Lesson 3 The Dynamic Structure of the Atmosphere

Goal To familiarize you with the factors that affect atmospheric motion including atmospheric pressure, wind, frictional influences, fronts and air masses. To explain how topographical features such as land, water, and mountains affect air movement.

Objectives Upon completing this lesson, you will be able to do the following:

1. Name and explain three forces that determine wind direction and speed within the earth's friction layer.

2. Explain why wind speed changes with height and why this is important in air pollution studies.

3. Describe the effect that pressure systems have on the transport of air pollution.

4. Identify the two basic properties of an air mass.

5. Distinguish between four different types of fronts.

6. Explain the phenomenon called “frontal trapping.”

7. Explain how different types of terrain affect air flow and consequently air pollution dispersion.

Introduction We are all familiar with the various forms atmospheric motion can take: gentle breezes, thunderstorms, and hurricanes, to name a few. Air moves in an attempt to equalize the air pressure imbalances that develop as a result of variations in insolation and differential heating. Differential heating is the main cause of atmospheric motion on the earth. This lesson will answer your questions about what causes the wind to blow from a certain direction and what causes the general global patterns of air circulation. You will learn

how winds aloft behave differently from surface winds and how surface winds are influenced by the earth’s topography.

Atmospheric Circulation Air moves in an attempt to equalize imbalances in pressure that result from differential heating of the earth's surface. While moving from areas of high pressure to low pressure, wind is heavily influenced by the presence or absence of friction. Thus, surface winds behave differently than winds aloft due to frictional forces acting near the earth's surface. The rotation of the earth modifies atmospheric motion but does not cause it, since the atmosphere essentially rotates with the earth. The movement of air helps keep concentrations of pollutants that are released into the air from reaching dangerous levels.

Air Pressure Even though you can't see it, air has weight. In any gas such as air, molecules are moving around in all directions at very high speeds. The speed actually depends on the temperature of the gas. Air pressure is caused by air molecules (e.g. oxygen, nitrogen) bumping into each other and other things and bouncing off. Air pressure is a function of the number of air molecules in a given volume and the speed at which they are moving. When air is confined within a certain boundary, heating the air increases its pressure and cooling the air decreases its pressure. Forcing air into a smaller space increases air pressure while allowing it to expand into a larger space reduces air pressure.

Air pressure at any location whether it is on the earth's surface or up in the atmosphere depends on the weight of the air above. Imagine a column of air. At sea level, a column of air extending hundreds of kilometers above sea level exerts a pressure of 1013 millibars (mb) (or 1.013 kP). But, if you travel up the column to an altitude of 5.5 km (18,000 feet), the air pressure would be roughly half, or approximately 506 mb (0.506 kP).

Areas of high and low pressure are depicted in Figure 3-1. The roughly concentric circles around the areas of highest and lowest pressure are called isobars, which are lines of equal pressure. Isobars may follow straight lines or form rings as they do around areas of high and low pressure. The pressure readings in the diagram range from 1008 to 1024 millibars (mb).

Figure 3-1. Isobars around areas of low and high pressure

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1 0 1 2 m b

1 0 0 8 m b1 0 1 6 m b

1 0 2 0 m b

1 0 2 4 m bL

Wind Wind is the basic element in the general circulation of the atmosphere. Wind movements from small gusts to large air masses all contribute to transport of heat and other conditions of the atmosphere around the earth. Winds are always named by the direction from which they blow. Thus a "north wind" is a wind blowing from the north toward the south and a "westerly wind" blows from west to east. When wind blows more frequently from one direction than from any other, the direction is termed the prevailing wind.

Wind speed increases rapidly with height above the ground level, as frictional drag decreases. Wind is commonly not a steady current but is made up of a succession of gusts, slightly variable in direction, separated by lulls. Close to the earth, wind gustiness is caused by irregularities of the surface, which create eddies. Eddies are variations from the main current of wind flow. Larger irregularities are caused by convection⎯or vertical transport of heat. These and other forms of turbulence contribute to the movement of heat, moisture, and dust into the air aloft.

Coriolis Force

If the earth did not rotate, air would move directly from high pressure toward low pressure. However, since the earth does rotate, to an observer standing on the surface of the rotating earth there is an apparent deflection of air. The Coriolis force causes this deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis force is an apparent force due to the earth rotating under the moving air. Observed from space, this movement of air (or any freely moving object for that matter) would appear to follow a straight line. But to an observer on earth this movement appears to be deflected.

A demonstration of the Coriolis force is shown in Figure 3-2. Imagine a spinning turntable rotating around its center axis like the earth (Figure 3-2a). If you were to hold a ruler still and draw a straight line across the spinning turntable you would see a straight line from your vantage point. If the turntable were the earth, your vantage point would be space. However, the line you would draw on the turntable would actually be curved. So from the turntable's point of view, the line was deflected (Figure 3-2c).

