highest in all the ranges. The monthly increase in min temperature from May to July is about 1-4 c while the fall from august to September is only 1-2 c

Rainfall

56. this is the rainiest season and the rainfall occur in the southern slopes of Himalayas. This season alone accounts 65% of the total annual rainfall. there is increase in rainfall from June to august in outer Himalayas and a decrease is noticed over inner ranges in the same period

Thunderstorm

57. There is a decrease of thunderstorm activity from may to June inner Himalayas do not get any thunderstorm during this season, only one thunderstorm is recorded over the outer Himalayas.

Fog

58. Fog increases from May to June and a decrease from august to September the fog reported in the outer Himalayas is only one day and in the inner Himalayas fog does not occur in this season.

AUTUMN

59. During this season the monsoon gradually retreats .the deflected monsoon current, which penetrate under the under the influence of the monsoon trough into north west India is replaced by dry north westerlies. The weather is mostly fine and this is the best period for flying.

Western Himalayas.

Maximum temperature

60. The mean daily max temperature over the lower ranges varies between25-29 c in October and between 19-23 c in November. The monthly variation is of the order of 4-6 c. Max temperature over outer ranges varies from 15-23 c. Over the inner ranges the mean daily max temperature lies between 13-16 c in Oct and between 4-11 c in Nov

Minimum Temperature

61. all stations indicate a fall in min temp from Oct to Nov over the lower ranges it varies from 8-15 c .It lies between –1to 12 c in the outer ranges .the monthly variation

is of the order of 5-6 c .the inner ranges exp sub freezing min temperature during this season . Mean daily minimum temperatures vary between 0to –12 c.

Rainfall

62. the rainfall over the lower and the inner ranges is below 2 cm .the outer ranges get comparatively more rainfall, the mean monthly rainfall over the outré ranges is between 1-4 cm per month.

Fog

63. the occurrence of fog is almost nil in this season over the entire region.

Central Himalayas.

Maximum temperature.

64. Over the lower ranges it varies from between 17 and 30 c and 13-25 c over the outer ranges. The inner ranges also show a drop of about 4 c in November. On the northern side of Himalayas the fall of maximum temperature is also quite well marked.

Minimum temperature

65. Mean daily minimum temperature over the lower ranges varies from 9-17 c the fall of min temperature in November is between 2-5 c .over the outer ranges it varies from 1-15 c in the inner ranges Walchumgola reported 4 c in October and fall to 0 c in November

Rainfall

66. Mean monthly rainfall shows fall from October to November on all the ranges. In lower ranges it varies between 4 to 20 cm and falls to 2cm in November. Over the outer ranges it varies from 3-17 cm in October and is about 2 cm in November. On the inner ranges the rainfall is about 11 cm in October and falls to 2 cm in November

Fog

67. Fog occurs very rarely over the lower ranges. The frequency of occurrence is about 2 days per month. Outer ranges are more susceptible to the occurrence of fog .the occurrence of fog drops down to below 4 days in the month of November

Eastern Himalayas

Maximum temperature

68. Over the outer ranges it varies from 12 to 20 c and hat over the inner range is 7 c to 11 c the decrease of temperature from September to October in all the ranges is about 2-3 c whereas fall from October to November is about 3-4 c

Minimum temperature

69. The monthly decrease in mean daily min temperature from September to November is more rapid than that of mean max temperature. Over the outer Himalayas it varies from –4 to 15 c and that over the inner Himalayas it is –8 to –15 c and over the inner ranges it varies from –17 to –30 c

Rainfall

70. The mean monthly rainfall varies from 4-14 cm in October and 2-4 cm in November. These large variations in rainfall are due to the withdrawal of monsoon in October from the area. Over the outer ranges it is below 5 cm. the rain fall over the inner ranges is less than 1 cm.

Thunderstorm

71. There is a considerable decrease in the thunderstorm activity, in all the ranges. The frequency of thunderstorm over the lower ranges is less than five days. Thunderstorm does not occur over the outer and the inner ranges.

Fog

72. The frequency of fog varies from 1-5 days over the lower and the outer ranges. Fog does not occur over the inner ranges.

Check Assimilation

State true or false / Fill in the blanks

1. (a) Tibet is marked by ________temperature, ________ pressure and __________absolute humidity in summer, ________pressure in winter and _______wind speed in annual average compared to the free atmosphere conditions at the same height over the other regions in the same latitudinal belt.

(b) PBL of Tibetan Plateau is the Lowest in the world by virtue of the height of the plateau (True/False)

2. (a) During the year, there are more thunderstorm days over the Tibetan Plateau than elsewhere in this zonal belt, (True/False)

(b) In addition to the thermal influence, this plateau exerts ______________influence on both the zonal and the meridional flow, which results in strong horizontal and vertical perturbations in the atmospheric flow..

(c) Most of the thunderstorm occur during winter season over western Himalayas are due to _________ and occur in _______ front.

Conclusion

73. Tibet is region of extreme climate, with large seasonal and diurnal variations of weather elements.

74. It plays major role in affecting the monsoon over Indian Subcontinent.

75. The major seasons are summer and winter. June is the hottest month and January is the coldest month.

76. In comparison to other regions of same latitude, Tibet has lowest temperature near ground. It has highest temperature in free atmosphere in summer. It receives more number of thunderstorms also.

Check Assimilation: The Key

1. (a) Highest, Lowest, maximum. Highest, minimum.

(b) False ; Highest

2. (a) True

(b) mechanical Blocking

(c) WD’s, Cold front.

(d) Lower , Higher

CLIMATOLOGY OF CHINA

Geographical Location

1. China, in Southeastern Eurasia, faces between Northwestern Pacific Ocean and South China Sea with two big offshore islands, Taiwan and Hainan and many small islands. China is separated from North Indian Ocean by India, Bengal, and Indo-China Peninsula. The country extends from 53ºN in Heilongjiang Province, to 4°N, James Shoal in the South China Sea. The Southern edge of Hainan Island lies at 18ºN. In the west- east direction, China extends from 73ºE to 135ºE. The total area, 9,600,000 Km2, is about a quarter of Asia and equals almost whole of Europe, China comprise 6.4% of Global area. There are more than 5000 islands in China.

2. China covers an area of sub continental dimension, if the extremely far distances in the horizontal direction are considered. The farthest air distances measured around 4,500 Km in a west to east direction and between 4,200 and 5,600 km in a north to south direction, depending on southern most locality referred. The shortest air distance measured in a north to south direction is close to 1600 Km, between 42ºN and 20ºN over central china.

Topography

3. Mountains occupy about 43% of China’s land surface; mountainous plateaus account for another 26%; and basins, predominantly hilly in terrain and located mainly in arid regions, cover approximately 19% of the area. Only 12% of the total area can be classified as plains. China can be divided into six major geographic regions, each of which contains considerable diversity in terrain and topographic-relief.

4. The Northwest. This region consists of two basins—the Junggar Pendi on the north and the Tarim Basin on the south—and the lofty Tian Shan mountain chain. The Tarim Basin has the vast, sandy Takla Makan Desert, the driest desert in Asia. Dunes in its interior rise to elevations of about 100 metres (328 feet). The Turpan Pendi, the largest area in China with elevations below sea level, commands the southern entrance of a major pass through the Tian Shan. The Junggar Pendi is primarily a region of fertile steppe soils supporting irrigated agriculture, although it contains some areas of sandy and,stony-desert.

5. The Mongolian Borderlands. Located in north central China, this is a plateau region of mainly sandy, stony, or gravel deserts that grade eastwards into steppe lands with fertile soils. The flat to rolling plains are partitioned by several barren, flat-topped mountain ranges. Along the eastern border is the higher, forested Da Hinggan Ling

(Hinggan-Mountains).

6. The Northeast. Comprising all of Dongbei Pingyuan east of the Da Hinggan Ling, the Northeast region incorporates the Manchurian Plain and its bordering uplands. The plain has extensive tracts of productive soils, and the uplands are hilly to mountainous, with numerous broad valleys and gentle slopes. The Liaodong Bandao (Liaodong Peninsula) extending to the south is noteworthy for its good natural harbours.

7. North China. This region lies between the Mongolian Borderlands on the north and the Yangzi River Basin on the south and consists of several distinct topographic units. The Huangtu Gaoyuan on the northwest is formed by the accumulation of loess. The region is extensively terraced and cultivated. The Huabei Pingyuan, the largest flat lowland area in China, consists of fertile soils derived from loess. Most of the plain is under intense cultivation. To the east, the Shandong Qiuling (Shandong Highlands) on the Shandong Bandao (Shandong Peninsula) consists of two distinct areas of mountains flanked by rolling hills. The rocky coast of the peninsula provides some good natural harbours. To the southwest are the Central Mountains, which constitute a formidable barrier to north-south movement.

8. South China. This region embraces the Yangzi Gorges and the topographically diverse regions to the south. The Yangzi Gorges consist of a series of basins with fertile alluvial soils. The Sichuan Basin, located to the west, is enclosed by rugged mountain spurs of the Central Highlands and constitutes a relatively isolated area of hilly terrain. The area is known for its intensive terrace farming. The highlands of South China extend from the Tibetan Plateau east to the sea. One of the world’s most scenic landscapes is found in eastern Guizhou Province, where the terrain is dominated by tall limestone pinnacles and pillar-like peaks. To the east are the largely deforested and severely eroded Nan Ling hills, and along the coast are the rugged Southeastern Highlands, where bays with numerous offshore islands provide good natural harbours.

9. The Tibetan Plateau. The high, mountain-rimmed plateau of Tibet occupies the remote southwestern extremity of China. The world’s highest plateau region, Tibet has an average elevation of about 4,877 metres (16,000 feet) above sea level. Bordering ranges include the Himalayas on the south, the Pamirs and Karakorum on the west, and the Kunlun and Qilian Shan on the north. The surface of the plateau is dotted with salt lakes and marshes, is crossed by several mountain ranges, and also contains the headwaters of many major rivers of southern and eastern Asia, including those of the Indus, Ganges, Brahmaputra, Mekong, Yangzi, and Huang He (Yellow River). The landscape is bleak, barren, and strewn with rocks. . The Tibetan Plateau contains many large saline lakes. The largest of these is the marshy Qinghai Hu

Major Rivers

10. The three longest river systems in China are the Yangzi, Huang He, and Xi Jiang, all of which flow in a generally west to east direction to the Pacific Ocean. The major

river of North China is the Huang He, called “China’s Sorrow” because of its devastating flooding throughout history.

11. The Yangzi River of central China is the longest river in Asia. A major transport artery, the Yangzi, has a vast drainage basin and enters the sea at Shanghai. The most important river system of southern China is the Xi Jiang.

Mountain chains

12. Of the mountain chains, one group is oriented from west to east, another from northeast to southwest. Many high west east oriented chains extends eastwards from the Tibetan Plateau – Altai, Tian Shan, Kun Lun, Tang Gu La, Gong Di si, Himalayas, Qi Lian, Qin Ling, Yin Shan, Nan Ling etc. NE – SW oriented mountains include da xing An Ling, Chang Bay, tai Hang, Wu sham Taiwan, hang Duan, etc.

Air Masses in China

13. The principle air masses that dominate China during different seasons are:

(a) Polar Siberian (from NE, dry and cold polar air mass by origin).

(b) Polar Pacific (from NE and ENE, dry and cold polar air mass by origin).

(c) Tropical Pacific (in summer from SE and E, warm and moisture laden by origin).

(d) Equatorial (in summer from SW, warm and moisture laden by origin).

Seasons

14. The four common seasons, winter, spring, summer and autumn normally also distinguished in the weather calendar of China. Winter and summer however regarded as dominating seasons. Due to enormous dimensions of china over space, the length of seasons and their exact dates are controversial. The four common seasons are normally applied in a statistically simplified way that divides the year in to four periods of 3- month duration each:-

(a) Winter: Dec – Feb

(b) Spring: Mar – May

(c) Summer: Jun – Aug

(d) Autumn: Sep - Nov

15. The four determined seasons in china were sometimes modified by two additional seasons, which are distinguished by particular weather conditions:

(a) Mei–yu (represents the late spring and early summer rains of April, associated with small depressions extending seawards along a trough which lies in an east-west direction between the Yangtze and the southern coast.)

(b) Crachin season (during Feb, Mar and early April the collision of maritime and continental air masses leads to the typical “crachin” formation of fog or low clouds along the coast of the China Seas)

Summer (Jun – Aug)

16. Pressure Distribution (Sea Level). The pressure field shows a relatively week pressure gradient so that for China only a moderate variation of pressure is experienced. As for Jul, the mean pressure at sea level, over the continent, drops to 995 hPa in the center of the low over Pakistan, while it rises to a maximum of 1025 hPa over N-Pacific Ocean. Over most parts of China, Pressure at sea level, mostly occurs at an intermediate level between 1000 and 1005 hPa.

17. Upper Air Features.The main features of the summer circulation may be summed up as follows:-

(a) At lower levels, Indian monsoon trough replaces the wintertime Mongolian cold high.

(b) In the middle troposphere splitting of the westerlies by the Tibetan plateau and the southern branch of westerlies vanish, while the subtropical high moves northward and strengthens considerable.

(c) In the upper troposphere, the Tibetan anticyclone and upper level easterly jet stream prevail and westerly jet stream greatly decreases.

Onset, Retreat and Duration of Summer Monsoon

18. Onset. The onset of monsoon from South to Northeast China starts in the first 10-day period of May and finally terminates in the last 10-day period of Jul. On set of the summer monsoon is usually defined by an abrupt increase of rainfall. The mean dates of the summer monsoon are therefore derived from long-term rainfall records.

19. The regions initially influenced by the summer monsoon in the beginning of May, are Hainan and southern parts of Taiwan islands. The monsoon advances to coast of China on May 10, and then advances slowly northwards. During the last 10 days of May, the monsoon front stagnates in southern China. Monsoon arrives in the middle and lower reaches of Yangtze in the mid Jun. Monsoon arrives in Northern and Northeastern China by late Jul. The northward advance of summer monsoon takes place in alternating phases of quick and slow development.

Air Mass Flow in Summer Monsoon

20. Summer monsoon in China has two sources of air masses, one from the subtropical anticyclone in the NW Pacific, another from the cross equatorial flow from the southern hemisphere. These two sources establish two different branches of Chinese summer monsoon, a southeast and southwest branch. The southeast monsoon component is much stronger; it extends further and also maintained longer. This leads to frequently used term “southeast monsoon” or summer monsoon.

21. The summer monsoon over China includes the following major components:-

(a) The anticyclone over Australia.

(b) The cross equatorial flow from the Southern Hemisphere.

