Meteorology 2 – SEMESTER 5 – UNIT 1

Climatology : Wind and Pressure systems over the oceans; Mean Surface Pressure and Wind distribution; Anabatic and Katabatic winds; Land and sea breezes; Local winds; Westerlies, Polar Easterlies, Doldrums and Inter- tropical Convergence Zones(ITCZ); General distribution of surface temperature, surface current; Monsoons, Dynamics of Indian monsoon; Seasonal weather & climatic characteristics in

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Various factors affecting atmospheric Motion ?(a) Earth rotation effects(Coriolis Force ): Due to

rotation of earth, whilst blowing from high pressure to low pressure areas, the wind is deflected to the right of its path in Northern Hemisphere & Left in Southern Hemisphere. The force which causes the deflection is called “Coriolis Force”. It is minimum at Equator and increases as latitude increases and becoming maximum at Poles. Coriolis force always acts at right angles to the direction in which wind is blowing. F= 2v pΩSin Q ( F – Coriolis force, V = Wind Velocity, p=Density of atmosphere.

Q = Angular velocity of earth ( 15 degrees per hour), Q - Latitude

(b) Pressure Gradients: Is the fall of pressure with distance, If the distance between consecutive isobars is small, the pressure gradient is said to be high and strong winds are expected to blow. If the distance between consecutive isobars is large, the pressure gradient is said to be small and winds of lower speed will be expected. For a given pressure gradient, stronger winds are expected in lower latitudes than in the higher latitudes. On the surface of the earth, winds always try to blow from an area of low pressure(LP), because of gradient force.

Coriolis

© Boundary Layer(Friction): Due to the roughness of the ground, the wind speed is reduced on account of the resistance. These effects are found up to 2000-3000 feet which is called friction layer or boundary layer. Beyond 2000 feet air flow is free without any resistance. The friction is more over the land than over sea. Over the land, the wind speed is reduced by half to 1/3, if the intended speed is 20 Kts, the realized speed only 10 Kts and there will be deflection of 25 degree from its path towards low pressure area. Over the sea, the reduction is about 1/3 means if the intended speed 20 Kts, the realized speed will be about 13 Kts and the deflection from the path will be about 10-15 deg. towards low pressure area side. During the day time, the frictional effect is more while during night time the frictional effect is less.

Friction: loss of air momentum to surface obstacles such as trees, buildings… exerted in opposite direction to the motion . Friction slows the wind relative to its geostrophic velocity. This slowdown decreases the Coriolis acceleration so that air is deflected towards the low pressure region.

Wind Pattern:

Winds:It is the horizontal motion of the atmospheric air from a high pressure area to a low pressure area. The wind velocity depends upon the pressure gradient which is the pressure difference between the two places over a given distance. Strong pressure gradient means strong winds & Weak means light wind. This depends on closeness of Isobars drawn on a surface chart. If Isobars are closely packed(Steep Pressure Gradient) which means strong surface winds. The direction of the wind is found out with the help of “Wind Vane” & the velocity of the wind is found out with the help of “Anemometer”. The wind direction denotes the direction from where it is coming eg. Westerly wind means wind is coming from the west and moving into an easterly direction.

Buys Ballots law: Buys Ballots Law states that if an Observer faces the wind, the low pressure area will lie on his right hand side in Northern Hemisphere and on his left hand side in the SH.

Caution when applying Buys Ballots Law : Buys ballots law should not be applied within a few degrees of the equator. This is because Coriolis force is negligible at the equator and therefore the winds blow directly across the isobars from HP to LP Areas. In the vicinity of land, the wind experienced may not be the free unobstructed wind. It may be wind defelected by the land.