Figure 3-2.The Coriolis force

(a) Let a spinning turntable represent the rotating Earth. (b) Draw a "straight" line

from the center to the edge, holding the ruler still. (c) Remove the ruler.

Your "straight" line on the moving turntable is curved.

This is the same thing that happens when the wind blows. This apparent force on the wind:

• Increases as wind speed increases

• Remains at right angles to wind direction (see Figure 3-3)

• Increases with an increase in latitude (i.e., force is greatest at the poles and zero at the equator)

The effect of this deflecting force is to make the wind seem to change direction on earth. Actually, the earth is moving with respect to the wind. As Figure 3-3 shows, winds appear to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Figure 3-3. The deflection of large-scale winds in the Northern and Southern Hemispheres

Pressure Gradient Force

Wind is caused by nature's attempt to correct differences in air pressure. Wind will flow from areas of high pressure to low pressure. The pressure equalizing force that attempts to move air from high pressure to low pressure is called the pressure gradient force.

North Pole Maximum deflection at pole

South Pole Maximum deflection at pole

Southern Hemisphere

Northern Hemisphere

Deflection to left

Deflection to right

No deflection at equator

60° S

30° S

0°

30° N

Latitude60° N

Equator

Direction of windviewed from space

Direction of wind viewed from Earth

LEGEND

The pressure gradient is the rate and direction of pressure change. It is represented by a line drawn at right angles to the isobars as shown in Figure 3-4. Gradients are steep where isobars are closely spaced. The wind will move faster across steep gradients. Winds are weaker where the isobars are farther apart because the slope between them is not as steep; therefore, wind does not build up as much force.

Figure 3-4. Pressure gradients

Figure 3-4 shows that wind moves from areas of high to low pressure, but because of the Coriolis force (effect of the earth's rotation), wind does not flow parallel to the pressure gradient. Also, notice that wind direction at the surface (solid lines) differs from wind direction high above the earth (dotted lines) despite the same pressure gradient forces operating. This is due to frictional forces as explained in the next section.

Friction

Friction, the third major force affecting the wind, comes into play near the earth's surface and continues to be a factor up to altitudes of about 500 to 1000 m. This section of the atmosphere is referred to as the Planetary or Atmospheric Boundary Layer. Above this layer, friction no longer influences the wind. The Coriolis force and the pressure gradient force are in balance above the Planetary Boundary Layer. As shown in Figure 3-5, the balanced forces that occur above the layer where friction influences the wind, create a wind that will blow parallel to the isobars. This is called the

High Low

PGF Steep

Upper air wind flow Surface wind flow

geostrophic wind. In the Northern Hemisphere low pressures will be to the left of the wind. The reverse is true in the Southern Hemisphere.

Figure 3-5. Balance of forces resulting in geostrophic wind (Northern Hemisphere)

Within the friction layer, the Coriolis force, pressure gradient force, and friction all exert an influence on the wind. The effect of friction on the wind increases as the wind approaches the earth's surface. Also, the rougher the surface of the earth is, the greater the frictional influence will be. For example, air flow over an urban area encounters more friction than air flowing over a large body of water.

Friction not only slows wind speed but also influences wind direction. Friction's effect on wind direction is due to the relationship between wind speed and the Coriolis force. Remember, the Coriolis force is proportional to wind speed. Consequently, as winds encounter more friction at progressively lower altitudes within the friction layer, wind speeds decrease and so does the Coriolis force. With friction, the Coriolis force lessens in relation to the pressure gradient force; the pressure gradient force no longer exactly balances the Coriolis force as it does with the geostrophic wind above the Planetary Boundary. Instead, the pressure gradient force predominates, turning the wind toward low pressure (see Figure 3-6). The wind direction turns toward low pressure until the resultant vector of the frictional force and the Coriolis force exactly balances the pressure gradient force. As frictional forces become greater, wind directions turn more sharply toward low pressure. This change in wind direction at different altitudes within the friction layer is depicted in Figure 3-7 and is referred to as the Ekman Spiral. The turning of the wind's direction lessens with height until friction no longer influences wind flow as in the case of the geostrophic wind.

High Pressure

Coriolis Force

Geostrophic Wind

Pressure Gradient Force

Low PressureP - 2

P - 1

P

P + 1

P + 2

Figure 3-6. The Coriolis force combines with friction to balance the horizontal pressure gradient force

High PressureCF

P

Low Pressure

1008 mb

F

V

1004 mb

P - Pressure gradient forceV - Wind velocityCF - Coriolis forceF - Friction

The lengths of the arrows are drawn proportionalto the magnifudes of the forces or velocity involved.