(c) The ITCZ.

(d) The tropical easterly jet.

(e) The subtropical anticyclone in the West Pacific.

(f) The Mei-yu frontal zones and the mid latitude disturbances.

22. The circulation pattern in the Southern Hemisphere has a close connection with the formation and the advance of summer monsoon over China. The dynamic processes of summer monsoon over China are ruled by the heat source, which is centered in the South China Sea and a heat sink with its center in Australia.

23. Retreat. The retreat of monsoon in China is a straight forward, rather quick process which only lasts around one month. The retreat of summer monsoon in East Asia is closely connected with the Southward migration of the Subtropical anticyclone. The ridge of the subtropical anticyclone starts to retreat southwards in early Sep, quickly moving southward and arriving at 25°N in late Sep, thereafter only slowly retreating to 17°N by late Oct.

Some Characteristics of Summer Monsoon

24. In case of a strong summer there are above normal rainfalls in N and S China, but below normal rainfall in Yangtze valley.

25. Onset and advance of summer monsoon in China have been found as events of a remarkably great variation over time. Significant inter-annual variations of the dates

of onset and withdrawal of summer monsoon were observed over China, and similarly the strength of monsoon varies from year to year.

26. A remarkable large intra-monsoonal variation of rainfall also represents a characteristics feature of summer monsoon in China. During summer monsoon, heavy precipitation also originates from thunderstorms in shallow, thermal depressions, orographic lifting on wind exposed slopes and typhoons.

Winter (Dec-Feb)

27. Sea Level Pressure Distribution. The Asia Pacific region is divided in to an intensely developed high pressure over mid-Siberia, Mongolia, and an established low pressure over the NW-Pacific Ocean. The continental high pressure is on Baikal Sea and Mongolia, while the centre of NW- Pacific low pressure lies over the Bering Sea and the Aleutian islands. Since both pressure systems practically lie in the same latitude of 50 to 55 N, a steep pressure gradient occurs which produces strong and persistent NE’lies. They carry dry continental and cold polar air masses over the entire East Asian region. The mean values of sea level pressure in January reach a maximum of 1040 hPa in the centre of the anticyclone and decrease to 1020 hPa only over southern China.

28. Upper Air Features. The characteristics feature of the winter climate is cold, dry air control. The upper tropospheric westerly expand to the northern part of South China Sea, the southern branch south of the Tibetan plateau intensifies. The jet stream over the East Asia main land - Japan attains its maximum intensity with a five day mean velocity always more than 50 -60 m/sec and stagnates near the winter average position of 30°N. The long wave trough of East Asia, the Mongolian cold high pressure and the Aleutian low pressure, all are strongest and most stable in winter.

Onset and Duration of Winter Monsoon

29. Winter monsoon, which is commonly considered as persistently invading continental, cold and dry air masses, associated with winds from northern quadrant, is quickly established over NE and N China during the first 10-day period of Sep when the summer monsoon has retreated to the Yangtze valley. During the second and third 10-day period of Sep, winter monsoon occupies nearly all eastern parts of China, East of 105°E longitude. During the first 10-day period of Oct, the coastal regions of South and Southeast China are also penetrated by the winter monsoon, and after Oct -10, the penetration of China Seas as well as Hainan and Taiwan Islands follows.

30. Winter monsoon is fully settled until the end of Mar. The transition from winter to summer takes place in Apr and May.

Air Mass Flow in Winter Season

31. The air mass which mostly affects China during winter are of a persistent dry and cold, polar origin, although they can be warmed and partly moistened when they move southward. As a typical character of the constantly invading continental air, which is commonly called winter monsoon, cold waves frequently travel from Mongolia up to southern most parts of China. The effects of cold wave, which is associated with a sharp drop in temperature and frequent rains, decreases, with increasing, distance from their origin. Continental air of dry and cold origin prevails from Oct through Mar and even extends to Apr, at times.

Aviation Weather Hazards

32. The main weather hazards during winter are as follows.

(a) Precipitation

(b) Frozen Rains . When rain strikes the surface, which has a temperature below 0°C, it rapidly freezes and may build up thick ice shells on trees, electrical wires, poles and buildings.

(c) Thunderstorms . However rare in cold season, they do occur over South China. These may develop after weather has been anomalously warm or in association with persistent rains.

Spring Season (Mar-May)

33. After bitter cold, spring comes with warm weather and everything looks fresh. The main characteristics of spring weather are as follows:

(a) Rainfall gradually increases.

(b) Weather, is variable and most weather systems are migratory.

(c) Frequent Extra tropical cyclone occurs over NE China.

Extra Tropical Cyclones

34. In china, they are classified according to their places of origin.

(a) NE China cyclones originate in Mongolia and adjoining region. Move east to northeastwards and attain peak over NE China.

(b) Yellow river cyclones develop over plains of North China. They move east-northeast wards towards Aleutian Islands.

(c) Yangtze river cyclones originate over the middle and lower reaches, of the yellow river and move eastwards.

(d) East China Sea cyclones are most southerly extra tropical cyclones originating from the China mainland; track is same as that of Yangtze River cyclones.

35. The extra tropical cyclone and its cold front always give extensive precipitation (rain or snow) over NE China and less weather activity over Inner Mongolia. Maximum intensity is observed in Jun.

Aviation Weather Hazards

36. The main aviation weather hazards during this period are as follows.

(a) Persistent Spring rains. Two main circulation types are associated with persistent rainy periods in spring, these are:-

(i) Eurasian blocking high.

(ii) Large low-pressure vortex, which dominates Middle and high Eurasia.

(iii) Gales, spring gales affect N china and coastal area.

Autumn

37. Over parts of China, a period of fine and comfortable weather occurs during autumn season. Crisp air, bright sunshine and gentle breezes prevails.

38. Weather. On certain occasions, high temperatures persist in to Sep. This hot and uncomfortable weather is known as tiger weather. During autumn, occasionally prolonged rains are seen over lower Yangtze River, under influence of frequent typhoons, and tropical depressions.

Transient Disturbances

39. Troughs in Westerlies. The main characteristic features of troughs in westerlies over china are as follows.

(a) Most of area stays under westerly regime for a majority of time.

(b) Troughs and ridges, with varying amplitude, moves across China.

(c) Their Average speed of propagation is 10-15º longitude per day.

(d) Enhance rainfall over north and sometimes over central parts of China.

40. Extra Tropical Cyclones and Anticyclones. The main characteristic features of extra tropical cyclones and anticyclones over Chinese region are as follows.

(a) The extra tropical cyclones move in the westerly regime across China.

(b) Their maximum intensity is observed during month of June.

(c) During winters, they extend up to Central China.

(d) The extra tropical anticyclones originate in Siberia and Mongolia during winters (Crachin).

(e) Their General direction of movement is towards E-SE.

41. Typhoons. China is one of the countries affected and damaged by typhoons. Main features of typhoons are:-

(a) Intense tropical storm, originates in NW Pacific, with maximum frequency of 4-6 per month, during July to October.

(b) Practically nil Typhoons form during December to April and frequency of formation is least during Feb.

(c) Associated with strong winds and heavy rainfall in SE and N China and contributes more than 50% of annual rainfall in the coastal regions.

(d) Do not persist more than 500 km inland after crossing the coast.

Precipitation

42. Precipitation is another climatic factor. Annual rainfall decreases from east and south to west and north. South China, Including Taiwan, Hainan Island and Guangdong experiences the maximum, more than 1750-2000 mm. In Yangtze river valley the amount is moderate, about 1000-1500 mm. Along the most important boundary of China climate, the Qin Ling mountain and Huaihe river, rainfall is about 750 mm. North of it rainfall is scanty with about 500 mm in N China, less than 250 mm in interior NE China and less than 50 mm over the Takla-Makan Desert.

43. From the point of view of annual variation, winter (Nov – Feb) is dry and cold and summer (Jun – Aug) is rainy. May and Sep are transitional months.

44. Since winter, spring, summer and autumn are considered as the principle seasons in China, a representative month from each season may reliably describe the seasonal precipitation pattern in China.

(a) April (Spring) . Maximum rainfall more than 100 mm is recorded over N and S China. Lowest rainfall less than 25 mm is recorded over Tibetan Plateau, NW China and over Mongolian borderlands.

(b) July (Summer) . Precipitation totals throughout China in Jul are not considerably higher than Jan and Apr, but variability is more. The North, South and Northeast China receive rainfall between 150 to 200 mm with scattered small pockets receiving 250-mm. rainfall towards Northwest is below 25 mm.

(c) Oct (Autumn) . The North, and South China receives rainfall below 50 mm with few small pockets receiving 100 mm. rainfall towards Northwest is below 10 mm. few small pockets towards Northeast China receives rainfall below 50 mm.

(d) Jan (Winter) . Northern china experiences very dry weather and persistently clear skies during winter, rainfall amount is below 50 mm. Northeast, Mongolian Borderlands, and Northwest china receives rainfall / snowfall between 50 to 100 mm.

Temperature

45. The mean annual temperature distribution demonstrates gigantic thermal differences in China. Considering mean distribution of isotherms China broadly may be divided in to a western and eastern part. The description of seasonal temperature distribution is as follows.

(a) Jan (Winter). Major parts of China experience a mean temperature below 0°C, Comparatively small sectors in Southern and Southeastern China record positive temperatures.

(b) Apr (spring) . From Jan to Apr temperature increases by 10°C in Southern China and up to 25°C in Northeastern China, a similar increase is also seen over western parts of China.

(c) Jul (Summer) . Jul shows further decreasing range of temperature in comparison to Jan and Apr, the spatial range of temperature amounts to 20°C, mountain regions above 4500 msl are excluded.

(d) Oct (Autumn). In comparison to Jul, the temperature in all of China drops considerably, to a greater extent in Western, Northern and Northeastern China than in southern and Southeastern China.

46. The regions with temperature extremes are mentioned below.

(a) Areas registering maximum temperatures more than 40ºC:

(i) Henan province.

(ii) North China plains.

(iii) Tarim Basin.

(b) Highest recorded: 49.6°C 13 July 75 at Turpan.

(c) Areas registering minimum temperatures less than -30°C:

(i) Heilungkiang province.

(ii) Northeastern parts of Inner Mongolia.

(iii) Peaks over Tibetan Plateau.

(d) Lowest recorded: - 51.5°C 21 Jan 60 at Xingjian.

Aviation weather Hazards

47. Dust storm. The most favourable areas for formation of dust storm are plains of North China and South Manchuria. The main period of occurrence is from March to May and again from July to September. Most favourable situation is passage of the fronts in the westerly wind regime and most favourable time of occurrence is noon to evening.

48. Fog. Advection Fog is common in summers and radiation type fog is common in winters (Crachin). Maximum frequency of formation is in the night and stays till morning hours. During Jan over land areas, fog persists till noon. The favourable areas of formation of fog are, South China Hills and Sichuan Basin (186 days per year). Over Qinghai-Xizang and Xingjian Frequency of fog, formation is less than five days and over coastal areas it is, 5- 25 days per year.

49. Thunderstorm. The most favourite regions for the formation of thunderstorm are Hainan and Luichow Peninsula with frequency over 90 -100 per year in the months from Mar-Sep. As we move towards North frequency decreases and is close to ten per year over Heilungkiang, over Southern Provinces frequency of thunderstorm formation is close to thirty per year, over deserts it is close to five per year and it is close to twenty over the Hills.

50. Hail. The formation of hailstorm is very rare over plains and in the Northwest, over South China it is close to two per year. Over North China plains (altitude more than 1000’) frequency of hailstorm formation is close to five. In Northwest China over Kansu frequency is close six per year and over Sensi in North China frequency is close to seven per year.

Conclusion

51. China is a large country with vast territory and extremely complex land forms and therefore considerably large variation of climate is expected. It is obvious from the climate classification that the main feature of climate of China is its diversity and contrariety which together leads to existence of a great number of climate types.

CLIMATOLOGY OF BANGLADESH

Chapter objectives

After reading this chapter, you should be able to:-

Assimilate the Physical Features, Climatic Elements and different Climatic Regions of Bangladesh.

Understand how the Pressure , Temperature, Rainfall and Humidity varies during different seasons in different regions.

Understand the different seasons of Bangladesh and Aviation weather Hazards.

Structure

1. Introduction

2. Season in Bangladesh

3. Pre–Monsoon Season

4. SW Monsoon.

5. Post Monsoon.

6. Aviation weather Hazards

INTRODUCTION

I. Bangladesh is the darling child of the nature and it is situated at the bank of Bay of Bengal in the northern hemisphere. It lies between 20° 34' N and 26° 38' N Lat as well as 88 ° 01' E and 92° 41' E Long. The Tropic of Cancer, the very important geographical line on the globe, passes over central part of Bangladesh.

2. Conical shape of Bay of Bengal in the southern side, the Himalayas in the northern side and a vast body of landmass in the west-north-west side are responsible for copious rainfall in Bangladesh. Bangladesh has a typical monsoon climate with the same threefold division of the year that occurs in India, but the cool season is generally

warmer than India. Generally, annual average rainfall varies from 1500 to 2500 mm, Although temperature during the hot season are rather lower than in some parts of India, the heat is made uncomfortable by the high humidity. This damp, muggy season continues throughout the main rainy season, but the heat is rarely dangerous. It is, however, very unpleasant for the unacclimatized visitor.

SEASONS IN BANGLADESH

4. Like India, Bangladesh has got the following seasons:

(a) NE Monsoon or Winter Monsoon.

(b) Pre-Monsoon or Spring Transition.

(c) SW Monsoon or Summer Monsoon.

(d) Monsoon or Autumn Transition.

North East Monsoon or Winter Monsoon (December to February)

5. This season is characterised by an anticyclonic pressure system dominating the country except in February. Mist and Fog are the main weather in December and January. Wind blows from northwest or north direction. Cold waves blow from almost north to south over Bangladesh. Mean temperature ranges from 18-21º C in the northern and central districts and lowest temperature come down occasionally 4-5º C over these districts. Rainfall during winter is scanty but western disturbance may cause 3-10mm of rainfall over the northern districts.

6. The main weather conditions during this period are:-

(a) Poor visibility due to mainly fog, mist and haze.

(b) Low clouds associated with fog lifting.

(c) Upper air jet.

7. In Bangladesh the advection-radiation fog is more common in the landmass other than coastal districts of Chittagong where advection fog is more common.

Pre-monsoon or Spring Transition (March to May)

8. During this season temperature increases rapidly due to normal sunshine approaches to our area from south to north. The mean temperature remains within 23°C to 30°C. April and May are the hottest months of the year. The highest temperature ranging from 44°C to 45°C is attained in the northern and northwest districts. Over rest of the country it ranges from 41°C to 43°C. Local severe storms Nor’wester, which move generally from northwest direction that are responsible for the welcome of precipitation.