Gradient wind/PGF/Coriolis force/Centripetal force:Gradient Wind/Pressure Gradient Force (PGF) : Air is a fluid. One important property of fluids is that the pressure at a point in the fluid acts in all directions. This means that air can exert a pressure horizontally as well as vertically. Imagine a parcel of air with different pressure on either side of it. Pressure is just the force acting on each square metre. So if the parcel has different pressures on either side of it, it has different forces on either side. There is therefore a resultant force acting on the parcel from higher pressure to lower pressure. This is the pressure gradient force. The size of the pressure gradient force depends upon the rate of change of pressure with distance. The Pressure Gradient Force is always directed towards the lower pressure. If the pressure gradient force were the only force acting, the wind would blow straight across the isobars from high to low pressure.Geostrophic Wind: It blows parallel to the straight line isobars. The wind moves from high pressure to a low pressure area on account of the rotation of the earth and it gets deflected to the right of its path at every stage. At one point, the wind starts blowing parallel to the straight line isobars instead of moving into low pressure area, such winds are called Geostrophic Wind. Geostrophic winds results on

account of the balance the two factors of Pressure Gradient force &Coriolis force & it stronger at lower latitude & lighter at higher latitude.These winds only occur in certain conditions. Geostrophic winds results on account of the balance between the two forces namely, Pressure Gradient force &Coriolis force. When Pressure Gradient force and Coriolis force are equal and opposite and these are the only forces acting. The isobars must be straight and parallel. The winds cannot occur less than 600 miles above the earth’s surface so that friction at the earth’s surface is negligible.

dpds = 2vrWSinf; where

dpds is pressure gradient

force.

Therefore VG= dp/ds

2PWSin φ, where VG = velocity of

Geostrophic wind.

• From the above equation it is clear that the Geostrophic wind is stronger at the lower latitudes as Sinf appears in the denominator, for a given pressure gradient force. Geostrophic wind is comparatively lighter at higher latitudes for a given pressure gradient force. TheWinds blow spirally inwards towards a depression (LP area surrounded by areas of HP) and clockwise in the NH. Winds blow spirally outward from the centre of an anti-cyclone (HP area surrounded by areas of LP), clockwise in the NH.

Backing of Wind:An Anticlockwise change of direction from which the wind is blowing eg. From N to NW, from E to NE, from SW to S, etc.

Veering of Wind : A Clockwise change of direction from which the wind is blowing eg. From N to NE, from S To SW, from W to NW, etc.

Thermal Wind :Vectorial difference between the higher level wind and lower level wind is called thermal wind. Thermal wind = Upper level wind – Lower level wind (vectorially).(Upper Level =230/30 Kts, Lower level=250/20 Kts& then Thermal wind 20/20 Kts).

The thermal wind blows keeping lower temperature (cool air) to the left side and higher temperature (warm air) to right side in northern hemisphere. Wind System:

Wind is caused by the movement of air which is caused by differences in air pressure.

Differences in air pressure are caused by the uneven heating and cooling of the Earth.

Warm air is less dense and creates an area of low pressure

Cold air is more dense and creates an area of high pressure

Global Winds: Wind movement of air caused by differences in air pressure, air rises at the equator and sinks at poles. The equator received more direct sunlight creating less dense, warm rising air toward the poles leaving an area of low pressure. At the poles, air is colder so it sinks toward the equator leaving an area of high pressure.Local Winds: Movement of air are also influenced by local conditions, and local temperature variations

commonly cause local winds.Local winds are not part of the global wind belts. Gentle winds that extend over distances of less than 100 km are called a breeze.Trade winds- prevailing winds that blow from east to west from 30º latitude to the equator in both hemispheres Like all winds, trade winds are named according to the direction from which they flow.In the Northern Hemisphere, the trade winds flow the northeast and are called the northeast trade winds. In the Southern Hemisphere, they are the southeast trade winds.

Westerlies prevailing winds that blow from west to east between 30º and 60º latitude in both hemispheres Between 30º and 60º latitude, some of the descending air moving toward the poles is deflected by the Corioliseffect. In the Northern Hemisphere, the westerlies are the southwest winds. In the Southern Hemisphere, they are the northwest winds.