Figure 3-7. The Ekman spiral of wind in the Northern Hemisphere

The effect that friction has on wind has a profound influence on the transport of air pollutants. As a plume of air pollutants rises from a stack it will likely rise through the boundary layer of the atmosphere where friction changes the direction of the wind with height. This will spread the plume horizontally in slightly different directions. Also, pollutants released at different heights in the atmosphere may move in slightly different directions.

Pressure Systems The horizontal movement of air is directed by many forces. Surface winds are directed in a counterclockwise fashion around low pressure systems (cyclones) in the Northern Hemisphere. This same balance of forces directs air in a clockwise fashion around high pressure systems (anticyclones) in the Northern Hemisphere. The resulting air flow associated with pressure systems near the earth’s surface is shown in Figure 3-8. At upper levels of the atmosphere where frictional forces are removed, the air moves parallel to the isobars as shown in Figure 3-5.

Above atmospheric boundary layer

Ekman spiralSurface

Z

Low pressure

High pressure

Figure 3-8. Surface air flow around low and high pressure systems

Effects of Pacific High and Bermuda High on Air Pollution

The presence of semipermanent, subtropical anticyclones over the major oceans influence air pollution dispersion in several areas of the world including the continental United States. The Pacific High and Bermuda High are two such examples of large-scale, high pressure systems that affect air quality in southern California and the southeastern U.S. respectively. These high pressure systems are referred to as semipermanent because they shift position only slightly from summer to winter. They are formed from air sinking in the region above the horse latitudes (around 30° latitude). Cold, subsiding (sinking) air aloft is compressed and is heated as it sinks in these areas of high pressure, establishing an elevated temperature inversion. An elevated temperature inversion occurs when a layer of warm air resides over a layer of cooler air, thereby restricting the vertical movement of air. The bottom of this inversion layer generally approaches the surface the further one gets from the center of the anticyclone. For more information about inversions in general and subsidence inversions in particular, see Lesson 4.

Pacific High On the eastern side of these semipermanent anticyclones, the inversion layer is strengthened by the clockwise air flow around the pressure system which brings in air from the north. The air cools from contact with the cool ocean water. This condition plagues southern California which is located on the eastern side of the Pacific High. Temperature inversions, which limit vertical mixing of air pollutants, are common in this area. Thus air pollutants can build up in the shallow layer of the atmosphere under the inversion layer to dangerous levels.

Bermuda High On the western side of the semipermanent anticyclones the conditions are less severe. Clockwise motion of air results in wind flow from the southern tropical areas where the air is warm and moist. Subsiding air in these areas of

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L

high pressure can still lead to elevated temperature inversions, but the frequency and strength of these inversions are not as great as those influencing the western coasts of continents, due to the advection of warm air. This situation is typical of the southeastern United States where the Bermuda High, situated in the Atlantic Ocean, influences pollution transport and dispersion in this region.

General Circulation The general circulation represents the average air flow around the world. While winds at any particular time and place may vary widely from the average, studying the average wind flow patterns can help you to identify the predominant circulation patterns at certain latitudes and to understand the causes for these patterns. As you learned in lesson 2, the driving force behind general circulation is the uneven heating of the earth's surface. The equatorial regions receive much more energy from the sun than the polar regions do. Horizontal temperature variations in the atmosphere, caused by unequal heating, leads to pressure differences that drive atmospheric circulation.

Because the global circulation of air is complex, we will start with a simple model that explains how air would circulate without the complications caused by the earth's rotation and its non-uniform surface. If the earth were nonrotating and composed of a uniform solid surface, we would see a very predictable circulation pattern extending from the equator to the poles (see Figure 3-9). Air at the equator, which receives more of the sun's radiation, would be hotter than air at the poles. The air at the equator would be warm and buoyant and would rise due to convection. As the warm equatorial air rises, thunderstorms develop which release more heat, causing the air to continue to rise until it reaches the upper atmosphere. At this point, the air would begin to move toward the polar regions, cooling as it traveled. At the poles, the dense cold air would sink to the surface and flow back toward the equator. In the Northern Hemisphere, the air flow near the surface would always be out of the north because the cooler air from the North Pole would replace the warm air ascending at the equator.

Figure 3-9. Hypothetical planetary air circulation for nonrotating earth of uniform surface

But, the earth does rotate⎯a fact that changes this relatively simple air flow into a very complex situation. The Coriolis effect is a major factor that explains the actual air flow patterns around the world.

The following explanation for planetary air circulation takes the Coriolis force into account. At the equator, warm air rises and often condenses into huge thunderclouds and storms. Thus a band of low pressure develops around the equator. These thunderstorms liberate heat which drives the air higher in the atmosphere. Here, the air starts to travel laterally toward the poles, cooling as it moves. The air starts to converge or "come together" aloft near the 30o latitudes. The convergence of air causes air to sink or subside at this latitude. This leads to air divergence at the earth's surface. As air sinks in this region, skies are generally cloudless and surface winds are light and variable. The 30o latitudes are referred to as the horse latitudes because sailing ships traveling to the New World were often becalmed here. According to legend, as food and supplies diminished, horses were often eaten or thrown overboard in this region.