9. Pre-monsoon or spring transition has got the following main weather activity:-

(a) Norwester.

(b) Tornado.

(c) Tropical Cyclone.

Nor’wester

10. Definition. In the month of March, April and May, that is during pre-monsoon period thunderstorms that usually originate in West Bengal and Chotanagpur, some of which reach severe thunderstorm or tornado violence, are termed as Nor’wester because of their pro-tendency to move from north-west. In Bengali Nor’wester are popularly known as ‘Kal Baishaki’ due to their black appearance and period of formation in the Bengali month Baishak.

11. Climatology of Nor’wester . Maximum frequency of Nor’wester occurs in the month of March, and decreases in the month of April and May. In Bangladesh, Nor’wester or thunderstorm may occur in the middle of February due to early withdrawal of winter and may continue till the onset of monsoon. During this period the severe thunderstorm turns into tornado and causes havoc to the man and material, specially in the central parts of the country.

12. Classification of Nor'wester . According to the place of origin, movement and frequency Nor’wester can be classified as Type 'A', Type 'B’, Type 'C’ and Type 'D'. They can be differentiated as follows:-

(a) Type 'A' . Type ‘A’ Nor’wester occurs simultaneously over a large area and generally in the afternoon. 70% to 80% Nor'wester are of this type. They usually start in West Bengal and Chotanagpur in the afternoon and proceed from northwesterly direction with a speed of 48-64 kph.

(b) Type ‘B’. They originate in the sub-mountainous districts of North Bengal and move southwards mostly during night and early morning. They move slowly at a speed of 16-32 kph (10-20 mph) to Meghna estuary from north. About 10% Nor’wester are of this type.

(c) Type 'C'. They originate over hills of Nagaland, Manipur and Mizoram and travel westward. They are very rare and come from northeasterly direction.

(d) Type 'D'. They are very similar to Type 'B' but their place of origin is near the Khashi hills. They move usually from north to south. They are about 5% only.

13. Frequency of Nor’wester. The frequency of Nor’wester with their duration is classified as:-

(a) In 30% cases, duration ranges from 1h to 1.5h.

(b) In 20% cases, duration ranges from 1.5h to 2h.

(c) In 40% cases, duration ranges from 2h to 4h.

(d) In rest 10% cases, duration ranges more than 4h.

Tropical Cyclone

14. The tropical cyclones are the most destructive nature's phenomenon. Most of them are violent in nature and few are weak and die out before they intensify into a matured storm. Many tropical cyclones have visited Bangladesh since long and killed many people in the coastal areas. The recent tropical cyclones that visited Bangladesh as giant killer are November 12 Cyclone, 1970, May 25 Cyclone, 1985, November 29 Cyclone, 1988 and April 29 Cyclone, 1991. The most destructive one hit Cox’s Bazaar on 29 April 1991 with a wind speed of 225 km/hour and maximum water surge of 20 feet. The cyclone killed 1,38,00,00 people and damaged many valuable physical infrastructures including very costly aircraft in BAF Base Zahurul Haque, Chittagong. In the recent years, the awareness of mass people about cyclone and development in forecasting cyclone signals has saved many people and material.

15. Tropical cyclone is called as killer weather in Bangladesh. Post monsoon tropical cyclones affects more than that of Pre-monsoon. The track of tropical cyclones in the month of May is as shown in the Figure 2.

Southwest Monsoon or Monsoon (June to September)

16. During the month of June the surface wind changes to southerly direction over the southern and central district and south-easterly to easterly over northern district of Bangladesh. Maximum rainfall occurs in this season. The monsoon usually sets in the south eastern corner of Bangladesh on 1st June and gradually approaches to the northwest direction with heavy to very heavy rain. During the first two months the rainfall is usually 700mm - 850mm per month over Sylhet and south eastern districts, 450mm - 600 mm per month over northern and southern districts, and the rainfall intensity is less over western part of Bangladesh compared to other regions. Monsoon depressions cause the copious rainfall over Bangladesh.

Onset and Withdrawal of Southwest Monsoon over Bangladesh

17. The onset and withdrawal dates of summer monsoon over Bangladesh are shown:-

Weather in Southwest Monsoon

18. The weather in SW Monsoon is mainly:-

(a) Poor visibility due to heavy to very heavy rain.

(b) Low ceiling with maximum amount of 6-8 oktas at 400 ft or below.

(c) Strong wind in coastal districts with speed of 30-40 kts.

Aspects of Monsoon

19. The following three features are very important in respect of SW Monsoon:-

(a) Break monsoon which causes flood over northern part of the country.

(b) Monsoon depression which provides sufficient amount of rainfall over south part of the country, this part would have insufficient rainfall.

(c) Cyclones do not form during the south west monsoon period.

(d) Retreat mainly accompanied by northerly or northwesterly wind.

Post monsoon Season or Autumn Transition (October to November)

20. It is the transitional period from summer to winter. During this period rainfall, temperature and speed of wind decreases while atmospheric pressure increases. During October, Sylhet gets 200-250 mm of rain and rest of the country gets 100-170 mm of rain. The mean temperature falls 28-29° C in November.

21. The following are the main weather phenomenon during the Post Monsoon or Autumn Transition:-

(a) Tropical Cyclone.

(b) Thunderstorms.

(c) Poor visibility with the fog, mist and haze.

Tropical Cyclone

22. Both the Pre-Monsoon and Post-Monsoon period are the favourable for the formation of tropical cyclones over Bay of Bengal. However, the topical cyclones that forms in Post-monsoon periods affects much than that of Pre-monsoon period. The number for storms and tropical cyclones formed in the Bay of Bengal for the period 1891-1998 are shown in Table-1. It also shows the intensification of storms into tropical cyclones in the month of November and second maximum in October.

23. During the Post-monsoon period the intensification of storms into tropical storms are more than that of Pre-monsoon season, and more so Bangladesh is mostly affected during the Post-monsoon period specially the Chittagong regions. The track of tropical cyclones in the Bay of Bengal during Post-monsoon period is shown in Figure 5.

WEATHER AND BANGLADESH

24. Bangladesh has one of the worst records in the world for natural disasters. Being a small country, it has all the variability and peculiarity of weather activities. The killer weather is tropical cyclones. Tornado and thunderstorm also bring disaster for the country. Hundreds of thousands of people have been killed by floods and cyclones, as well as tidal surges, tornado, droughts and even cold spells. Continuous heavy rainfall for few weeks especially in the south-eastern part of the country is very common. Poor visibility condition with visibility less than 1000m is usual in the morning of winter months over all the country.

AVIATION WEATHER HAZARDS

25. The following are the aviation weather hazards in Bangladesh:-

(a) Low ceiling during SW Monsoon especially over Chittagong regions.

(b) Poor visibility during winter monsoon and Post-monsoon season almost over whole country.

(c) Strong/gusty/squally wind with Nor’wester during Pre-monsoon season and strong wind with SW monsoon period over southeastern parts of the country.

(d) Thunderstorms and hail storms over central and western part of the country.

Check Assimilation

Fill in the Blanks / State True or False. If False, give the correct Statement.

1. Maximum frequency of Nor’wester occurs in the month of March, and decreases in the month of April and May (True/False)

2.According to the place of origin, movement and frequency Nor’wester can be classified as ______, _______, ______ & _____.

3. Break monsoon which causes flood over northern part of the Bangladesh. (True/False)

4. Generally, annual average rainfall varies from _____ to _____ mm,

Check Assimilation : The Key

1. True

2. Type A, Type B, Type C, and Type D

3. True

4. 1500, 2500

CLIMATOLOGY OF MYANMAR

Introduction

1. The Republic of the Union of Myanmar (also known as Burma) is the Northwestern-most country on the mainland of Southeast Asia. It is strategically located near major Indian Ocean shipping lanes. Born on 04 Jan 1948, after a long British colonial ruling. The union of Myanmar, occupies the largest territory in South-East Asia. Its capital is Yangon which was earlier known as Rangoon.The standard time of Myanmar is 6.30 hrs ahead of GMT. Ethically, the diamond-shaped Myanmar is a deeply religious Buddhist country and is well known by tourists for its abundance of sights and sounds. Historically, it's been called the golden land (Suvarnabhumi); one of the poorest countries in the world, yet boasting temples gilt with more gold than the finest cathedrals in Europe.

Geography

2. Location. Geographically, the union of Myanmar is situated in Southeast Asia between latitudes 09° 32' N and 28° 31' N and longitudes 92° 10' E and 101° 11' E. With an area of 676,557 sq. km; Myanmar is almost twice as big as –Germany or the state of Arizona or France and UK-combined. Its south to north length is approximately 1920 km. Where as, the width (east to west) is about 905 km. The highest point is Khakaborazi (in the northern Kachin state, near Tibet/china) at 5881 meters above sea level.

3. Coastal Belt. Myanmar’s territorial waters are demarcated up to 12 nautical miles from base line on 15 November 1968. It has a 2,832 km long coastline on the Indian Ocean.

4. Neighbors. It is bordered by china, Laos and Thailand in the east, and by Bangladesh, India and the Indian Ocean in the south and west. it is sandwiched between India and Bangladesh on one side and china, Laos and Thailand on the other, while to the south is the Andaman sea with beautiful unspoiled beaches.

States and Divisions

5. Today’s Myanmar has been divided into 14 states and divisions, they are;

(a). Thaninthayi

(b). Mon

©. Yangon

(d). Ayeyarwaddy

(e). Kayin

(f). Bago

(g). Rakhine

(h). Magwe

(i). Mandalay

(j). Kayah

(k). Shan

(l). Sagaing

(m). Chin

(n). Kachin

Valleys and Plains

6. Valleys and plains, of a country play an important role in determining its climatology and Myanmar is not an exception to it. The valleys and plains of Myanmar can roughly be divided into three parts .They are:

(a). The Western hills region.

(b). The Central valley region.

(c). The Eastern hill region.

7. The Western hill region. The Himalayas rise in the north west, where the khakabo razi, right on the border between myanmar and tibet, is the highest mountain in south-east Asia at 19,290 ft./5881m.The western hills region serves as a wall that separates Myanmar from India.

8. The Central valley. The Central valley region consists of the broadest valley of the ayeyawady. The first part of the river ayeyawady is from the origin of river to Mandalay; the second part constitutes from mandalay to pyay and the third is the part from pyay to the mouth of the river. The central valley region consists of sittaung valley

and chindwin valley. In the centre lie the small mountain ranges such as zeebyu taungdan, minwun taungdan, hman-kin taungdan and gangaw taungdan. There also lies the low range of bago yoma that slopes down from north to south.

09. The Eastern valley. The eastern hill region is the shan plateau which is average 3,000 to 6,000 feet above sea-level. Unlike the plain, the plateau has high mountain ranges and the river than lwin flows through the shan plateau to the northern taninthayi coastal strip. The rivers of shweli, myitnge, zawgyi and pan-laung which have their sources at the shan plateau flow in to the river ayeyawady. The 4,981-foot mount popa, an extinct volcano, is famous.

Mountains and Peaks

10. To the north of it, Myanmar has got series of mountains that largely influences its climate all around the year. Major mountains peaks are:

(a). Mt. kakaborazi (5886m height ) highest peak in southeast Asia..

(b). Mt.phonyin (4,560m height) .

(c). Mt. bota (3800m height).

Meteorological Sub Divisions

11. The country is divided into six meteorological sub divisions. Namely:

(a). The arakan coast and neighbouring hills

(b). Northern Burma

(c). Central Burma

(d). The delta region

(e). Eastern Burma and Shan states

(f). Tenasserim

Rivers

12. Rivers are the symbol of life and lay foundations for civilizations. A country’s climate is significantly determined by the number and volume of rivers flowing across that country. Major rivers of Myanmar are:

(a). Ayeyarwaddy

(b). Chindwin

(c). Mekong

(d). Sittoung

(e). Thanlwin

13. In the diamond – shaped Myanmar they all flow from north to south. the most important watercourse of Myanmar is the Ayeyarwaddy, which originates in the eastern part of Tibet. it flows for more than 2,000 kilometers from north to south and thus divides the country into a western and an eastern half. The Ayeyarwaddy is passable for ships for a length of about 1,450 kilometers.

Climate

14. The tropic of cancer runs through areas near tiddim, kyunhla and hsenwi in northern Myanmar. This puts two thirds of Myanmar lies in the tropical zone and the remaining one third in the sub-tropical zone. Hence, most of Myanmar enjoys a tropical climate. The seasons vary from region to region due to the country's diverse geography.

15. Temperature. The hottest period of the country is in April and May when temperatures can reach about 43°C in central Myanmar, 36.1°C in northern Myanmar and between 29.4°C and 35°C on the Shan plateau. The coolest month is January where the average temperature falls to 16°C. in the highlands of over 3,000 feet, temperature can drop below 0°C however , The extremes of temperature are rare in the Myanmar. For example its capital Yangon records min temp of 16 ° c in the month of Dec ; and max temp of 33 ° c in the months of May and Jun. Monthly average temperature in Yangon:

Table-1: Average High Temperature (in °C) of Four Cities of Myanmar

City

Jan Feb

Mar Apr May Jun Jul Aug Sep Oct Nov

Dec

Putao 19.3

19.9

23.6 24.9 28.5 28,5

28.2 29.3 28.5 27.5 24.3

20.4

Mandalay

28.5

32.1

35.8 38.4 36.8 34.2

34.3 32.3 33.1 32.2 30.2

28.2

Yangon 32.2

34.5

36.0 37.0 33.4 30.2

29.7 29.6 30.4 31.5 32.0

31.5

Myeik 31.4

32.1

33.1 33.6 31.8 29.4

29.1 28.8 29.5 30.7 31.6

31.5

Table-2: Average Low Temperature (in °C) of Four Cities of Myanmar

City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Putao 6.9 9.3 12.6 15.8 19.6 21.5 22.7 22.5 22.0 18.4 12.2 8.1

Mandalay 13.3 14.9 19.7 24.4 25.8 25.8 25.8 25.2 24.9 23.5 19.4 14.8

Yangon 17.9 19.3 21.6 24.3 25.0 24.5 24.1 24.1 24.2 24.2 22.4 19.0

Myeik 20.7 21.6 23.1 24.6 24.2 23.6 23.4 23.3 23.5 23.2 22.7 21.0

16. Rain Fall The country receives most of its rainfall during its monsoon season. Monsoon rains begin to fall towards the end of Jun and lasts until September. Rain can be torrential but usually falls in short bursts during the afternoon. It rains almost throughout the year (Dec to Mar) over South and SW Myanmar.