Polar easterliesprevailing winds that blow from east to west between 60 and 90 latitude in both hemispheres. Surface winds created by the polar high pressure are deflected by the Corioliseffect and become the polar easterlies. Where the polar easterlies meet warm air from the westerlies, a stormy region known as a front forms.

Doldrums/Horese Latitudes: The trade wind systems of the Northern Hemisphere and Southern Hemisphere meet at the equator in a narrow zone called the doldrums. As the air approaches 30º latitude, it descends and a high-pressure zone forms. These subtropical high-pressure zones are called horse latitudes. Surface winds are weak and variable in both of these zones.

General circulation of Atmosphere or Planetary Wind Circulation:

\ITCZ : Between latitude 0o – 30o,North-East winds from Northern hemisphere and South-East winds from southern hemisphere converge at the equator which is called equatorial low pressure area or Inter-Tropical Convergence Zone (ITCZ) or doldrums. The easterly winds are called Trade winds as they were favourable for trading sailors in olden times

Hadley Cell: The air that converges at the doldrums rises upto higher levels in the atmosphere, moves pole wards in both hemispheres and sinks down to the surface at latitude 30o and again moves towards the equator on the surface. This circular motion of wind from surface to above at the equator and from latitude 30o back to the equator is called Hadley cell.

Hadley Cell

Ferrels Cell

Polar Cell

Ferrels Cell: At Lat 30o in both hemispheres Subtropical high pressure belt is situated with dry deserts of the world (Sahara desert). From sub tropical high pressure belt, westerly winds blow to Lat 60o where lies the sub polar low pressure area. From subpolar low pressure area the air goes up and a portion moves equatorwards and sinks down at latitude 30o and moves polewards on the surface which is called Ferrels cell.

Polar Circulation: From the arctic region where there is a high pressure area easterly winds blows equator wards to lat 60o where a sub polar low pressure area is situated. It can be seen that the winds are deflected to the right in northern hemisphere and to the left in southern hemisphere on account of coriolis force. The motion is called polar circulation.

The latitude 25o – 35o is called Horse latitude. A strong westerly wind at latitude 40oS is called Roaring fourties.

Various Local winds around the world :1. Levanter: From East in West Mediterranean during summer.2. Vendevale: From SW in West Mediterranean during Winter,Katabatic Cold Wind.3. Mistral:From NW in Gulf of Lyons during Winter, Katabatic Cold Wind.4. Bora: From NE in Adriatic Sea during winter, Katabatic Cold Wind.5. Sirocco: From SE in Malta and Italy during summer (Sandy Wind).6. Gregale: From NE in Malta in Winter.

7. Etesian: From North in Aegeana Sea in Summer, Katabatic Wind.8. Khamsin: From South in Egypt in Spring Hot, Dusty.9. Simoon: From South in Arabia in Spring Hot, Dusty.10. Shamal: From NW in Persian Gulf in Summer, Hot Wind.11. Elephanta. From South to South East Malabar coast in Sep/Oct. at the end of South West Monsoon.12. Brickfielder: From N Australia in summer, Hot Wind.13. Southerly Buster: From SE to South Australia in Spring, Cold Wind.14. Pampero: From Northern South West in River Plate in Winter, Squally wind.15. Harmattan: From East, West Africa in Winter, Sand Laden Wind .16. Nor’wester: From Northward or North Eastward, mainly experienced during hot months occurs occasionally in West Bengal and neighbouring regions and may Bay of Bengal .

17. Roaring Forties: is another name for the prevailing westerly winds of the Southern Hemisphre around 40 Deg SSea Breeze : During the day, the land gets extremely hot and the air in contact with it gets heated, resulting in a low pressure over land. The temperature of the sea surface, and hence the temperature of the air over it, remains fairly constant resulting in a relatively high pressure over

sea. The warm air on the land rises giving way for the air from sea(high pressure) to blow to land is called

Sea Breeze, It sets about at 1000/1100 hrs local time, reaches maximum force 3-4 about 1400 hrs and dies down about sunset. Land Breeze: At night, the process is reversed. The land gets cooled(High Pressure) more quickly than the surrounding sea area, which is still warmer(Low Pressure), the air of warm sea rises to fill up its place, the cool air from land blows to sea called Land Breeze. It sets in a couple of hrs after sunset and blows until about half-hour after sunrise.