From the horse latitudes, some of the surface air moves back toward the equator. Because of the Coriolis effect, the winds blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. These steady winds are called the trade winds because they facilitated sailing ships on their voyages from Europe to America. As you can see in Figure 3-10, the trade winds converge around the equator in a region called the Intertropical Convergence Zone (ITCZ). This converging equatorial air heats and rises, continuing the cycle.

North Pole

South Pole

Figure 3-10. General atmospheric circulation cells

Instead of moving toward the equator, some surface air at the 30o latitudes moves toward the poles. The Coriolis force deflects these winds to the east in both hemispheres. These surface winds blow from west to east and are called the prevailing westerlies or westerlies in both hemispheres. Between 30 to 60o latitudes, traveling pressure systems and associated air masses (discussed later) help transport energy. Moist air from southerly regions are transported northerly. This moisture condenses, liberating energy that helps heat the air in the northerly latitudes.

In the areas between the 60o latitudes and the poles, the polar easterlies prevail. This easterly wind forms a zone of cold air that blows to the southwest (Northern Hemisphere) and to the northwest (Southern Hemisphere) until it encounters the warmer westerly winds. The interface between the polar easterlies and the westerlies is the polar front that moves as these two air masses push back and forth against each other. The polar front travels from west to east helping to move cold air southward and warm, moist air northward (N. Hemisphere), thereby bringing heat energy toward the polar regions. As warm, moist air characteristics of the westerlies push up and over the cold, drier easterlies, stormy weather develops. Therefore, clouds and precipitation typically accompany the polar front.

As shown in Figure 3-10, narrow bands of high speed winds, referred to as jet streams, develop where horizontal temperature differences are great. Although the jet stream varies in size and strength it is generally found between 7.6 and 12.2 km (25,000 and 40,000 ft) above the earth and has speeds typically between 129 and 193 km (80 and 120 mph) depending upon latitude and season. These high altitude winds affect surface winds as they help "steer" surface weather systems. Although the jet

N

S

0°

30°

60°

30°

60°

Intertropical Convergence Zone

Polar easterliesPolar jet

Subtropical jet Northeasttrade winds

Westerlies

stream generally runs east-west around the globe, it often dips north-south as it follows the boundary between warm and cold air.

Air Masses Air masses are macroscale phenomena, covering hundreds of thousands of square kilometers and extending upward for thousands of meters. They are relatively homogeneous volumes of air with regards to temperature and moisture, and they acquire the characteristics of the region over which they form and travel. The processes of radiation, convection, condensation, and evaporation condition the air in an air mass as it travels. Also, pollutants released into an air mass travel and disperse within the air mass. Air masses develop more commonly in some regions than in others. These areas of formation are known as source regions, and they determine the classification of the air mass. Air masses are classified as maritime or continental according to their origin over ocean or land, and as arctic, polar, or tropical depending principally on the latitude of origin. Table 3-1 summarizes air masses and their properties. Figure 3-11 shows typical trajectories of air masses into North America. The boundary between air masses with different characteristics is referred to as a front. A front is not a sharp wall but a zone of transition which is often several miles wide. Fronts are discussed later in this lesson.

Figure 3-11. Trajectories of air masses into North America

Table 3-1. Classification of air masses

Name Origin Properties Symbol Arctic Polar regions Low temperatures, low

specific but high summer relative humidity, the coldest of the winter air masses

A

Polar continental* Subpolar continental areas

Low temperatures (increasing with southward movement), low humidity, remaining constant

cP

Polar maritime Subpolar area and arctic region

Low temperatures, increasing with movement, higher humidity

mP

Tropical continental Subtropical high-pressure land areas

High temperatures, low moisture content

cT

Tropical maritime Southern borders of oceanic subtropical, high-pressure areas

Moderate high temperatures, high relative and specific humidity

mT

Note: The name of an air mass, such as Polar continental, can be reversed to continental Polar, but the symbol, cP, is the same for either name.

Temperature is a basic property of air masses. The temperature of an air mass depends on the region where it originates. Arctic air masses are the coldest and tropical air masses are the warmest.