Table-3: Average Rainfall in mm of Four Cities of Myanmar

City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Putao 14 38 75 147 176 738 998 912 645 220 22 16

Mandalay 4 2 1 40 138 116 83 135 150 125 38 6

Yangon 5 2 7 15 303 547 559 602 368 205 60 7

Myeik 4 51 55 128 422 783 740 868 482 302 73 13

17. Human Comfort Index Although there is a slight drop in temperature, the rise in humidity makes the very uncomfortable conditions during wet season.

Seasons of Myanmar

18. The country’s climate is divided into 4 seasons, namely:

(a). The SW or wet monsoon (Jun to Sep) ,

(b). The post monsoon (Oct to Nov) ,

(c). The NE or dry monsoon (Dec to Mar),

(d). The pre monsoon (Apr to May)

19. The SW Monsoon

(a). Pressure Pattern. In Jul low pressure area lies over north- Myanmar and high- pressure area over Bay of Bengal.

(b). Upper Winds. During this season, the equatorial easterly approaches N’wards over north-Myanmar above the monsoon currents. Easterlies are established normally above 18000ft.The boundary between upper easterlies & Westerlies is at about 30°N both during Sep it moves south again to about 25°N.

(c). Weather. During this season the central Myanmar experiences rain many during this season .Fog over hills is frequent and St Clouds below 600’ is common during this season. Such St Clouds of patchy nature occurs chiefly in

late night and begins to lift an hour or so after the sunrise. Country receives about four fifths of the total annual precipitation during this season.

20. Post monsoon

(a). Pressure Pattern. During this season, in Myanmar, the pressure gradient falls to practically nil. Further to the north, high pressure starts developing and low pressure over Bay of Bengal.

(b). Upper Winds. The boundary between upper E’lies and W’lies continuously shift s-wards during this season. By mid Nov, the boundary lies approximately over s- Myanmar with strong & steady W’lies over north and central Myanmar.

(c). Weather. As the season progresses the rain fall decreases in the north but remain continuous in tenasserim. Arakan coast is affected by post monsoon cyclones forming in s- bay and Recurving north or Ne-wards. During Nov, the frequency of fog rises and mainly confined to valleys and near rivers.

21. NE monsoon

(a). Pressure pattern. Permanent belt of high pressure lies to the north of Myanmar and India. pressure decreases southward to reach its maximum value south of equator at about 10o s , which results general light to moderate NE’ly current in the lowest 3000’.

(b). Upper Winds. Two main upper air systems affect Myanmar; temperate W’lies and equatorial E’lies. W’lies reaches their farthest south and at 10000’ & above normally covers all Myanmar and India north of 15o n. W’ly are strong &steady. South of 13o n the winds aloft are the equator e’lies.

(c). Weather. During this season, generally fine weather with clear sky prevails. However, fog is observed in late night and early morning hours. Rain th’showers occurs over n-parts during passage of WD. Occasional showers are experienced especially in December over tenasserim area in association with e’ly wawes and bay of cyclones.

22. Pre monsoon:

(a). Pressure Pattern. In the end of March the pressure gradient becomes very weak. In April low pressure area develops over north Myanmar and high pressure area over Bay of Bengal.

(b). Upper winds. Very light W’ly persists over the area. The boundary E’ly and W’ly move slowly farther north. By May the higher level winds over s- Myanmar become light and variable with E’lies prevailing further south.

(c). Weather. During this season, generally fine weather prevails over the area except N-Myanmar where dust storms occur frequently. During later part of the season, scattered Th’storm activity occurs over most parts of the area. Late in May, monsoon air penetrates Myanmar, resulting cold and rain over these areas. In Apr, 4-8 days Th’storms and 1 dust storm for 1 to 2 days occur; in May, 10-16 days Th’storm occur.

Tropical Cyclones

23. Tropical cyclones of great severity are often experienced in Myanmar, however; a few of them hit the country in the post monsoon season. Arakan coast is affected by post monsoon cyclones forming in S- bay and recurving north or NE-wards.

Aviation Weather Hazards

24. General.

(a). Tropical cyclones of bay

(b). Thunder storms

(c). Mountain waves

(d. Poor visibility in rain and th’showers

(e). Fog, mist, haze near valley and rivers.

25. During SW Monsoon. Under mentioned are the aviation weather hazards of this season:

(a). Low clouds as low as 600’.

(b). Thunder storms (max over arakan coast).

(c). Poor visibility in rain and th’showers.

(d). Frequent fog over hills.

.

26. During Post Monsoon. Aviation weather hazards of this season are chiefly:

(a). Thunder storms (in Nov max no of days; 25 days); over Arakan coast; associated with bay cyclones.

(b). Fog over many places in valley and near rivers.

27. During NE Monsoon. Under mentioned are the aviation weather hazards of this season:

(a). Morning mist over valley, river and near coast.

(b). Fog (late night).

(c). Rain fall associated with WDs over n-Myanmar.

(d). Occasional thunder showers over tenasserim in association with E’ly waves and bay cyclones.

28. During Pre- monsoon. Under mentioned are the aviation weather hazards of this season:

(a). Thunder storms (max 10-16 during May) over S- Myanmar.

(b). Dust storms over N Myanmar.

(c). Poor visibility in th’shower.

(d). Generally fine weather.

Climate Change

29. Burma ranks among the world’s top countries most at risk from the combined effects of climate change, according to a new report by the Asian Development Bank. The report spotlights the Asia-Pacific as the most disaster-prone region on earth, and warns that new patterns of migration are likely to emerge as populations escape humanitarian crises brought about by global warming. More than 42 million people in the region have been forced to leave their homes due to environmental disasters during the past two years alone, and that figure is set to increase. It says that a rise in sea levels presents the most pressing concern, with eight of the 10 countries with the greatest number of people inhabiting low-lying coastal zones living in the Asia-Pacific. In Burma, 10 percent of the country would be affected by a sea-level rise of between one and five meters.

30. Poverty and weak governance compounds the vulnerability of those living in areas prone to the negative effects of climate change. In Burma this is most evident in the Irrawaddy delta and the stretch of coastline in central Arakan state around Ramree Island – both areas are of low-elevation, and decades of neglect by the central government has meant that disaster warning and flooding prevention systems are woefully lacking.

31. Burma also comes in second place for the dangers that will accompany a rise in sea levels. According to a dataset created by Wheeler, nearly 4.5 million Burmese inhabit areas designated at risk from storm surges. Other countries have higher figures for this, namely China with 23.7 million, but Burma ’s coping ability so poor that only Somalia tops it in terms of susceptibility to sea level rises.

32. Regarding the fallout from extreme weather, Burma ranks fifth on Wheeler’s index. According to the ADB report, “many more people” in Southeast Asia died as a result of natural disasters between 2001 and 2010 than during the previous decade, primarily due to the 2004 India Ocean tsunami and 2008’s Cyclone Nargis, whose aftermath showcased the Burmese government’s inability to respond to extreme weather.

Conclusion

33. Most of Myanmar enjoys a tropical climate. Apr-May hottest months and Jan is the coldest month. Surface winds are W’ly to SW’ly from Apr-Nov; where as, NE’ly Dec – Mar.

34. Rainy season is the most uncomfortable season as the human comfort index is max during this season. Country receives about four fifths of the total annual precipitation during rainy season’s season.

35. Max no of thunderstorms occur during post monsoon (over Arakan coast), normally fine weather prevails during NE and pre monsoon.

36. Myanmar ranks among the world’s top countries most at risk from the combined effects of climate change and sea level rising. Its needs to build-up a comprehensive disaster management system to negate the negative effects of climate change and natural disasters.

CLIMATOLOGY OF SRI LANKA

"To me the beauty of Ceylon lies not so much in its blue seas and golden beaches, its jungles and its mountain peaks, as in its ancient atmosphere. There is no nation, from Egypt of the Pharaohs to modern Britian, in whose literature this island has not at some time been mentioned by one or other of its many names - Lanka, Serendib, Taprobane, Cellao, Zellan, to recall a few. History lies buried in its sands, for Lanka is very, very old."

- D. J. G. Hennessy, 1949

Chapter objectives

After reading this chapter, you should be able to:-

Assimilate the Physical Features, Climatic Elements and different Climatic Regions of Sri Lanka.

Understand how the Pressure , Temperature, Rainfall and Humidity varies during different seasons in different regions.

Understand the different seasons of Sri Lanka and Aviation weather Hazards.

Structure

1. Introduction

2. Topography

3. Climatic Regions

4. Climatic Elements.

5. Seasons.

6. Aviation weather Hazards

7. Conclusion.

Introduction

1. Sri Lanka is a beautiful island country in the Indian Ocean which lies between a chain of tiny islands known as Adam's Bridge. Srilanka is a compact island lying off the southern tip of peninsular India. It is shaped like a teardrop falling from southern tip of peninsular India. It is separated from the Indian sub-continent by a strip of sea, which at its narrowest is about 25 miles wide. Capital of Srilanka is Colombo. The country was formerly called Ceylon. Sri Lanka is a blend of tropical splendor and a rich cultural heritage and is known to the world as the "pearl" of the Indian Ocean.

2. Location. It lies about 32 kilometres off the southeast coast of India. It is separated from India by the Palk Strait and the Gulf of Mannar. Sri Lanka lies between 5° 55' & 9° 55' N and 79° 42' and 81° 52' E.

3. Area. From North to South it has a maximum length of 435 km (N-S extent) and at its widest point it measures 225 km (E-W extent), giving it a land area of 65,600 sq.km. (25,000 sq miles).

4. Time. The country is six hours ahead of GMT.

TOPOGRAPHY

5. Sri Lanka is dominated by the astonishingly varied features of topography, making it one of the most scenic places in the world. Three zones can be divided by its distinguished elevation as follows :-

(a) The Central Highland

(b) The plains and

(c) The coastal belt.

Central Highlands

6. An outstanding feature of the topography of Sri Lanka is a mountainous mass in the south central part of the country, with the highest point being the peak of Pidurutalagala at 8,281 feet (2524 m), Kirigalpotha at 7837 feet, Tatapola at 7733 feet and Adam peak at 7341 feet. The Adam's peak lying to the west, at the southern end of the plateau is better known for its spectacular scenery and sacred pilgrimage site with its elevation of 2224 meters.

7. Two plateau - Nuwara Eliya and the Horton plains extend across the upland area. These are major centers of commercial tea plantation. The plateaus are noted for their cool, healthy climate. North of the mountains, and extending south, is an arid and gently rolling plain known as the dry zone.

8. Adam’s Peak. Adam’s Peak also called Sri Pada is situated 7,360 feet above sea level in the Central Hills of Sri Lanka. Sri Pada is a cone shaped peak which can be seen from many miles away from the sea coast.

The Plains

9. Sri Lanka has an irregular surface with low lying coastal plains running inland from the northern and eastern shores. The northern lowlands region has flat, broad, and fertile plains drained by several small rivers and marked by occasional ridges. The land descends from the Central Highlands to a series of flat plains between 30 and 200 meters above sea level, dominating the east and the north of the island.

Coastal Belt

10. A coastal belt surrounds the island, consisting of scenic sandy beaches and lagoons. Best beaches line along the southern coast, southwestern coast and eastern coast. In the northeast and the southwest, the coasts cut across the stratification of crystalline rocks, cliffs, bays, and offshore islands, creating one of the world's best natural harbors at Trincomalee on the northeastern coast and a smaller rock harbor at Galle on the southwestern coast. In the northwest, Mannar Island which is joined with

the mainland by a bridge is almost connected to the southern India by a long chain of sandbanks and islets called Adam's Bridge.

Major Rivers and Lakes

11. Rivers in Sri Lanka originates in the Central Highlands, near Adam's Peak and flow through the gorges, broad valley and plains and finally empty in the sea near Trincomalee, creating the different landscapes of escarpments, waterfalls and deep gorges. Most of the rivers are short and frequently interrupted by the discontinuities of terrain. The Mahaweli Ganga river with its length of 335 kms, flowing eastward to the north of the Central Highlands, is the country's largest river. The upper reach of the river is wild and non-navigable while the lower reaches are prone to the seasonal flooding. The river is the most important water necessity for the irrigation system in the northeast region in which natural rainfalls are deficient.

12. Major rivers and streams are found in the mountainous south central region of Sri Lanka. The Mahaweli Ganga is the longest river, emptying into the Indian ocean south of Trincomalee. Other rivers are the Kelani ganga, the mouth of which is near Colombo; the Kaluganga, which reaches the sea near Kalutara on the southwest coast; and the Aruvi Aru, which flows northwest across the dry zone to a point near Mannar.

13. Many of Sri Lanka's so-called "rivers" are really small, narrow brooks that are dry except during the rainy season. On the other hand, the rivers and streams that flow through the southwestern Wet Zone are full year-round and carry half of all the freshwater on the island.

Climate

14. Sri Lanka, a tropical island has varied climatic conditions from sunny hot beach

levels to the cool conditions of the hills, the temperature change constantly. Three factors are responsible for Sri Lankan climatology, its geographical position, topography and Insolation.

15. Sri Lanka's position between 5 and 10 degree north latitude endows the country with a warm climate, moderated by ocean winds and considerable moisture. The climate of Sri Lanka is typically tropical.

16. Its location near the equator made Sri Lanka as an extremely hot and humid climate. However, at the higher elevations, the climate is cool, and in the dry zone the humidity is relatively lower. November to January is the coolest time of the year, with January as the coldest month whereas February through May is the hottest period, with May as the hottest month.

17. Humidity. Humidity is generally high throughout the year. The island’s humidity level varies from region to region and due to seasons each year. The humidity and heat are at their most uncomfortable in April. Humidity is typically higher in the southwest and mountainous areas and depends on the seasonal patterns of rainfall. At Colombo, for example, daytime humidity stays above 70 percent all year, rising to almost 90 percent during the monsoon season in June. During the months June to August, the relative humidity falls to 0% by day time in the central hills.

18. Temperature. The average annual temperature in the lowlands is 32°C (90°F) and 21°C (70°F) in the higher mountainous regions. Upland areas are cooler and more temperate, and coastal areas are cooled by sea breezes. At Nuwara Eliya, in the hill 1800m above sea level, the annual average temperature is 15.8oC.

Seasons

19. Lying in the equatorial and tropical zone, Sri Lanka is influenced by the monsoons, allowing two distinct seasons: wet and dry or SW and NE monsoon season. These are the two monsoonal seasons in Sri Lanka, making its climate more complex and varied in different regions. The rainfall pattern is influenced by the monsoon winds of the Indian Ocean and Bay of Bengal and is marked by four seasons.