(This is the large difference in diurnal range of atmosphere temperature over land and over sea.)

Differentciatethe Land& Sea Breeze : Land Breeze Sea Breeze

1COOL AIR SINK ON LAND - WARM AIR EXPAND&RISE ON LAND (HIGH PRESSURE) (LOW PRESSURE)2. WARM AIR EXPAND AND RISE ON

- COOL AIR SINK ALONG SEA SEA(LOW PRESSURE) (HIGH PRESSURE)

3 WIND BLOWS FROM THE LAND TOWARDS - WIND BLOWS FROM SEA TO LAND (HIGHTHE SEA(HIGH TO LOW PRESSURE AREA) PRESSURE TO LOW PRESSURE AREA

4 LAND BREEZE SPEED MAY GO UPTO 7 KTS - SEA BREEZE SPEED MAY GO UP TO 30-35 KTS.

5 VERTICAL DEPTH OF LAND BREEZE - VERTICAL DEPTH OF SEA BREEZE ABOUT

ABOUT 200 M 500 – 1000 M 6 LAND BREEZE IS WEAK -

SEA BREEZE IS STRONGER

7 SETS AFTER SUN SET TO SUN RISE - SETS 1000/1100 HRS TO SUN SET

(a) Katabatic Wind: At night, the slope of a hill or mountain cools more rapidly than the surrounding free air. Hence, the air close to the slope gets colder, denser and heavier than the air at the corresponding level. The heavier air close to the slope slides down the slope in the night which is called.(b) Anabatic Wind: During day time, the hill or mountain slope gets heated more quickly than the free air. Heated air on the slope is lighter and rises up along the slope. This wind is called Anabatic Wind. Katabatic wind is stronger than Anabatic wind

Atmosphere Pressure System:The Pressure of Atmosphere is the force exerted

on surface of unit area by the air. When the air is at rest, the pressure exerted is called “Static Pressure” or simply “Barometric Pressure”.

When the air is in motion, the pressure exerted is called “Dynamic Pressure” or “Wind Pressure”. In meteorology, we are concerned only with static pressure or Barometric Pressure.

The air is held close to earth by gravitational attraction. The pressure would decrease with height in the atmosphere.

The pressure observations are reduced to mean seal level for comparison purpose. The Pressure values recorded at different heights will be different. Reduction of pressure to mean sea level is important in order to find out areas of Low/High pressure.

Isobars are lines joining the places of equal pressure.

Unit of pressure is millibar or HectaPascal(HPA)1 HPA=1000 dynes. One dyne is the force required to produce an acceleration of 1 cm per sec.2 in mass of one gram.

Isobars are drawn an interval of 2 HPA(mb) in India.

Nomenclature of low pressure area against wind speed ?

Low Pressure Area (L) is a region of relatively low pressure with nearly circular and concentric isobars around the centre where the pressure is the lowest. When the low pressure is intense, it is called depression or cyclone. Weather will be bad. Winds will be anticlockwise in Northern Hemisphere. Depending up on the wind speed, low pressure areas have different names aqs follows:

Nomenclature Wind Speed

Low pressure Area (LPA) Upto 16kts

Depression (D) 17 to 27 kts

Deep Depression (DD) 28 to 33 kts

Cyclonic Storm (CS) 34 to 46 kts

Severe Cyclonic Storm (SCS) 47 to 63 kts

Severe Cyclonic Storm with core of hurricane winds 64 to 119 kts

Super cyclone > 120 kts

Atmospheric Pressure: Atmospheric pressure is the force exerted, per unit area, by air. It is thus the weight of the column of air above a unit area. SI Units of atmospheric pressure: Hectopascals or Millibars.