Moisture is the second basic property in an air mass. Moisture plays such a significant role in weather and climate that it is commonly treated separately from the other constituents of air. In one or more of its forms, atmospheric moisture is a factor in humidity, cloudiness, precipitation, and visibility. Water vapor and clouds affect the transmission of radiation both to and from the earth's surface. Through the process of evaporation water vapor also conveys latent heat into the air, giving it a function in the heat exchange (as well as in the moisture exchange) between the earth and the atmosphere. Atmospheric water is gained by evaporation but lost by precipitation. Only a minute fraction of the earth's water is stored as clouds and vapor in the atmosphere at any one time. The net amount of water in the atmosphere at the end of any given period for a particular region is an algebraic summation of the amount stored from a previous period, the gain by evaporation, the gain or loss by horizontal transport, and the loss by precipitation. This relationship expresses the water balance of the atmosphere.

Fronts Four frontal patterns⎯warm, cold, occluded, and stationary⎯can be formed by air of different temperatures. The cold front (Figure 3-12) is a transition zone between warm and cold air where the cold air is moving in over the area previously occupied by warm air. Cold fronts generally have slopes from 1:50 to 1:150, meaning that for every kilometer of vertical distance covered by the front, there will be 50 to 150 km of horizontal distance covered. The rise of warm air over an advancing cold front and the subsequent expansive cooling of this air lead to cloud cover and precipitation following the position of the surface front. (The surface front is the location where the advancing front touches the ground.)

Figure 3-12. Advancing cold front

Warm fronts, on the other hand, separate advancing warm air from retreating cold air and have slopes on the order of 1:100 to 1:300 due to the effects of friction on the trailing edge of the front. Precipitation is commonly found in advance of a warm front, as can be seen in Figure 3-13.

6 km

600 0 km 1000

Surface front

Warm air

Cold air

Figure 3-13. Advancing warm front

When cold and warm fronts merge (the cold front overtaking the warm front) occluded fronts form (Figure 3-14). Occluded fronts can be called cold front or warm front occlusions, as shown in Figure 3-15. But, in either case, a colder air mass takes over an air mass that is not as cold.

Figure 3-14. Occluded front

Cold air

Cold airColder air

Warm air

Occluded front

6 km

6000 km1000

Warm airCold air

Figure 3-15. Cold and warm front occlusions

As either type of occluded front approaches, the clouds and precipitation resulting from the occluded front will be similar to those of a warm front (Figure 3-13). As the front passes, the clouds and precipitation will resemble those of a cold front (Figure 3-12). Therefore, it is often impossible to distinguish between the approach of a warm front and the approach of an occluded front. Regions with a predominance of occluded fronts have a great deal of low cloud cover, small amounts of precipitation, and small daily temperature changes.

The last type of front is the stationary front. As the name implies, the air masses around this front are not in motion. It will resemble the warm front in Figure 3-13 and will manifest similar weather conditions. Figure 3-16 shows how a stationary front is represented on a map. The abbreviations cP and mT stand for continental Polar and maritime Tropical air masses. A stationary front can cause bad weather conditions that persist for several days.

Figure 3-16. Stationary front

Cold air

Cold front occlusion

Colder air

Warm front occlusion

Warm air

Warm air

Cold airColder air

c P

m T

Migrating areas of high pressure (anticyclones) and low pressure (cyclones) and the fronts associated with the latter are responsible for the day-to-day changes in weather that occur over most of the mid-latitude regions of the earth. Mid-latitude low pressure systems form along frontal surfaces that separate air masses of different origin having different temperature and moisture characteristics. The formation of a low pressure system is accompanied by the formation of a wave on the front consisting of a warm front and a cold front, both moving around the low pressure system in a counterclockwise motion. This low pressure system is referred to as a cyclone. The life cycle of a typical cyclone is shown in Figure 3-17. As you recall, the triangles indicate cold fronts and the semicircles indicate warm fronts. The five stages depicted here are:

1. Beginning of cyclonic circulation

2. Warm sector well defined between fronts

3. Cold front overtaking warm front

4. Occlusion (merging of two fronts)

5. Dissipation

Figure 3-17. The life of a cyclone

Frontal Trapping Frontal systems are accompanied by inversions. Inversions occur whenever warm air rises over cold air and "traps" the cold air beneath. Within these inversions there is relatively little air motion, and the air becomes relatively stagnant. This frontal trapping may occur with either warm fronts or cold fronts. Since a warm front is usually slower moving than a cold front, and since its frontal surface slopes more gradually, trapping will generally be more important with a warm front. In addition, the low level and surface wind speeds ahead of a warm front (within the trapped sector) will usually be lower than the wind speeds behind a cold front. Most warm frontal trapping will occur to the west

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1)

3)L

5) H

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through north from a given pollutant source, and cold frontal trapping will occur to the east through south of the source.

Topographical Influences The physical characteristics of the earth's surface are referred to as terrain features or topography. Topographical features not only influence the way the earth and its surrounding air heat up, but they also affect the way air flows. Terrain features, as you would expect, predominantly affect air flow relatively close to the earth’s surface. As shown in Figure 3-18, these features can be grouped into four categories: flat, mountain/valley, land/water, and urban.