20. The climate of Srilanka has been classified into four seasons. They are :-

(a) The Hot season or First Inter Monsoon season (Mar – May)

(b) The SW Monsoon season (Jun – Sep)

(c) The Post monsoon or Second Inter Monsoon season (Oct – Nov)

(d) The Cool Season or Northeast monsoon (Dec – Mar)

First Intermonsoon Season or Hot Season (Apr-May)

21. First intermonsoonal period occurs from March till mid-May, with light, variable winds and evening thundershowers.

22. Pressure Pattern. During this season, the pressure pattern is generally weak. (Fig. 1)

Fig 1. First Intermonsoon Season : April Pressure Pattern

23. Surface winds. Winds are dominated by the land and sea breezes until the onset of the SW monsoon and are light and variable in land. At the onset of monsoon, squalls occur with gusts upto gale force.

24. Cloud. The amount of cloud varies from an average of 6 octas in the west coast to less than 4 octas on the east cost. The low clouds is in the form of broken patches and only becomes a continuous sheet during rain.

APRIL PRESSURE APRIL PRESSURE

PATTERNPATTERN

FIRST INTER SEASONFIRST INTER SEASON LL

1009

10081008

1009

25. Upper winds. Until the monsoon begins winds are light and variable upto 10,000’ with Ely 10-15 kt. When the monsoon is in, the winds are SW-Wly 20 –25 kt upto about 5,000’ with light variable winds from 5000 and 10,000’. Above that Ely wind prevails. (Fig. 2)

Fig 2. First Intermonsoon Season : April Upper Wind

26. Rain . Over West Coast the average number of rain days in Apr is 13-14 in the south and 3 – 4 in the north. In May, the number in the south increases to 18. Rain is most frequent in the afternoon and evening and is also frequently associated with thunderstorms. The onset of monsoon at the end of May gives a considerable amount of rain associated with thunderstorms in the south, but hardly affects the north. At the end of Apr and during May, squalls with showers of rain occur at night. The average rainfall varies from 2-5 inches in the north to 5-10 inches in the south in Apr and there is some increase in May. (Fig.3)

Fig 3. First Intermonsoon Season : Rainfall Distribution

The Southwest Monsoon Season (Jun – Sep)

27. In mid-May to October, the Southwest monsoon, called in Lankan language as "Yala season", brings moisture from the Indian Ocean, resulting in the heavy rains in the south and west coasts as well as in the Central Highlands. Some windward slopes receive up to 250 centimeters of rain per month. In this period, the northern and

eastern parts of the island are left drier with a little quantity of rainfall as lying in the leeside.

28. Pressure Pattern. During this season, the pressure gradient starts strengthening. (Fig. 4)

Fig 4. Monsoon Season : July Pressure Pattern

29. Surface Winds. These are usually light to moderate, but on the west coast, during the early months of the monsoon, may average 30-40 kt for several hours. On end with short gusts upto 70 kt. The direction is SW in Jun but changes to Wly in Jul-Sep.

30. Clouds. The amount of cloud is least in the NE where the average is less than 4 octas and greatest in the west. On the west coast the average in the south is 6 octas. The clouds are usually in the form of rapidly moving patches frequently as low as 1,500 or 1,000’.

31. Upper winds. The wind increases rapidly from the ground values to a steady 20-30 kt at 3000’. From 3000’ to 15,000’ the wind remains between WSW and WNW 20-30 kt. Above 15,000’ are light easterlies. In Jun the winds, in the lower levels tends to be some what more SWly and at the beginning and end of the season the height at which the light easterlies come in is rather lower than during the middle of the period . (Fig. 5)

Fig 5. Monsoon Season : July Upper Wind

32. Rain. The highest rainfall occurs on the hills in the south-west of the island where rain falls occurs on 20-25 day per month. Along West Coast, South of Puttalam the Rainfall is high decreasing until August and increasing again in Sep. Rain may occur at any time during the day with a slight max at dawn. The number of Rainfall days decreases again to 13 in Sep at Colombo. The rain on the coast occurs chiefly in the form of sharp squally showers and periods of continuous rain are not frequent except perhaps at the beginning of Jun. North of Puttalam, rain is rare in this month. Along East Coast there is a little rain in first two months but in latter half of the season showers and T’ST occurs on about 4-7 days in the month. (Fig. 6)

Fig 6. Southwest Monsoon Season : Rainfall Distribution

Second Intermonsoon Season or Post Monsoon Season ( Oct-Nov)

33. The third season occurs in October and November, the intermonsoonal months. During this season, periodic squalls occur and sometimes tropical cyclones bring overcast skies and rains to the southwest, northeast, and eastern parts of the island.

34. Pressure Pattern. During this season pressure gradient is weak. (Fig. 7)

Fig 7. Second Intermonsoon Season : October Pressure Pattern

35. Surface winds. In Oct the winds are light and moderate SWly with frequent calms in the morning, but as soon as the season advances, the winds become light and variable and the land and sea breezes assume control. Off the NW coast in Nov the winds vary between NW and NE with freq heavy squalls. In Oct the island is sometimes affected by the Bay depression and there may be high winds, particularly in the north.

36. Cloud. During the early part o the season, the distribution is similar to that of the SW monsoon. When monsoon begins to withdraw, the tendency is for clear mornings and cloudy afternoons. There may however be sheets of low cloud lasting for several days during wet, or threatening weather associated with depression.

37. Upper winds. In the beginning, the upper winds are similar to those of the SW monsoon but the westerlies usually disappear at 7000’ and are replaced by light variable wind. Above 10,000’ the wind are light Ely. In Nov, the wind are variable upto 10,000’ with easterlies 10-15 kt above. (Fig. 8)

1010

1010

1009

1009LL

HH

OCTOBER PRESSURE OCTOBER PRESSURE

PATTERNPATTERN SECOND INTER MONSOON

Fig 8. Second Intermonsoon Season : October Upper Wind

38. Rain. Along West coast, heavy rains lasting for several days, associated with depressions may sometimes be experienced. Rainfall mostly occurs every 2 days out of 3. It is also the wettest period in the north where the average falls may be 12-16 inches and where the number of rainfall days averages 15-16 in Nov. (Fig. 9)

Fig 9. Second Intermonsoon Season : Rainfall distribution

Northeast Monsoon or The Cool Season (Dec- Mar)

39. During the fourth season i.e. Northeast monsoon ("Maha season" in Lankan language) which lasts from December to March, the moisture brought from the Bay of Bengal causes downpours in the northeast region of the island. The northeastern slopes of the mountains may be inundated with up to 125 centimeters of rains during these months.

40. Pressure Pattern. During this season the pressure gradient starts getting established. (Fig. 10)

Fig 10. Northeast Monsoon Season : Jan Pressure Pattern

41. Surface Winds. NEly wind of 20kts in Dec and it weakens towards end of the season over east coast. Over west coast wind are weak near the coast with marked land and sea breezes.

42. Clouding. Along the West Coast the cloud amount decreases from 4 to 5 octas in Dec to 2 to 4 octas in Mar. The mornings are usually free from clouds but a sheet of high stratus frequently reaches the coast from the hills in the afternoon. Low clouds are usually not experienced except in rainy or thunder weather. Over East Coast the cloudiness is usually controlled by the monsoon,. When the monsoon is strong, the skies are generally overcast, and they are clear or only partially clouded when the monsoon weakens. There is a general increase in rain and cloud over the hills.

43. Upper winds. In the first 1500 feet there is a transition to N-NEly 10-15 kt. From 1500 – 1000 feet the wind changes to NE-Ely 10-15 kt with Ely 10 kt wind above. (Fig. 11)

1011

1011

1012

1012

LL

HHJANUARY PRESSURE JANUARY PRESSURE

PATTERNPATTERN1013

1013

NORTHEAST MONSOON

Fig 11. Northeast Monsoon Season : Jan Upper Wind

44. Rain. Over West Coast rain occurs mostly in the afternoon, about 7-10 days per month in Dec and Jan and 5-6 days in Feb & Mar. Feb is the driest month, with an average of 5 rain days in the south and 2 in the north. The rainfall is 5-10 inches in Dec falling to less than 2 inches in Feb. (Fig. 12)

Fig 12. Northeast Monsoon Season : Rainfall Distribution

45. Over East Coast during Dec, the rainfall averages 15-20 inches decreasing to 8 – 14 inches in Jan. In Feb – Mar the average is 2-5 inches. Rain may occur at any time and there is a slight max at dawn. The number of rainfall days decreases from 16 in Dec to 7 in Mar.

Precipitation

46. Sri Lanka’s rainfall varies from year to year and the average annual rainfall for the country is around 1836mm. However, in recent times the annual rainfall is decreasing per year at the rate of 35mm for Colombo, 20mm Galle and 34mm for Batticaloa. The rainfall depends on the rain from the two monsoon rain periods as well.

47. Precipitation is characterized by wide seasonal and regional variations. The monsoon season in the southwest lasts from May to November, during which time the rainfall is exceptionally heavy. In the northern dry zone, most of the precipitation—roughly 1,020 millimeters (about 40 inches)—annually occurs during the monsoon season, which begins in the first week of November. Most crops in the dry zone require irrigation. The hills and lowlands of the southwest section, which is known as the wet zone, normally have some rainfall throughout the year, but heavier precipitation occurs in May and June and again in October and November.

Tropical Cyclones and Depressions.

48. Tropical cyclones of great severity are not often experienced in Srilanka but depression affects the country. Depression occurs in Oct & Nov. Maximum in the month of Nov and occasionally between Jan & March but rare in Apr-Sep

49. Depression usually skirts the northeast coast of the island in a Nly or NWly direction the effect are therefore felt more on the north & east coast.

50. Weather- Maximum precipitation occurs in the north & east coast

51. Direction- Generally move ESE to WNW.

Aviation Weather Hazards

52. Thunderstorm. Thunderstorms are very frequent in Srilanka and occur in all months. However they are more frequent during Mar to May also in Oct and Nov. Thunderstorm are sometimes violent and may occasionally be accompanied by hail & strong surface winds gusting up to 50-70kts. Thunderstorms are most frequent during afternoon.

53. Waterspouts. Waterspouts occur in the vicinity of the coast. Between 1920 to 1931 as many as 45 waterspouts occurred over off west coast and 16 off the east coast.

54. Gale and squall. Gale is most frequent in May and June when they are recorded on 7-8 days per month and least from Dec to April when average frequency is 1-2 days per month.

55. Low clouds. Stratus lower to 150 to 200m agl under moderate to strong monsoon condition is observed in both SW monsoon and NE monsoon.

56. Visibility. Visibility is generally good except in heavy rain the visibility may reduce to few hundred meters.

57. Mist/haze. Mist/haze occurs generally during morning hours during NE monsoon over west coast for 2-3 hr

Check Assimilation

Fill in the Blanks / State True or False. If False, give the correct Statement.

1.In Sri Lanka humidity is typically higher in the southwest and mountainous areas and depends on the ___________ of rainfall.

2.Over West Coast the average number of rain days in Apr is ______ in the south and _______ in the north. .

3. Tropical cyclones of great severity are not often experienced in Srilanka but depression affects the country (True/False)

4. Average annual rainfall for the country is around ________mm.

Conclusion

58. The following broad inferences can be drawn from climatological tables :-

(a) Surface winds are mainly SWly from May to October and NEly in the remaining months for all stations.

(b) The frequency of number of cloudy days increases with the advance of the SW monsoon particularly for stations on the west coast.

(c) The variation of temperature is between 3-5 oC. Coldest months are Dec & Jan (24 oC) and warmest in Apr & May (28o C)

(d) Thunderstorm is almost throughout the year but maximum are in May & Nov.

(e) Visibility generally remains good except in rain/mist. No fog occurs.

(f) Rainfall varies from 120 to 400 cm.

(g) Squall and gale are maximum in Jun and least in Dec.

59. For a relatively small country Srilanka’s climate is remarkably varied. From dry desert lie areas to verdant rain forests, from sunny ocean beaches to misty mountain tops. And since Srilanka is in the monsoonal tropics, it has a dry and a wet season which takes place simultaneously on opposite sides of the island.

Check Assimilation : The Key

1. Seasonal Patterns

2. 13-14: 3-4

3. True

4. 1836

CLIMATOLOGY OF INDIA AND NEIGHBOURHOOD

1. The climatology of the Indian subcontinent and its neighbourhood is to a

large extent determined by the geographical location. India is an extension of the

great Asiatic continent with the vast expanse of the Indian Ocean to the south. To the

north of India stretches the loftiest mountain ranges of the world, radiating from the

Pamir Knot near the intersection of latitude 30°N and longitude 74°E where the

boundaries of the USSR, China, Afghanistan, Pakistan and India come close to one

another.

Physiography Relief

2. From the Pamir Knot in Central Asia, three mountain ranges run westwards—the

Alai Tag along latitude 40 °N, the Hindukush to the south of it along the northern

borders of Afghanistan and Iran joining the Elburz mountains and the third the

Sulaiman mountains through Pakistan to the Persian Gulf, almost parallel to the

Indus river. From the Pamir Knot, again a number of mountain ranges radiate

eastwards.

3. The southernmost is the Himalayas, constituting a massive mountain wall

extending eastwards over 2500 Km, the width varying between 150 and 250 Km. The

Everest (8848 m), Kanchenjunga (8585 m), Makalu (8470 m), Dhavalagiri (8172 m),

Nanga Parvath (8126 m), Nanda Devi (7817 m) are all peaks situated on the Himalayas.

To the north of the Himalayas, almost parallel to it lies the Karakoram range. Mt. Godwin

Austin or K2 (8611 m), the second highest peak after Everest, lies on the Karakorams.

4. From the eastern end of the Himalayas runs southwards, a chain of mountains

— the Namkai — Patkai — Naga — Barail — Lushai chain along the boundary between

Assam and Burma. The chain extends further south through Burma as the Arakan

Yomas, parallel to the coast. The Pegu Yoma, parallel to it runs through the central parts

of Burma. The Garo-Khasi-Jaintia hills of Assam run east-west as an offshoot of the

main Patkai-Lushai chain.

5. South of the Himalayan wall and stretching from the Arabian Sea to the Bay of

Bengal lies the Indo-Gangetic plain, about 3000 Km long and 250 to 400 Km wide.

INDIA-PHYSICAL

6. Further south, the Vindhya and Sathpura ranges with an average elevation of 1

Km run east-west roughly near the tropic of Cancer and demarcate the Peninsular

plateau from the Indo-Gangetic plains. The Peninsular plateau is the highest on the

western edge with the Western Ghats on the west coast and slopes towards the east.