1 Hectopascal = 1 Millibar, 1000 mb = 1 bar = 750.1 mm of mercury

1 bar = 1.02 Kg per cm2 or 10.2 t per m2.

The average atmospheric pressure at the earth’s surface is about 1013 mb.

Lapse Rate of atmospheric pressure: Atmospheric pressure always decreases as height increases. The graph of the lapse rate of atmospheric pressure against height above sea level is a curve. The average lapse rate is about 115 mb per Km height in the lower levels of the atmosphere (Upto 5 Km height). At higher levels, the lapse rate is higher.

Semi-Diurnal range of Atmospheric Pressure: The difference between the maximum and minimum values is called the semi-diurnal range of atmospheric pressure. The average semi-diurnal range is more in the tropics than in middle latitudes. In Tropical regions it is about 3 mb (ieupto =+-1.5 mb from normal) and in UK (lat 51 deg North) it is about 0.8 mb (ie. Upto +-0.4 mb from normal). In high latitudes, it is negligible and frequently masked by fronts and frontal depressions.

Barometric Tendency: Barometric tendency is the difference between the atmospheric pressure at the time of observation and the atmospheric pressure three hours earlier. It is expressed in millibars and up to one decimal of a millibar.

Barometric tendency gives the forecaster a good idea of the rate of change of pressure, which is useful for

predicting the movement of pressure systems (also called isobaric patterns). It is for easy comparison by the forecaster that barographs and barometer readings should be in accordance with UTC(GMT) and not ship’s time.

Where a barograph is not working, or is not provided on board, barometric tendency can easily be obtained by the aneroid barometer – align the fixed pointer to the dial pointer every three hours, starting from 00 UTC, and note the change each time.

Isobars: An isobar is a line drawn, on a weather map, joining all places having the same atmospheric pressure at the time for which that weather map as drawn. It is important to note that atmospheric pressure is dynamic. It may change frequently. Hence, its value, when state, holds good only for that particular time.

Isollobars: While the pressure pattern is changing rapidly air does blow across the isobar. Rate sof change of pressure can be measured, and lines joining places with a equal rate of change of pressure are called Isollobars.

Diurnal Variation of Pressure :The pressure is maximum at 1000 hrs& 2200 hrs. Minimum at 0400 hrs& 1600 h rs.(Local time). The range of oscillation is high in the tropics being 3 to 4 HPA in places like

India. At poles, the oscillation of range is minimum. At mid latitude, daily range is somewhat moderate (1-2 HPA). In winter, the pressures are high. In summer, the pressures are low.

Seven Basic Isobaric Patterns

1. Straight Isobars : Straight isobars are said to exist when the isobars run straight and nearly parallel for a few 100 miles. The pressure gradient is usually low, resulting in low wind speeds. Wind direction and force remain constant so long as the isobars remain unchanged. The weather associated with straight isobars cannot be defined as it depends on the properties of the air mass in which these isobars exist.

2. Cyclone or Low : Cyclone or low is an area or low pressure surrounded by areas of high pressure. The isobars form closed shapes. The winds blow spirally in wards, anti-clockwise in NH & clockwise in SH. The pressure gradient is usually high, resulting in strong winds. A low is an area of convergence of air. On reaching the centre, the air moves up as a strong upward current, resulting in cumulus or cumulonimbus clouds of very high vertical extent and heavy precipitation. A cyclonic pattern of isobars is called a low or depression if the wind speed is 33 kts or less, a cyclonic storm if the wind speed is from 34 to 47 kts or over.

3. Secondary Cyclone Or Secondary Low : Sometimes a low is closely followed by another, within its pattern of isobars. The first one is called the Primary and second one, the secondary. The latter is so named only because it formed later but it possesses all the qualities of the primary and may either fill up and disappear or it may intensify and swallow up the primary.

4. Trough of Low Pressure (T ) :It is a line extending outwards from a low pressure area so that the pressure is lower in the trough line than on both sides. Along the trough line weather will be bad. There are two forms of trough, In Non frontal

trough, the isobars curve gently(change direction gradually) & Frontal trough which exists at the boundary between two different air masses.