Figure 3-18. Topography

Topographical features affect the atmosphere in two ways as shown in Figure 3-19: thermally (through heating) and geometrically (also known as mechanically). The thermal turbulence is caused by differential heating. Different objects give off heat at different rates. For example, a grassy area will not absorb and subsequently release as much heat as an asphalt parking lot. Mechanical turbulence is caused by the wind flowing over different sizes and shapes of objects. For example, a building affects the wind flowing around it differently than a cornfield would affect it.

Figure 3-19. Topographical effects on heat and wind flow

Flat Terrain Although very little of the earth's surface is completely flat, some terrain is called flat for topographical purposes. Included in this category are oceans, even though they have a surface texture; and gently rolling features on land (Figure 3-20).

Figure 3-20. Flat terrain

Turbulence in the wind over flat terrain is limited to the amount of roughness of either natural or manmade features that are on the ground. Table 3-2 presents a listing of surface elements from smooth surface features with little frictional influence to rough features with a large frictional influence.

Table 3-2 Examples of different surface roughness (Listed in order of very smooth to very rough)

Mud Flats, Ice Smooth Sea Sand Plain, Snow Covered Mown Grass Low Grass, Steppe Flat & Fallow Ground High Grass Beets Palmetto Low Woods High Woods Suburbia City

These features induce a frictional effect on the wind speed and result in the well-known wind profile with height (Figure 3-21). Figure 3-21 shows that wind speed increases with altitude for each of the three types of terrains represented. Urban settings with dense construction and tall buildings exert a strong frictional force on the wind causing it to slow down, change direction, and become more turbulent. Therefore, gradient winds (i.e. those not affected by friction) are reached at higher altitudes above urban areas than above level terrain. Figure 3-21. Examples of variation of wind with height over different surface

roughness elements (Figures are percentages of gradient wind.) Source: Turner 1970.

Thermal turbulence over flat terrain is due to natural or manmade features. For example, water does not heat very quickly during the day but concrete heats exceptionally well. The concrete then releases large amounts of heat back into the air at night; water does not. Air rises over heated objects in varying amounts (Figure 3-22). As you learned in lesson 2, rising air is called convection.

Figure 3-22. Differential heating

Wind Speed (m/sec)

Urban area Suburbs Level countryGradient wind

Gradient wind

Gradient wind

0 5 10 0 5 10 0 5 10

600

500

400

300

200

100

030

40

61

51

68

75

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9094

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4556

68

77

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86

7865

Mountain/Valley The second type is mountain/valley terrain. This combination shown in Figure 3-23 is also called complex terrain.

Figure 3-23. Mountain/valley complex terrain

All air pollution investigators agree that atmospheric dispersion in complex terrain areas can be very different from, and much more complicated than, that over flat ground. The effects of complex terrain on atmospheric dispersion have been investigated in fluid modeling labs and by field experiments.

Mechanical turbulence over mountain/valley terrain is invariably connected to the size, shape, and orientation of the features. The numerous combinations of mountain/valley arrangements include a single mountain on flat terrain, a deep valley between mountains, a valley in flat terrain, or a mountain range. However, as in Figure 3-24, air tends to flow up and over an obstacle in its path with some air trying to find its way around the sides. If an elevated temperature inversion (warm air overlying cooler air) caps the higher elevation, then the air must try to find its way around the sides of the mountain. If the air flow is blocked, then trapping or recirculation of the air occurs. At night, hills and mountains induce downslope wind flow because the air is cooler at higher elevations. Usually downslope winds are light. However, under the right conditions, much faster wind speeds may result.

Figure 3-24. Wind flow over and around mountains (mechanical turbulence)

Thermal turbulence in mountain/valley terrain is also connected to the size, shape, and orientation of the features. Again, while every combination of mountain/valley effects cannot be explained, some generalizations can be illustrated. Mountain/valleys heat unevenly because of the sun's motion across the sky (Figure 3-25). In the morning, one side of a mountain or valley is lit and heated by the sun. The other side is still dark and cool. Air rises on the lighted side and descends on the dark side. At midday both sides are "seen" by the sun and are heated. The late afternoon situation is similar to the morning. After dark, as the air cools due to radiational cooling, the air drains down into the valley from all higher slopes.

Figure 3-26 depicts upslope and downslope winds that occur during the day and night respectively. In a true valley situation, downslope winds can occur on opposite slopes of the valley causing cool, dense air to accumulate or pool on the valley floor. This cooler air can move down the valley resulting in air movement due to cold air drainage. Also, since the cooler air drains to the valley floor the air aloft is warmer. This results in a temperature inversion which restricts vertical transport of air pollutants (discussed in Lesson 4).