A lower and more broken ridge known as the Eastern Ghats form the eastern boundary

of the plateau. The Western Ghats reach the highest point in the south in the Nilgiris

and Cardamom hills.There are coastal plains between the Western and Eastern Ghats

and the seas, the eastern plains being broader.

River Systems

7. The Himalayan chain constitutes the watershed for a number of rivers originating

from it and then flowing in southerly and easterly directions. These rivers are fed by the

melting of snow and glaciers as well as by the rainfall over their catchment areas. The

more important of these rivers are: Indus, Ganges and Brahmaputhra. In the

Peninsula, the main rivers are Narmada, Tapti, Krishna, Godavari and Kaveri.

Surface Climatology

Geographically the subcontinent covers tropical, sub-tropical and temperate zones.

Due to altitude, the climate of the Western Himalayas approaches sub-arctic

conditions, particularly in winter. Except for the southern part of the Peninsula, the

region does not come directly under maritime influence.

The sub-continent presents wide contrasts in climatic conditions. The annual variation

of rainfall is large, being less than 10 cm in the Thar desert and over 1000 cm in

Cherrapunji. The weather phenomena vary from the snowstorms of the Himalayas to

the sand/duststorms of Rajasthan. Temperatures vary between —45 °C in Dras in

winter and +49°C in Rajasthan in summer.

The major common climatic feature is the alternation of the monsoons. Between the

two monsoons are transitional seasons.

Extra-Tropical Disturbances

The winter season in the subcontinent is characterised by the dominating influence of

continental air mass of low humidity and is, therefore, the season of lowest rainfall,

taking the region as a whole, outside the Peninsula. However, extra-tropical cyclones

originating in West Asia, Mediterranean or sometimes as far west as the Atlantic move

across North India and cause cloudiness and precipitation. These have preferred

tracks.

Though the regions immediately to the west of India are not preferred regions of

marked cyclogenesis, the tracks of the migratory cyclones of the middle latitudes,

come down to the Indian region from October to June in the process of translation from

west to east. In the other months of the year, the tracks shift far too much poleward

and do not affect any portion of northern India. The tracks of these extra-tropical

cyclones and the attendant weather are considerably affected by topography. The

normal wind flow pattern of the winter monsoon gets immensely modified under the

influence of these migratory cyclones. These are known as western disturbances, in

India.

The sequence of weather associated with each western disturbance is: rise in

temperature, fall of surface pressure, appearance of high, medium and low clouds,

light to moderate precipitation, clearing followed by rise in surface pressure and fall in

surface temperature. After the passage of the disturbance, the normal winter monsoon

pattern is restored. Some of these disturbances show the typical structure of extra-

tropical cyclones viz. warm front, warm sector and cold front.

However, most of the disturbances which affect the Indian region are secondaries and

are mostly in the occluded stage. Often, a closed surface low may not be seen, but

only troughs in the upper westerlies are seen on the charts. The tracks of these

cyclones come farthest to the south upto latitude 23° in February. These cause rain in

the plains and snowfall in the hills over northern India. Some of the cold fronts cause

thunderstorms and hail. Immediately after the passage of the disturbance, most areas

are affected by radiation fog. In the wake of the disturbance, northern India usually

gets a spell of cold wave with the incursion of cold continental air.

Tropical Cyclones

Tropical cyclones of varying intensity and magnitude originate in the Bay of Bengal and

Arabian Sea and travel generally in a westerly direction. Some of them recurve and

move north/northeastwards. Also, some of the typhoons of the China Seas travel

westwards across Indo-Chinese Peninsula, enter the Bay of Bengal and revive as

cyclones. Again, some of the Bay cyclones moving across the Peninsula emerge into

the Arabian Sea, revive there and become cyclones. The maximum incidence of these

cyclones is from April to December.

The cyclones of the southwest monsoon period (June to September) are generally of

small intensity. During this period, the main area of formation is northwest Bay of

Bengal.

The cyclones of the pre-monsoon (April-May) and post-monsoon (October-November)

periods are of greater intensity. These generally form in the latitude belt 10° to 14°.

After an initial west/northwest

movement, they often recurve north/northeast. Some of these cyclones cause

immense damage and destruction, particularly when they cross coast. Inland, they

cause heavy rain.

Interaction Between the High and Low Latitudes

Sometimes the synoptic systems of the lower and higher latitudes interact with each

other. The flow patterns, though predominantly zonal (latitudinal) have at times

significant meridional (longitudinal) components.

In summer, in between the sub-tropical anticyclones can be seen troughs of the waves

in the westerlies extending from the subpolar regions to the tropics. When translatory

waves in the easterlies approach the waves in the westerlies near about the same

longitude belt, there is interaction between the tropical troughs and polar troughs.

When the flow pattern is predominantly zonal, the axes of the sub-tropical high cells

are oriented east-west and then the easterly troughs and westerly troughs bypass each

other without any interaction. When the meridional component in the flow pattern is

pronounced, both the easterly and westerly waves slow down and their amplitudes

increase when they reach the same meridian in their respective latitude belts. When

they move away, the amplitude and speed return to their original values. On occasions

the easterly trough gets captured, thereafter becoming stationary or retrograde. After

sometime, a fracture takes place and then both the waves progress in their own paths.

The superposition causes intensification of both the waves.

During the southwest monsoon season, the Bay depressions recurve when a trough in

the westerlies moves into the same longitude belt. When middle latitude westerlies

have large amplitude troughs, they draw the monsoon trough to the foothills of the

Himalayas, giving rise to a break in the southwest monsoon flow over the subcontinent.

A high level westerly trough in June may cause incursion of the monsoon current into

the Punjab and Kashmir.

In the northeast monsoon season, when westerly troughs extend to lower latitudes,

tropical cyclones decelerate, weaken or recurve. The progression of the wave in the

easterlies which causes rainfall in south Peninsula also gets affected under such

conditions.

Climatological Features Surface Pressure

January is the typical winter month. The Siberian anticyclone with a central pressure of

about 1035 mb is the predominent feature. Pressure gradients are the strongest over

the eastern region. By February there is a slight weakening and westward

displacement of the anticyclone. By March this anticyclone weakens and is displaced

further westwards. An isobaric bulge develops over north India, including the tendency

for low pressure formation over central India.

In March-April, the anticyclone is completely disrupted and exists only as an elongated

ridge from Sinkiang to Mangolia and a low pressure area appears over central India. In

May, the thermal low extends from east Madhya Pradesh to Rajasthan with a central

pressure of about 1004 mb. Another low pressure area appears over the extreme north-

eastern parts of China. By June, there is further fall of pressure over northwest India and

Pakistan where an elongated low appears and conditions are set for the onset of the

southwest monsoon.

July is a typical summer monsoon month. A surface low extending from northwest India

to the Bay of Bengal is the predominent feature. The lowest pressure of about 998 mb

over the northern hemisphere can be seen over Pakistan. Pressure gradients are the

strongest in the western region. The August pressure distribution is more or less the

same. By September, the pressure gradient starts decreasing in the west and increasing

in the east and signs of the establishment of the winter high can be seen.

By October, a high forms over northern Sinkiang and adjoining Mangolia. The

anticyclone and winter monsoon conditions get established by November and attain

their full features by December.

Temperatures

The subcontinent presents vast diversities of temperature. Northwest India presents a

pronounced continental climate, with extreme hot summer and freezing cold winter. In

southern Peninsula, the annual variation of temperature is small and it is warm and

humid. The main factors which determine the temperature are the altitude of the Sun,

latitude, elevation, distance from the sea and type of prevailing airmass which in turn

governs the cloudiness and weather. The June-July temperature maximum is modified

due to the cloudiness and precipitation of the southwest monsoon season. Hence we

find a maximum in the temperature in May-June and a secondary maximum in

September at the cessation of the rains.

In the winter period (December-February), continental winds prevail over most of India.

The temperature increases from north to south. The isotherms are more or less along

the latitude parallels. The daily mean maximum is of the order of 20 to 25°C over the

plains of north India and about 30°C in the Peninsula while the mean minimum is of the

order of 5 to 10cC in the north and 20 to 25°C in the Peninsula. February is generally the

coldest month. In the wake of some western disturbances, there is incursion of polar air

giving rise to cold waves when the temperatures are 5° to 10 °C below normal and sub-

freezing temperatures occur in the plains of north India. The cold waves sometimes

extend down to as low a latitude as 20 °N.

In the hot months from March to May, the hottest region extends from east Madhya

Pradesh to Pakistan. The western and northwestern parts of India have daily

maximum temperatures of the order of 35 to 45 °C. In north India, heat waves occur

at times. The mean minimum temperatures over the country range

between 20 to 25 °C. With the advent of the southwest monsoon, due to clouding and

precipitation, there is a drop in temperature but the temperature generally starts

decreasing only from October onwards. The range of temperature is naturally larger in

the interior of the country and is largest in pre-monsoon and smallest in the rainy

season. On the average, the diurnal range is 10 to 15°C in northwest India, decreasing

towards the east and south.

Annual Rainfall Pattern

India, as a whole, gets abundant rainfall, but the disparity in distribution is so large that

some areas are eternally dry while in some others the major portion of the rain goes

unutilised. Again, the year to year variation is so large that the same area may have

drought in one year and floods in another. The mean annual rainfall of India is 105 cm.

The distribution of rainfall over the sub-continent over the year is given in the map

below:-—

The prominent feature of the distribution is the orographic effect The western slopes of

Western Ghats and eastern India receive rainfall of the order of 200 cm. A narrow strip

on the west coast between latitudes 11°N and 13°N as well as the southern slopes of

Khasi hills have a rainfall of over 400 cm. Rajasthan desert has the least amount of

rainfall. It will be seen that apart from the frequency, intensity and tracks of translatory

synoptic disturbances, relief and topographical configuration have profound influence on

the rainfall pattern.

Annual March of Precipitation

The largest amount of rain for the major portion of the country is received during the

southwest monsoon period. The rainfall distribution depends on the strength of the

monsoon, the depth and orientation of the monsoon trough over the Indo-Gangetic plain

and the number, intensity and tracks of the tropical depressions moving inland from the

Bay of Bengal. The maximum rainfall in this season is in the west coast of the Peninsula

and eastern India (roughly 150 cm.) In the Peninsula the rainfall falls off sharply towards

the east of the Western Ghats. In northern India, the maximum rainfall is in the eastern

states, Bengal and the sub-Himalayan regions, falling off westwards.

The southwest monsoon commences its withdrawal from the north in September and

the rainfall decreases and ceases progressively over the whole subcontinent outside

south Peninsula. The rainiest season in most parts of the country outside Tamil Nadu is

the southwest monsoon season. Southwest monsoon rainfall, however, exhibits wide

variations in time and space due to irregular distribution. Abnormal dates of onset and

withdrawal and breaks in the monsoon give rise to floods and droughts.

Generally the northeast monsoon season is the dry period over the country outside

northwest India and Tamil Nadu. Tamil Nadu has the maximum rainfall in the retreating

monsoon season under influence of tropical cyclones or easterly waves. In northwest

India, the dry spell is broken when western disturbances or westerly waves move across

the country from west to east. There are on the average, about 4 to 5 disturbances per

month from December to March, causing cloudiness and precipitation. The precipitation

is in the form of snow over the hills and rain or thundershowers over the plains. All the

disturbances are not equally active and as such this type of cyclonic precipitation occurs

at intervals in the otherwise dry, clear period. At times the rain extends to the Gangetic

plains and occasionally to central India.

In the later part of the pre-monsoon period i.e. from May to June, the temperatures

increase and the circulation pattern also undergoes change. There is often influx of

moisture particularly in eastern India. Here, violent thundersqualls often accompanied by

hail occur. In northwest India, there is insufficient moisture and dust and thunderstorms

are common and small amounts of rain occur at times. In the Peninsula,thundershowers

occur, the frequency increasing with the advance of the season. The rain is of convec-

tional type rather than cyclonic.

Thunderstorms

Thunderstorms of the tropics are much more violent than those of the higher latitudes.

The tops of the cumulonimbus clouds often pierce the tropopause and extend a

kilometre or more into the stratosphere. Thunderstorms occur in all parts of the

subcontinent and there is no month when some part or other of the country is not affect-

ed by thunderstorms.Taking the country as a whole, the period of maximum incidence of

thunderstorms is March to September with the peak in March to May. The period of least

thunderstorm activity is December-February. The areas with highest thunderstorm

activity are Assam, Meghalaya, Bengal, Orissa, Chota Nagpur and adjoining areas of

central India and extreme south Peninsula. In northeast India the maximum activity is

during March-September while in southwest India it is September-December. Although

severe thunderstorms occur in the late afternoon, they can occur at night and early

morning hours also. In northeast India and the coastal areas of the Peninsula, especially

during northeast monsoon, thunderstorms occur mostly at night and during early

morning hours.

Hail

Thunderstorms are accompanied by hail especially in March-May in north India,

particularly in and near the sub-montane regions. Hailstorms occur in northwest India in

December-February also, in association with western disturbances. Hail is rather rare

during the southwest monsoon season. Thunderstorms of Bengal, Assam and Chota

Nagpur andadjoining areas during March to June are often accompanied by violent

destructive northwesterly squalls reaching to 60 to 80 kt. and hail. These are known by

the name nor'westers or Kalbaisakhis. The hailstorms cause widespread damage to

crops, cattle and sometimes even humans.

Dust Storms

During March-June, dust or thunderstorms occur in northwest India. These are known

as andhis. These are thundersqualls raising walls of dust or sand reducing visibility

practically to nil

Fog

India experiences mostly radiation fog, except in the coastal areas where advection fog

also occurs. In mountainous regions, in the valleys and hills, fog or mist is possible

throughout the year, during night or early morning. In the plains, however, mist/fog is

confined to the winter months, say November to March, mainly north of latitude 20 °N. In

winter, in the coastal belts advection fog may occur anywhere. The pre-condition for

radiation fog is the influx of sufficient moisture. In the north, this happens with the

passage of western disturbances. The clearance, cooling and onset of cold air after

overnight precipitation in the wake of a disturbance creates conditions conducive to the

formation of radiation fog. When the fog lifts, low stratus cloud persists for some time.

Local peculiarities like topography, wind direction and speed, atmospheric pollutants in

the nature of industrial effluents and smoke are factors which determine the

intensification and the time of dissipation of fog. Though fog may not occur in the plains

in the absence of a favourable synoptic situation, haze or mist occurs in winter near

areas where large expanses of water are present, e.g., lakes, rivers, canals, dams and

big tanks. Apart from this, in and near industrial and inhabited areas, atmospheric

obscurity in the nature of haze occurs at dusk and dawn due to smoke etc. stratifying in

the inversion layer near the ground.