5. High Pressure (H)/Anticyclone : Is an area of high pressure surrounded by areas of low pressure. The isobars from closed shapes. Highest pressure is at this centre. The wind blow spirally outwards, clockwise in the NH and anti-clockwise in the SH. There is a total absence of any cloud or precipitation over the anticyclone therefore, a sign of good weather – light winds, no clouds(blue sky), no precipitation , good visibility, etc.

6. Ridge of High Pressure (R): It is a line extending outwards from a high pressure area such that the pressure is higher in the ridge line than on the two sides. A ridge may form by itself or it may be the outer fringes of an anticyclone far away.

7. Col Region : It is an area that lies between two Lows and two Highs where pressure is uniform and winds are light. Relative humidity is fairly high and lightning may be seen.

Horizontal & Vertical Distribution atmosphere/The Pressure – Height relationship:

• Pressure Gradient : The horizontal rate of change of pressure over a given distance is known as pressure gradient. That is the pressure difference between two places divided by the distance between the places is pressure gradient. When isobars are closely packed together, pressure gradient is steep. When Isobars are far apart pressure gradient is weak or slack. Steep pressure gradient means strong wind speed. Change of pressure at a station during a given period is known as pressure tendency. The line joining places of same pressure tendency is called isollabar.

• Pressure and Height : Pressure decreases with height. This principle is made use of in the Construction of Altimeter with the help of an Aneroid Barometer. The scale is graduated in terms of height instead of pressure. As one goes upwards, the atmosphere will become thinner and thinner so that for every 10 m(30 FT) 1 Mb pressure falls, it may become equal 40 FT or 50 FT or even more (Thickness wise) at higher levels. Even though atmosphere extends over hundreds of KMS, 50% of the atmosphere is within 6 Km,

only which means at 6 Km the pressure value is half of that at the surface.

• The formula for finding the thickness of upper atmosphere at various levels is given by formula:

96 x T (Ambient temp. in Kelvin)

• h(Thickness of air in feet) =-------------------------------------------

P (Atmosphere pressure in Mb(at given level)

Seasonal Variation of Temperature:

Global surface temperatures have risen by about 0.6°C since 1900

Global surface temperature 1855-2010

How is this curve calculated?

It is likely that this warming is larger than for any century since 200AD, and that the 1990s were the warmest decade in the last millennium.

The warming differs in different parts of the world, but over the last 25 years, almost everywhere has warmed, and very few places have cooled.

Other changes have occurred, e.g.: Sea level has risen by about 20 cm, Ocean heat content has increased, Almost all mountain glaciers have

retreatedCoincident with this global warming, levels of CO2

(and other ‘greenhouse’ gases) have dramatically increased, to levels higher than those experienced for maybe millions of years.

Next: are temperatures and atmospheric composition linked?

Climate:

Climate is the average weather at a given point and time of year, over a long period (typically 30 years).

We expect the weather to change a lot from day to day, but we expect the climate to remain relatively constant.

If the climate doesn’t remain constant, we call it climate change.

The key question is what is a significant change – and this depends upon the underlying level of climate variability

Crucial to understand difference between climate change and climate variability…

•

Diurnal variation of atmospheric temperature: It has been observed that atmospheric temperature reaches its maximum at about 1400 hours local time and reaches its minimum at about half-hour after sunrise. Since this happens once per day, this is called diurnal variation of atmospheric temperature. The difference

General Distribution of Temp

• Temp. Zone of the World: Torrid zone lies between Tropic of Cancer(23.5 Deg N) & Tropic of Capricorn(23.5 Deg S) across the equator where the rays of the sun fall directly throughout the year.

• Temperate Zone: The North Temperate Zone lies between Lat 66.5 Deg N(Artic Circle) & 23.5 Deg S), South Temperate Zone lies between Lat 66.5 Deg S(Antarctic circle) & 23.5 Deg S). This has the moderate climate. Weather changes are more frequent. This zone is the best and most comfortable for living.