Figure 3-25. Thermal turbulence in valley (air rises when land is lighted)

Figure 3-26. Diurnal variations in mountain/valley flow due to solar heating

Also, the winds in a valley location are channeled by the shape of the valley. Winds predominately blow up valley or down valley. This can lead to high ground level air pollutant concentrations since variations in wind direction are restricted by the geometry of the valley.

The other heating effect is due to land features. Tree covered areas will heat less than rocky slopes or bare ground. A detailed knowledge of specific terrain areas is important to interpret the complex terrain's effect.

Upslope wind(Daytime)

Downslope wind(Night time)

LightDark

AirAir

p.m.

AirAir

noon

Light

Air Air

a.m.

Light Dark

Land/Water The third type of terrain is a land/water interface (Figure 3-27). Partly because of convenience, a number of large cities are located next to bodies of water. The land and water not only exhibit different roughness characteristics but different heating properties. The air flow and thus plume dispersion and transport can be very difficult to predict.

Figure 3-27. Thermal turbulence at land/water interface

The thermal properties of land and water are radically different. Land and objects on it will heat and cool relatively quickly. However, water heats and cools relatively slowly. Water temperatures do not vary much from day-to-day or from week-to-week. Water temperatures follow the seasonal changes, being delayed by as much as 60 days. For example, the warmest ocean temperatures are in late summer to early fall, and the coolest ocean temperatures are in late winter to early spring.

As the sun shines down on the land/water interface, solar radiation will penetrate several feet through the water. On the other hand solar radiation striking land will only heat the first few inches. Also, as the sun shines on the water surface, evaporation and some warming take place. The thin layer of water next to the air cools due to evaporation and mixes downward, overturning with the small surface layer that has warmed. This mixing of the water layer close to the surface keeps the water temperature relatively constant. On the other hand, land surfaces warm quickly, causing the adjacent air to heat up, become less dense, and rise. The cooler air over the water is drawn inland and becomes the well-known sea breeze (Figure 3-28). At night, the air over the land cools rapidly due to radiational cooling, which causes the land temperature to fall faster than that of the adjacent water body. This creates a return flow called the land breeze (Figure 3-29). The wind speeds in a land breeze are light; whereas the wind speeds in a sea and land breeze can be quite fast. Differential pressure over land and water causes sea breezes. With sea breezes

Sea

Heated land

(during the day), the pressure over heated land is lower relative to the pressure over the cooler water. With land breezes (during the night), the reverse is true.

Figure 3-28. Sea breeze due to differential heating

Sea

Heated land

Lowpressure High

pressure

Figure 3-29. Land breeze due to differential heating

The roughness features of land and water are also different (Figure 3-30). The water appears to be quite smooth to the flow of air. As the wind speed increases, the water surface is disturbed, and waves form. With waves induced by strong wind the water surface is no longer as smooth as it was with a light wind. However, water is still smoother than most land features. Because of the change from relatively smooth water to rougher land, the air flow changes direction with the increased frictional influence (increased turbulence). The amount of direction change depends on the amount of roughness change.

Figure 3-30. Mechanical turbulence at land/water interface

W arm erSea

Cooler land

Lowpressure

Highpressure

Urban Urban areas have added roughness features and different thermal characteristics due to the presence of man-made elements. The thermal influence dominates the influence of the frictional components (Figure 3-31). Building materials such as brick and concrete absorb and hold heat more efficiently than soil and vegetation found in rural areas. After the sun sets, the urban area continues to radiate heat from buildings, paved surfaces, etc. Air warmed by this urban complex rises to create a dome over the city. It is called the heat island effect. The city emits heat all night. Just when the urban area begins to cool, the sun rises and begins to heat the urban complex again. Generally, city areas never revert to stable conditions because of the continual heating that occurs.

Fig 3-31. Thermal and mechanical turbulence of cities

The mechanical turbulence over urban areas is much like complex terrain. The buildings, separately and collectively, alter the air flow: the larger the buildings are, the more the air is distributed. Also, the street areas channel and direct the flow in intricate ways. Just as the details of flow in mountain/valley terrain could not be accurately predicted, the flow in urban areas defies accurate description.

Review Exercise 1. Heating air ____________________ its pressure.

a. Increases b. Decreases

2. Lines that represent points of equal pressure are called ____________________.

3. Since the earth rotates, the Coriolis force makes the wind appear to turn to the ____________________ in the Northern Hemisphere. a. Right b. Left

4. In reality, with respect to the Coriolis force, the wind follows a ____________________ path while the earth rotates underneath. a. Curved b. Straight

5. Strong winds are associated with ____________________ spaced isobars.

6. The steepness between isobars is reflective of the ____________________ ____________________.

7. The geostrophic wind: a. Occurs above the Planetary Boundary Layer b. Blows perpendicular to the isobars c. Is influenced by friction d. a and b, only e. a, b, and c

8. The section of the atmosphere closest to the earth’s surface where friction influences the wind is called the ____________________ ____________________ Layer.