Upper Air Climatology Upper Air Temperatures

In the winter season, the temperature decreases steadily northwards in the lower levels

from the equatorial belt. At the 700 mb level, the temperature difference is of the order of

12°C between the latitude belts 20°N and 40°N. The same pattern is kept up to the 300

mb level. At the 200 mb level the In the summer, the region of maximum temperature

which was to the south of latitude 10°N in winter, shifts northwards. The highest tem-

peratures are found from 700 to 200 mb along a belt between latitudes 20°N and 40°N.

The warm region is a conspicuous feature over the Asian continent during the

southwest monsoon season and coincides with the sub-tropical high pressure ridge.

Temperature decreases both to the north and south of this ridge. At the 100 mb level,

however, the coldest air is still over the equatorial belt, the temperature decreasing

rapidly northwards The transitional, periods have features nearest to the winter or

summer pattern, according to the actual month of the year.

Tropopause

During the winter season, the sub-tropical or the middle type of tropopause also can be

seen to the north of latitude 25 °N, presumably associated with the westerly jet core.

During this period, say from November to April, the transition from the troposphere to the

stratosphere over northern India (north of latitude 25 °N) takes place in two stages on

many occasions. The lower transition or the middle tropopause is characterised by an

inversion or isothermal layer above it and occurs at about 10 to 12.5 Km. The upper

transition or the tropical tropopause is not very sharp or well-marked and has an

inversion or isothermal above it and occurs at about 16 to 17 Km.The middle tropopause

is seen between 225 mb and 195 mb at a temperature of about-55°C, north of latitude

25°N and the tropical tropopause between 115 mb and 100 mb at temperatures about -

70° C, the values being lower to the south of latitude 25 °N. The frequency of incidence

of middle tropopause decreases as the summer approaches.In summer, only tropical

type of tropopause occurs. The height of the tropopause is highest between latitudes

25°N and 30°N where it occurs between 190 mb and 95 mb. Towards the south, the

level is between 115 mb and 200 mb and the temperature of the order of -75°C.

Upper Air Circulation

The upper air flow patterns between the 700 mb and 100 mb levels show well-defined

characteristics which undergo seasonal variations. Winds are predominantly easterly

south of latitude 15°N and predominantly westerly north of latitude 34CN. Separating

these two predominant wind systems lie the semi-permanent cells of high pressure

where the winds are variable and the temperatures comparatively higher. The sub-

tropical high pressure cells together with the two predominant zonal wind systems on

either side undergo seasonal changes with the Sun's apparent annual movement north

and south of the equator. The zonal westerlies attain the maximum strength at about the

200 mb level during the winter while the zonal easterlies reach their peak strength at the

150 mb level in the summer.

In winter, the sub-tropical high pressure belt is located roughly along latitude 15°N at the

700 mb level and along latitude 10°N at the 200 mb level and aloft and the westerly

winds increase steadily in strength unto about 200 mb level, thereafter decreasing

slightly. The core of the sub-tropical jet stream, at about the 200 mb level, lies between

latitudes 25°N and 30°N. The average speeds of the jet is about 80 kt and when

pronounced 150 to 200 kt. In the transitional pre-monsoon season, wind speeds

decrease progressively as the season advances and also the jet axis shifts polewards.

In summer, the upper air flow pattern undergoes profound changes. The sub-tropical

high pressure belt shifts northwords abruptly, the shift often coinciding with the onset of

the summer monsoon in India. At the 300 and 200 mb levels, the high pressure ridge is

located between latitudes 25°N and 30°N and at the 100 mb level further north between

latitudes 30°N and 35°N. Easterlies prevail to the south of latitude 25°N above the 300

mb level. The speed of the easterlies increases with height from 200 mb, attaining

maximum strength between the 150 and 100 mb levels. They are most prominent

between latitudes 10°N and 15°N with average speeds of 60 to 70 kt, sometimes

reaching 120 kt at the core. This is the easterly or tropical jet stream.

In the southwest monsoon regime, the upper air circulation is generally westerly below

the 500 mb level. These monsoon westerlies strengthen from surface upwards and

reach an average speed of 25 to 30 kt at the 850 and 700 mb levels. When the

monsoon is strong, they attain speeds of about 50 kt at these levels and sometimes a

low level jet stream can be seen at about the 900 mb level. To the north of the sub-

tropical high pressure ridge the winds are westerly and of the order of 30 to 40 kt at the

300 mb and 200 mb levels.

In the next transitional period—the post-monsoon season, the subtropical high pressure

cells shift southwards progressively but slowly till winter conditions are established.

CHAPTER 4

SEASONS OF INDIA

The climate of India is dominated by the monsoon circulation. In one half of the year the

wind blows from the cooler humid ocean to the warmer dry land, while in the other half

there is a seasonal wind blowing from the cold dry Asian land mass to the warm Indian

Ocean-There is a spectacular reversal of the pressure and wind patterns between the

two halves, the change-over taking place in gradual stages. Thus the main seasons are

two: the summer southwest monsoon and the winter northeast monsoon. The transition

between these two main seasons during which the gradual reversal of pressure and

wind circulation takes place constitutes two other intermediate seasons.

The summer monsoon sets in over the extreme southwest of the Peninsula by the end

of May, gradually spreading over the entire subcontinent by July and withdraws from the

extreme northwest by the middle of September. Thus June to September is designated

as the southwest monsoon when the country gets the maximum rainfall.

December to March is the period when full-fledged winter northeast monsoon conditions

are established throughout the country and this period is designated as the cold or

winter season when the lowest temperatures occur over the country.

The transition periods, when the change-over takes place are de-signated as post-

monsoon (October to November) and pre-monsoon(April to May) seasons.The

temperature variations due to the influence of the wet southwest monsoon is such that

the highest temperatures are experienced in spring rather than summer. Hence, except

probably in the extreme north of the subcontinent, to call April to May as spring will be a

misnomer and therefore generally this season is referred to as the hot season and not

as spring.The post-monsoon period is the cool season when temperatures start

decreasing and is the autumn or fall season. The trees do shed their leaves and give

credence to the fall phenomenon. However, in the south of the subcontinent, rainfall

occurs during this season.However, the division into sharp seasons is not universally

applicable for all the areas of the subcontinent. For example, the monsoon current can

be seen over the Peninsula beyond September and in Kashmir, winter conditions exist

much earlier than December and extend beyond March, even to April.

Classification

The seasons, can however, appropriately be designated on the basis of temperature

and rainfall as: —

Cold Season . . December to March (winter)

Hot Season . . April and May (pre-monsoon)

Wet Season . . June to September (southwest monsoon .

Cool Season . . October and November (post- monsoon)

Deviations may occur in individual years such that the seasons are either delayed or

commence too early as compared to the normal dates or months. Herein, therefore,

dealing with Indian weather, we have come to the borderland between climatology and

synoptic meteorology and often the two merge. Once in a while the seasons march in a

perfect cycle with textbook precision. The fourfold seasonal division is based on low

level features, mainly winds, temperature and precipitation which are often highly

variable and even capricious.

Winter (Cold) Season (December to March)

This is the coldest portion of the year and hence called the cold season. The winter

northeast monsoon prevails over the subcontinent. Subsiding anticyclonic air is

prevalent over the major portion of the country, giving rise to mainly clear weather. The

core of the sub-tropical westerly jet stream now shifts well to the south of the Himalayas

and comes down farthest south to about latitude 22°N in February.

The northeast monsoon starting from the heart of the vast cold Asian land mass is

prominent only in the lower levels and as such the Himalayas exercise a beneficial

blocking effect on the polar continental Siberian outflow, preventing it from extending to

India. South of latitude 18°N, apart from the moisture picked up by the northeasterlies

during their travel over the Bay of Bengal, maritime Far East Trades also join the

circulation. This increased moisture gives rise to some precipitation in south Peninsula.

In the north, on an average about 4 to 5 western disturbances travel from west to east,

breaking the spell of clear weather. These cause clouding and precipitation.

Pressure Distribution

In the winter, on the surface there is an extensive belt of high pressure extending

from Sahara to Siberia. This belt is not a continuous one and is often broken up into

distinct cells due to the effect of land and sea and also orography. These are regions of

comparatively low pressure areas in the belt, in areas like Arabia, Caspian Sea, Black

Sea, Nile Basin, Mediterranean Sea, Red Sea and Persian Gulf. These low pressure

areas in the high pressure belt facilitate the formation and movement of cyclones or of

the secondaries when primary disturbances move across more northern latitudes. As far

as the Indian region is concerned, the secondaries that form in the coastal areas of Nile,

Upper Egypt, Sudan, Red Sea, Gulf of Oman and Northeast Arabian Sea are of im-

portance. These move eastwards and polewards, in other words, in a northeasterly

direction and affect India.

The pressure distribution for January is shown in the map below:

Upper Air Circulation

The upper air temperatures and flow patterns at different levels are shown in the

following pages.

JANUARY

Upper Air Isotherms ( °C )

60 70 80 90 100 110 60 70 80 90 100 110

Weather

In this season, generally cold dry north or northeasterly continent-tal air prevails over the country outside southeast Peninsula. Clear skies,fine weather, low humidity and low temperatures with large diurnal rangeare the normal features. However, these dry spells are broken at inter-vals when migratory extra-tropical disturbances move across northern India. y

The rainfall distribution of the season and the tracks of disturbances are shown below:

The western disturbances have a frequency of 5 to 6 per month, the maximum number

and the southernmost tracks being in January-February. Most of these are in the

occluded stage by the time they enter India.

The sequence of weather follows the extra-tropical cyclone pattern with cirrus, fall of

pressure and rise in temperature heralding the approach of the warm front. The clouds

lower to altostratus and stratus with drizzle and rain as the depression comes over the

station. Change

of wind direction, cumuliform clouds, thundershowers, rise of pressure and fall in

temperatures signify the passage of the cold front.

In the wake of the disturbance, strong cold dry continental norther-lies or northwesterlies

set in. With the high humidity from the precipitation, overnight clearing and sudden

cooling, radiation fog develops over the area. The fog often lasts upto roughly 1000 to

1100 hrs IST. While lifting up, the low stratus persists for another hour or so. Thereafter,

there is a spell of cold wave lasting for about 2 to 3 days. The precipitation in the

Himalayan ranges is mostly in the form of snow.In the south, there is a trough extending

from Kerala to Gujarat along the west coast of the Peninsula. Sometimes this trough

accentuates, extending right upto Rajasthan. Often, a cut-off low forms and moves in an

easterly direction causing rainfall in the central parts of the country and south Uttar

Pradesh. In southeast Peninsula, the northeasterly current after acquiring moisture

during its travel over the Bay gives rise to a few thunderstorms. In the Peninsula

westward moving easterly waves also cause cloudiness and precipitation. Sometimes a

cyclonic storm also may cause heavy rain in extreme southeast Tamil Nadu.

A rough pattern of the distribution of weather phenomena in this season can be seen in

the map below:

Aviation Weather Hazards

The main aviation weather hazards in this season are: —

(a) Visibility: Radiation fog or mist is common in many parts of the country. It forms

mostly in situ, but in valleys and river beds, they also drift in. In the adjoining plateaus

the advected lifting fog or stratus of the valleys appears as ground fog or mist. Moist

airnear the ground, clear skies for radiational cooling and weak winds for turbulent

mixing are conditions conducive for the formation of radiation fog. Conditions

immediately after the passage of a western disturbance in northern India satisfy these

requirements. The most susceptible areas are: northwest India, Uttar Pradesh, Bihar,

north Bengal and Assam (particularly south bank of the Brahma-puthra valley).

Advection fog occurs in the coastal belts and also where large water bodies are present.

The Brahrnaputhra valley, Sunderbans, the coastal belts and the hills and valleys of the

Peninsula experience this type of fog/mist.

Apart from radiation or advection type of fog or mist, mist or haze is common during

winter mornings and evenings generally over north and central India and near the hills of

Deccan plateau. The smoke near the towns and villages gets stratified round about dusk

and dawn and reduces the visibility to about 2 Km or so due to stratification in the

inversion layer.

(b) Icing: Freezing levels are the lowest in winter. In the north they vary between 2000

metres and 4000 metres above sea level while in the south they are of the order of

5000 metres. Ice accretion on aircraft is experienced at comparatively lower levels

in the north in this season.

(c) Sub-tropical jet stream: The core of the sub-tropical jet stream now lies between

latitudes 25 °N and 30 °N at a level bet-ween 300 mb and 200 mb. The average speeds

are of the order of 80 kt, though when pronounced, speeds of 150 to 200 kt are at-

tained.

Pre-Monsoon (Hot) Season (April and May)

The hot season is a transitional season when the winter monsoon pattern of pressure

and winds gets disrupted prior to the establishment of the summer monsoon pattern and

is thus referred to as the pre-mon-soon period. The season is characterised by

widespread dust haze and extremely high temperatures over north India. There are dust

storms (andhis) over western India and thundersqualls (norwesters or kalbai-sakhis)

over eastern India. These are triggered by the influx of warm moist air, sometimes in

association with western disturbances. The tracks of western disturbances move north

with the progress of the season.Land and sea breeze effect is prominent over the

coastal areas. The frequency of thundershowers increases progressively in south India

with increased influx of moist air from the sea as the season advances.

A few tropical cyclones form in the Indian seas. They move north or northeast and strike

the Bengal, Bangladesh or Burma coasts. The sub-tropical westerly jet stream weakens

and moves north of the Himalayas by the end of the period.

Pressure Distribution

The winter pattern gradually changes to be replaced by the summer pattern. The north-

south pressure gradient of the winter months gets disrupted. The Sun crosses the

equator northward and the Indian land mass gets progressively heated. Thus, this is a

period of continuous and rapid rise of temperature and decrease of pressure. Simul-

taneously, in the southern hemisphere, the reverse process takes place and the

southern anticyclone gets intensified. The pressure field over the entire country is diffuse

and the difference of pressure between the extreme north and south of the subcontinent

is often of the order of 2 to 3 mb only. As a consequence, the winds are light and

variable.

Along the sea coasts, land and sea breeze effect is regular and prominent. A few

tropical cyclones form in the Indian seas. A few western disturbances still affect the

extreme north of the subcontinent, though the tracks are now far north.

A trough of low covers almost the central region of the subcontinent and comparatively

high pressure cells exist over the Arabian Sea and Bay of Bengal. Sometimes strong

pressure gradients develop over Rajasthan and Gujarat.

The pressure distribution for April is shown below:

Upper Air Circulation

The upper air temperature and flow patterns for the various levels are shown in the

following pages.