• Frigid Zone: Artic circle to North Pole (66.5 Deg N to 90 Deg N) called North Frigid Zone. The South Frigid zone from Antartic Circle to South Pole ( 66.5 Deg S – 90 Deg S.) Here the rays of the sun are slanting during the summer and hardly any sunshine is received during the winter months. The areas are always icy cold.

circle

between the maximum and minimum values in a day is called the diurnal range of atmospheric temperature of the day. Diurnal range of air temp. over land is large (as much as 20 Deg C) whereas over sea, it is very small(less than 1 Deg C) because Land being solid has a low value of specific heat and so heats up or cools very quickly, Heat received from the sun is retained by the top layer of land, as land is a poor conductor of heat and negligible evaporation. Whereas, Sea being a liquid, has a higher value of specific heat and so heats up or cools slowly. Heat received from the sun is distributed over a large mass of water by convection currents and evaporation of water during day causes adiabatic cooling which balances some of the heat received from the sun.

The temperature of the land surface, therefore varies greatly between day and night. Consequently the air in contact with it has a large diurnal range. Since the temperature of the sea surface does not vary much between day and night, the air in contact with it has a practically negligible diurnal range

Surface Ocean Currents

Surface Ocean Currents: An ocean current can be defined as a horizontal movement of seawater at the ocean's surface. Ocean currents are driven by the circulation of wind above surface waters. Frictional stress at the interface between the ocean and the wind causes the water to move in the direction of the wind. Large ocean currents are a response of the atmosphere and ocean to the flow of energy from the tropics to polar regions. In some cases, currents are transient

features and affect only a small area. Other ocean currents are essentially permanent and extend over large horizontal distances.On a global scale, large ocean currents are constrained by the continental masses found bordering the three oceanic basins. Continental borders cause these currents to develop an almost closed circular pattern called a gyre. Each ocean basin has a large gyre located at approximately 30 degrees North and South latitude in the subtropical regions. The currents in these gyres are driven by the atmospheric flow produced by the subtropical high pressure systems. Smaller gyres occur in the North Atlantic and Pacific Oceans centered at 50 degrees North. Currents in these systems are propelled by the circulation produced by polar low pressure centers. In the Southern Hemisphere, these gyre systems do not develop because of the lack of constraining land masses.A typical gyre displays four types of joined currents: two east-west aligned currents found respectively at the top and bottom ends of the gyre; and two boundary currents oriented north-south and flowing parallel to the continental margins. Direction of flow within these currents is determined by the direction of the macro-scale wind circulation. Boundary currents play a role in redistributing global heat latitudinally2.

Draw neatly world map and identify all major ocean currents of the world.

The four monsoon season of IndiaThe climate of India: is classified into the following four seasons based on the calendar months. Over large land masses the atmospheric pressure is low during summer &High during winter. This seasonal change of atmospheric pressure over large landmasses results in seasonal winds.

(a )Winter (Dec, Jan, Feb)(b) Pre-monsoon or Summer or Hot Weather Season

(Mar, Apr, May)(c) South West Monsoon or Monsoon (June, July, Aug, Sept)(d)Post Monsoon or North east Monsoon (Oct, Nov)

(a)Winter: It is the coldest part of the year. The subtropical westerly jet stream appears at about lat 22oN and at an altitude of about 30000 – 35000 feet, above sea level. Sometime in the speed of sub-tropical Jet stream in North India exceeds 100 kts. About 4 to 5 western disturbances move across North India. They are low-pressure areas, which give winter rainfall over north India. These low pressure areas come from the west (from the warm waters of the Mediterranean sea). Hence they are called Western Disturbances. After the passage of western-disturbance, cold wave conditions and fog are common in North India. In other parts of India also, visibility hazards like haze, mist or fog are common. The frequency of fog is maximum in NE India. Extreme South Eastern coastal areas receive light rainfall on account of the passage of easterly waves from Bay of Bengal.

