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1. 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

2. 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).
1 0 1 6 m b
1 0 2 0 m b
L 1 0 2 4 m b
H
1 0 1 2 m b
1 0 0 8 m b
Figure 3-1. Isobars around areas of low and high pressure

3. 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.

4. 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.
North Pole Maximum deflection at pole
Northern
Hemisphere
Equator
Southern
Hemisphere
Deflection
to right
Latitude
60° N
No deflection at equator
Deflection
to left
60° S
30° N
0°
30° S
South Pole Maximum deflection at pole
Direction of wind
viewed from space
Direction of wind
viewed from Earth
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.
LEGEND

5. 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.
Upper air wind flow
Surface wind flow
High Low
PGF Steep
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

6. geostrophic wind. In the Northern Hemisphere low pressures will be to the
left of the wind. The reverse is true in the Southern Hemisphere.
Low Pressure
Pressure Gradient Force
Geostrophic Wind
Coriolis Force
High Pressure
P - 2
P - 1
P
P + 1
P + 2
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.

7. Low Pressure
CF
P
High Pressure
Figure 3-6. The Coriolis force combines with friction to balance the
horizontal pressure gradient force
1008 mb
F
V
1004 mb
P - Pressure gradient force
V - Wind velocity
CF - Coriolis force
F - Friction
The lengths of the arrows are drawn proportional
to the magnifudes of the forces or velocity involved.

8. Above atmospheric
boundary layer
Z
Low pressure
Surface Ekman spiral
High pressure
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.

9. H
L
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

10. 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.

11. North Pole
South Pole
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.

12. N
60°
Westerlies
Subtropical jet Northeast
30°
0°
Intertropical Convergence Zone
30°
60°
S
Polar easterlies
Polar jet
Figure 3-10. General atmospheric circulation cells
trade winds
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

13. 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

14. 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.

15. 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.)
Warm air
Surface front
Cold air
600 0 km 1000
Figure 3-12. Advancing cold front
6 km
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.

16. Cold air Warm 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 air
Colder air
Warm air
Occluded front
6 km
1000 0 km 600

17. Colder air Cold air
Cold air
Cold front occlusion
Warm air
Warm front occlusion
Warm air
Colder 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.
c P
m T
Figure 3-16. Stationary front

18. 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
5) H
Figure 3-17. The life of a cyclone
L
H
1)
3)
L
Frontal Trapping
H
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

19. 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

20. 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

21. Urban area Suburbs Level country
Gradient wind
Gradient wind
56
95
90
77
Wind Speed (m/sec)
40
90
85
75
68
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
Gradient wind
0 5 10 0 5 10 0 5 10
600
500
400
300
200
100
0
30
61
51
80
94
98
45
68
84
95
91
86
78
65

22. 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.

23. 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).

24. Air
Air
p.m.
Dark Light
Air
Air
noon
Light
a.m.
Air Air
Light Dark
Figure 3-25. Thermal turbulence in valley (air rises when land is lighted)
Upslope wind
(Daytime)
Downslope wind
(Night time)
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.

25. 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.
Sea
Heated land
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

26. (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.
Low
pressure High
Sea
Heated land
pressure
Figure 3-28. Sea breeze due to differential heating

27. Warmer
Sea
High
pressure
Cooler land
Low
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

28. 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.

29. 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

30. 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

31. 23. Match the following symbols with the fronts they denote.
a) b) c) d) - occluded
• 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
- warm
- stationary
- cold

32. 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

33. 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.

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