APRIL

Weather

Rainfall occurs mostly in eastern India with 5 to 25 cm of rain per month. The weather is mostly of convectional type. There is large-scale thunderstorm activity whenever there is an influx of moisture under any favourable synoptic situation. When the moisture content is less, dust-storms occur as in the case of the Punjab, Jammu, Haryana, Rajas-than, Uttar Pradesh and north Madhya Pradesh with a frequency of 3 to 5 a month. These duststorms are locally known as andhis.

Violent local thundersqualls often accompanied by hail and rain occur in the regions where deep humid winds from the sea meet hot dry land winds. The areas where this type of thunderstorms occur are Bengal, Assam, Orissa and east Bihar. They are locally known as norwesters or kalbaisakhis.

In the Peninsula, before the onset of the southwest monsoon, frequent incursion of moisture gives rise to pre-monsoon thundershow-ers.

Tropical storms forming in the Arabian Sea mostly move westwards and do not affect the Indian weather, though occasionally a storm recurves and crosses the north Mysore coast causing damage in Maharashtra and Gujarat.

In the Bay of Bengal, tropical storms develop between latitudes 10°N and 15°N. The storms move mostly north or northeast and strike the Bengal, Bangla Desh or Burma coast causing extensive damage and devastation. Some of the storms, however, move northwest and cross the Tamil Nadu coast. Some of these, after travelling over the Peninsula as weak depressions or lows, emerge into the Arabian Sea and revive.

The seaso

nal rainfall distribution and the tracks of disturbances shown below:

A rough pattern of the distribution of weather phenomena in this,season is shown in the map below.

35°ir

Aviation Weather Hazards /

(a) Thunderstorms: These are mostly violent convectional typeof thunderstorms caused by the intense insolation, when there isinflux of moisture. These are, however, local in nature and com-paratively short-lived and occur in the latter part of the day orearly evening and are not restricted to any particular area of thecountry. The most destructive of this type of convectional thun-dersqualls are the norwesters of eastern India. They occur frommid-March to June, mainly during afternoons and evenings. Themost susceptible areas of formation are Chota Nagpur bills andthe sub-montane districts of eastern Himalayas. They often reachtornadic violence. In the Peninsula, particularly in Kerala, thefrequency of thunderstorms increases as the season progresses.

(b) Dust Haze: With dry loose soil and strong winds, on mostof the days an extensive area covering Pakistan, Rajasthan, Jammu,Punjab, Haryana, north Madhya Pradesh and Uttar Pradesh getsenveloped in dust. At times the dust belt extends eastwards uptoBihar, Bengal, and upper Assam. Dust-in-suspension reduces visibi-lity to 2 Km or less for days together. At times, visibility may bereduced to a few metres when the phenomenon can be termed dustfog. The vertical extent of the dust layer varies between 3 to 5Km though on occasions it may extend even to 7 to 8 Km.

(c) Dust-raising Winds: Apart from the reduction in visibilitycaused by the suspended dust, there are sometimes, also strongsurface winds of the order of 30 kt gusting to 40 to 50 kt. Thishappens in the areas affected by the dust haze and is confinedmostly to the day time, say between 0900 and 1700 hrs IST. Thevisibility is often reduced to a few hundred metres in blowing dust.On occasions, dust-raising winds continue during night when thereis a very strong pressure gradient.

(d) Dust storms: Dust storms (andhis) of north India are theresult of dust scooped up from the earth by the squalls from thun-derstorms. In these thunderstorms, there is not enough moistureand as such only the downdraught without much precipitation en-sues In some cases, there is some rain, the rest of the rain havingevaporated before reaching ground.The squall speeds often reach40/50 kt. occasionally 60/70 kt. and the visibility deteriorates to

a few metres. Cu and Cb clouds are thus a pre-condition for these dust storms. Strong gusty surface winds which give rise to very poor visibility due to blowing dust, sometimes appear similar to dust storms, but are the result of strong pressure gradient and do not have Cu and Cb clouds. The squall-type dust storms are of short duration and are often followed by a few drops of rain. There is a general fall of temperature with this type of dust storms, because of the downdraught. The direction of the squall is usually between southwest and north, though sometimes it may take any other direction according to the location of the Cb cell with respect to the station. The regions affected are Pakistan, Jammu, Rajasthan, Punjab, Haryana, north Madhya Pradesh and Uttar Pradesh. At times dust storms occur in Bihar and Bengal also.

Southwest Monsoon (Wet) Season

(June to September)

This is the summer monsoon period when the southwest monsoon holds sway over the country. The country (outside western and eastern Himalayas and southwest Peninsula), receives nearly 75% of its rainfall during this period.

Southwest monsoon sets in at the extreme southwest tip of the Peninsula by the end of May and progresses inland in stages and extends to the northwest portions of the country by the middle of July. It starts retreating from the extreme northwest, by the beginning of September, progressively receding southwards. The period June to September is referred to as the southwest monsoon period and is the wet season. Often people refer to the southwest monsoon simply as monsoon since rainfall is commonly associated with monsoon.

Though the period June to September is termed southwest mon-soon season, southwest monsoon current is hardly in evidence in north-ern India in June where it is still parching dry and burning hot. Thesummer heat spell is broken and the temperatures drop as the monsooncurrent sets in.

The tropical depressions from the Bay of Bengal travel inland through central and northern India.

The sub-tropical westerly jet stream shifts north of the Himalayas and the tropical easterly jet stream makes its appearance over the Peninsula.

The main characteristics of the seasons are moderate to heavy rains, very low clouds, high humidity and sultry weather. This is also the period when devastating floods occur in many parts of the country.

283

Pressure Distribution

July pressure distribution is shown below :

The pressure gradient and wind flow in this season show

a complete reversal as compared to the winter. An intense

low pressure system (the deepest in the hemisphere)

develops over the northwestern part of the subcontinent

and adjoining Afghanistan, spreading eastwards as an

extensive trough upto the northwest angle of the Bay of

Bengal, covering the entire Indo-Gangetic plain. The

pressure gradually increases southwards to the Indian

Ocean, well into the southern hemisphere. In the Indian

Ocean, there is a high pressure area to the south. The

trough of low pressure in the Indo-Gangentic plain is usually

referred to as the monsoon trough. This is often modified

when depressions form in northwest Bay of Bengal.

The strengthening and breaks in the monsoon are

associated with the formation and movement of Bay

depressions. These depressions move west or northwest,

MEAN SEA LEVEL PRESSURE

(mb)

55

usually along the axis of the monsoon trough. They move

upto Rajasthan, weaken and merge into the seasonal low or

recurve north or northeast at some intermediate stage-move

towards the western Himalayas and break up over the hills.

Upper Air Circulation

The upper air temperatures and flow pattern for the various levels are shown in the following pages.

JULY

Upper Air Isotherms (°C )

60 70 80 90 100 110 60 70 80 90

285

UPPER WINDS - JULY

Weather

The southwest monsoon current sets in over the seas extending from the

south of Sri Lanka to the Andaman Sea and Tenasserim by the middle of May. By

the beginning of June it extends further northwards as two distinct branches—the

Arabian Sea branch and the Bay of Bengal branch, the former to Kerala and the

latter to Burma, Ban-gla Desh and Bengal. In the next, two weeks the monsoon

extends to the whole country outside northwest India. By the middle of July the

whole subcontinent comes under the sway of the monsoon flow.

The seasonal distribution of rainfall and the tracks of disturbances are shown below:

65° 75° 85° 95°E

More or less general rain occurs throughout the country, though the intensity

and duration of each rainy spell and the rainfall distribution are controlled by the

strength of the monsoon, the orientation of the monsoon trough and the tropical

depressions.

The chief features of the rainfall are as follows: The monsoon current meeting the

Western Ghats yields heavy rain on the west coast due to orographic effect, the

rainfall averaging 250 cm. To the east of the Western Ghats, there is rapid

decrease of rainfall due to the orographic desiccation. The Arabian Sea branch of

the monsoon gives some rain in Gujarat and west Rajasthan. Here again, the

maximum rainfall is in the Aravalli region due to orographic effect. The Bay of

Bengal branch causes heavy rains in the Burmese coastal belt and Eastern

Himalayas. From Bengal, this branch is deflected westwards as easterlies, more

or less parallel to the Himalayas. In north Madhya Pradesh, Uttar Pradesh, Punjab,

Haryana and Himachal Pradesh, the southwesterly Arabian Sea branch and the

easterly Bay of Bengal branch of the monsoon meet along the monsoon trough

and give rise to moderate to heavy rain.The axis where the fresh Arabian Sea

monsoon air and the modified Bay monsoon air meet, is the monsoon front. The

distribution of rain in the Indo-Gangetic plain is, therefore, naturally governed by the

orientation of this monsoon front.

Depressions forming in the Bay of Bengal move westwards along the trough of low

pressure. Widespread and locally heavy rain is caused by these depressions.

When a western disturbance or westerly wave is moving eastwards across the

north of the subcontinent in the same longitude belt, the depressions are pulled

polewards and recurve. The rainfall is usually heaviest in the southwestern sector of

the depression.When, the monsoon front moves north and lies along the foothills

of the Himalayas, the rainfall over the plains decreases and there is an increase of

rainfall over the hills. This is termed break-monsoon. The revival of the monsoon

subsequently takes place when another depression forms in the Bay and the

monsoon front moves south again, its eastern end dipping into the head Bay.Under

strong monsoon conditions, layer type of clouds predominate. Southeast Tamil

Nadu is a sheltered area.The southwest monsoon starts withdrawing by the middle

or the third week of September from northwest India, bringing in its wake continental

dry air and fine weather. The retreat of the monsoon air continues gradually and

progressively southwards and eastwards.

A rough pattern of the distribution of weather phenomena in this season is shown in the map below :

Aviation Weather Hazards

(a) Low clouds: Stratus clouds lower usually to 150 to

200 m

agl under moderate to strong monsoon conditions.

However,

when depressions affect a place, the cloud base may

lower to 30

to 60 metres agl.

(b) Strong surface winds: Under strong monsoon

condition

surface winds are usually of the order of 25/30 kt.

When de-

pressions affect an area, surface wmds may reach

speeds of 40 to

50 kt.

(c) Visibility: Visibility is generally good. However, in

heavy

rain the visibility may deteriorate to a few hundred

metres.

(d) Easterly or tropical jet stream: During the monsoon

season,

the easterly or tropical jet stream lies between latitudes

10° and

15°N with its core between 150 to 100 mb levels.

The core

speeds are on the average 60 to 70 kt but occasionally

reach 100

to 120 kt.

Post monsoon (Cool) Season

(October and November)

The southwest monsoon starts withdrawing from

northwest India by September itself. The withdrawal

proceeds in stages, eastwards and southwards. There is a

transitional period when the summer pattern of pressure and

winds undergoes slow but steady modification prior to the

establishment of the winter pattern. Consequently, the pattern

is rather diffuse and ill-defined. This period is, therefore

referred to as the post-monsoon or retreating monsoon season.

The temperatures start decreasing all over the country

and by October, it is cool in most places. The nights

particularly are cool and pleasant. Hence the season can

appropriately be called the cool season.This is the season

when Tamil Nadu gets its maximum rainfall.

The zonal westerlies shift south, of the Himalayas and the

subtropical westerly jet stream makes its appearance over

the extreme north of the country. The easterly jet stream

disappears. The tracks of western disturbances move south

and a few of them start affecting northwest India.

Pressure Distribution

The summer pattern of pressure and winds undergoes

modifications prior to the establishment of the winter

pattern and as such the patterns are rather diffuse and

ill-defined. The retreat of the southwest monsoon is often

in jerks.The tracks of the extra-tropical cyclones of middle

latitudes start shifting southwards and as the season

progresses, a few of them start affecting the extreme

north of the country.In the south, October is the month

of maximum frequency of cyclonic storms in the Bay of

Bengal. They usually form in the neighborhood of the

Andamans and move towards the Tamil Nadu, Andhra and

Orissa coasts. In November also there are severe

cyclonic storms in this area. These normally strike the

Tamil Nadu coast between Cuddalore and

Masulipatinam. Some of the storms move in north or

northeasterly direction and strike the Bengal or Bangla

Desh coasts. Some of the cyclones which revive in the

Arabian Sea may recurve and strike the Maharashtra-

Gujarat coast.

The October pressure distribution is shown below:

Upper Air Circulation

The upper air temperatures and flow pattern for the various levels are shown in the following pages.

60°

MEAN SEA LEVEL PREAAURE

Weather

With the retreat of the southwest monsoon, cold dry

continental air sets in over northwest India, ushering in fine

clear weather with good visibility. With the advance of the

season, dawn and dusk mist or haze causing deterioration in

visibility is often experienced.

During the periods when western disturbances affect the

extreme north (1 to 2 per month), clouding and precipitation may

occur, particularly in Kashmir and Punjab. The farther north the

place, the greater the effect of these disturbances. Snowfall

may start in the Great Himalayas as early as October in

association with these disturbances.In northeast India also, the

weather generally improves with the withdrawal of the monsoon.

However, when the Bay of Bengal storms cross the Bengal or

Bangla Desh coast, influx of moisture causes clouding and

weather in this region also.When the Bay storms cross either

the Tamil Nadu coast or move northeast skirting the Orissa

coast, weather in the whole coastal belt along the Peninsula,

Andhra, Orissa and Bengal gets affected. Extensive clouding

with very low clouds, heavy rain and thundersqualls are

associated with the storms crossing the eastern coast. Some of

these storms cause very, high seas and extensive damage in

the Andhra and Tamil Nadu coastal belt.

The seasonal rainfall distribution and the tracks of storms are shown below :

A rough pattern of the distribution of weather phenomena in this season can be seen in the map below:

Aviation Weather Hazards

(a) Low visibility: As the southwest monsoon

withdraws, win-

ter conditions progressively set in and normally poor

visibility due

to stratified smoke occurs particularly near towns and

inhabited

areas during dawn and dusk. In the extreme north

of the coun-

try, after the passage of a western disturbance, mist,

fog or lifting

stratus may occur for a day or two. In the south, poor

visibility is

experienced only with precipitation.

(b) Tropical storms: Severe cyclonic storms develop

in the Bay

of Bengal and in the Arabian Sea in the latitude belt

10°N to 15°N

and affect the coastal belts.

(c) Sub-tropical jet stream: The mean position of the

sub-tro-

pical jet core starts shifting south and may be

located between

latitudes 30°N and 35°N near about the 200 mb level.

The ave-

rage speeds are 60 to 70 kt, the maximum at times

reaching 100 to

150 kt.