(b) Pre Monsoon: It is a very hot season with extremely high temperature because of which a low

pressure area forms over land. Convectional rain during afternoon and evening is common on account of excess solar heating. There is wide spread thunder shower activity whenever there is an influx of moisture from ocean area. Where the moisture contents are less, dust storms occur which are commonly known as “Andhi” in North India.The most destructive thundershower that occur over N-E India, especially in and around West Bengal are called “Norweaters” or locally known as “Kalbaishagis”. They are very violent thunderstorms which originate in Chota Nagpur plateau in the forenoon and travel south-eastwards to West Bengal and give heavy showers, strong squally winds accompanied by thunder and lightning with Tornadic Violence during afternoons and evenings, due to the incursion of moisture from Bay of Bengal . In the afternoons, over North India, very hot surface westerly wind blows across, which is called “Loo”. People living in coastal areas enjoy the sea breeze during daytime. Sometimes, cyclonic storms from the Bay of Bengal strike the east coast or recurve and move towards West Bengal, Bangladesh or Myanmar coast, during April and May.

The contrasting features between South West monsoon& North East monsoon ?© Southwest Monsoon: Our country receives 80% of the total annual rainfall during these 4-months (June to September). It sets over Kerala by 1st of June and slowly progresses northwards. Trivandrum – 1st June, Kolkata – 7/8th June, Mumbai – 10th June & Delhi 29th

June. During Northern summer, the continent of Asia gets very warm and the resultant low pressure over it centre over the Thar Desert with a pressure of about 994 mb. This low is considerably lower than the equatorial low of 1012 mb and hence a pressure gradient exists from the equator towards NW India. The SE Trade winds, blowing from oceanic high of 30 Deg S towards the equatorial low, cross over the equator and blow, as a strong SW wind called SW Monsoon, towards the low over NW India. The SW direction is the result of gradient force and Coriolis force. The SW Monsoon brings heavy rain to the West Coast of India, West Bengal, Bangladesh & Myanmar. The wind force is about 7/8 in Arabian Sea & about 6 or 7 in Bay of Bengal and is also experienced in China sea.

Low pressure areas or depressions form in the Bay of Bengal at the eastern end of the monsoon trough axis and move northwestwards along the trough axis. There will be very heavy and wide spread rainfall all over the India especially over the west coast and NE India. Tropical easterly jet stream is found between Lat 10 -15 deg N in South India at a height of about 45000 feet. The easterly winds at that level will be 60-70 kts from easterly direction. Occasionally it may reach even 100 – 120 kts. Over the peninsular India, surface wind will be very strong from the westerly direction. Navigation of ships in Arabian sea is a

difficult job because of very strong westerly winds and consequent high values.

(d)North East Monsoon: This season is also known as Post monsoon or NE monsoon or cyclone season. Tamil Nadu and South Andhra coast receives maximum annual rainfall. During Northern winter, the continent of Asia gets cold and the resultant high pressure over it centre over Siberia with a pressure of about 1036 mb. The equatorial low of 1012 mb, being oceanic, remains practically unaffected by the change of season. The anti cyclonic winds, around the Siberian high, reach the Bay of Bengal and Arabian sea as the NE monsoon with a force of 3 to 4. Heavy rain falls on the East coast of India In china sea the pressure gradient is larger, resulting in wind force between 5 & 7. The wind direction in this region is between North and North East. On an average 2 to 3 cyclones form over Bay of Bengal and very rarely over the Arabian Sea. The Arabian Sea cyclone is the one that moved across the peninsular India and emerged out in the Arabian Sea. These storms have a NW track. Sometimes they have parabolic track while recurring and striking the Maharastra or Gujarat coasts.

What is meant by break condition of Monsoon: The South West monsoon sets in over SW coast of India during June/ - Sep and maximum rain received this time. It is not uniformly distributed over space and time in a year. Further the monsoon activity various year to year in typical months July & August. In some years there may be periods of interruption in the monsoon rain over major part of country lasting several days known as ‘breaks’ in the monsoon.