9. Which of the following statements about the relationship between friction and the Coriolis effect is correct? a. As friction increases, the Coriolis effect on wind direction decreases. b. As friction increases, the Coriolis effect on wind direction increases. c. There is no relationship between friction and the Coriolis effect on wind.

10. The change in wind direction at different altitudes within the friction layer is referred to as the ____________________ ____________________ .

11. In the Northern Hemisphere, air flow around a cyclone is ____________________. a. Clockwise b. Counterclockwise

12. The surface airflow in a low-pressure area ____________________. a. Converges b. Diverges

13. True or False? Converging air at the equator is referred to as the Intertropical Convergence Zone. a. True b. False

14. Bands of high velocity winds in the upper atmosphere are referred to as ____________________ ____________________.

15. Fronts generally separate ____________________ ____________________.

16. The uniformity of an air mass is based on two physical properties. What are they? ________________________________________ ________________________________________

17. Air masses are named by their source regions based on their origin over ____________________ or ____________________ and their ____________________.

18. List two land-based air masses. ________________________________________ ________________________________________

19. ____________________ fronts approach as a sharp wedge of air. a. Warm b. Cold c. Occluded

20. ____________________ fronts separate advancing warm air from retreating cold air. a. Warm b. Cold c. Occluded

21. Generally, ____________________ fronts have a cloud cover and precipitation following the position of the surface front. a. Warm b. Cold

22. Precipitation is generally found in advance of a(an) ____________________ front. a. Warm b. Occluded c. Stationary d. Warm, occluded, or stationary

23. Match the following symbols with the fronts they denote.

• Occluded • Warm • Stationary • Cold

24. True or False? Frontal trapping is usually worse with warm fronts than cold fronts because warm fronts usually move slower and their surfaces slope more gradually. a. True b. False

25. True or False? Gradient winds are not affected by friction. a. True b. False

26. True or False? Urban areas have added roughness features due to the presence of manmade features. a. True b. False

a) b) c) d) - occluded

- warm

- stationary

- cold

Review Exercise Answers 1. a. Increases

Heating air increases its pressure.

2. Isobars Lines that represent points of equal pressure are called isobars.

3. a. Right Since the earth rotates, the Coriolis force makes the wind appear to turn to the right in the Northern Hemisphere.

4. b. Straight In reality, with respect to the Coriolis force, the wind follows a straight path while the earth rotates underneath.

5. closely Strong winds are associated with closely spaced isobars.

6. Pressure gradient The steepness between isobars is reflective of the pressure gradient.

7. a. Occurs above the Planetary Boundary Layer The geostrophic wind occurs above the Planetary Boundary Layer. It blows parallel to the isobars.

8. Planetary Boundary (or Atmospheric Boundary) The section of the atmosphere closest to the earth’s surface where friction influences the wind is called the Planetary or Atmospheric Boundary Layer.

9. a. As friction increases, the Coriolis effect on wind direction decreases. As friction increases, wind speed decreases. Since the Coriolis force is proportional to wind speed, the Coriolis effect on wind direction decreases as wind speed decreases.

10. Ekman Spiral The change in wind direction at different altitudes within the friction layer is referred to as the Ekman Spiral.

11. b. Counterclockwise In the Northern Hemisphere, air flow around a cyclone is counterclockwise.

12. a. Converges The surface airflow in a low-pressure area converges.

13. a. True

Converging air at the equator is referred to as the Intertropical Convergence Zone.

14. Jet streams Bands of high velocity winds in the upper atmosphere are referred to as jet streams.

15. Air masses Fronts generally separate air masses.

16. Temperature Moisture The uniformity of an air mass is based on two physical properties: temperature and moisture content.

17. Land Sea Latitude Air masses are named by their source regions based on their origin over land or sea and their latitude.

18. Continental Polar Continental Tropical Two land-based air masses are continental Polar (cP) and continental Tropical (cT).

19. b. Cold Cold fronts approach as a sharp wedge of air.

20. a. Warm Warm fronts separate advancing warm air from retreating cold air.

21. b. Cold Generally, cold fronts have a cloud cover and precipitation following the position of the surface front.

22. d. Warm, occluded, or stationary Precipitation is generally found in advance of warm, occluded, or stationary fronts.

23. a. Cold b. Warm c. Occluded d. Stationary

24. a. True Frontal trapping is usually worse with warm fronts than cold fronts because warm fronts usually move slower and their surfaces slope more gradually.

25. a. True Gradient winds are not affected by friction.

26. a. True Urban areas have added roughness features due to the presence of manmade features.