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Chapter 7 Global Scale Winds
Mariners of centuries ago were well aware of geographic regions that lacked winds and those
regions with prevailing winds good for sailing. The route of Christopher Columbus' first voyage across the
Atlantic took advantage of prevailing wind directions. The Portuguese used steady northeast winds to sail
to Africa more than 50 years before Christopher Columbus set sail on his journey. Columbus used these
winds at the start of his voyage by sailing to the Canary Islands and then across the Atlantic. These ocean
goers were also aware of the existence of westerly winds to the west of Portugal. They consistently
experienced this wind pattern regardless of season. Mariners attempted to sail west from the Azores but
were confronted by westerly winds-winds coming from the west blowing toward the east. Attempts to sail
west from the Azores confronted mariners with westerly windswinds coming from the west blowing
toward the east. On his return voyage Columbus first sailed northward and then used these westerly winds
in the middle latitudes to return to Spain.
Nearly 3 and 1/2 centuries after Columbus's first voyage, the HMS Beagle sailed around the world
making scientific observations. Throughout its five-year voyage, which carried Charles Darwin, the HMS
Beagle took advantage of the prevailing wind directions to circle the world. The crossings of the Pacific
and Indian Ocean occurred around 20 S where the winds are steady from the southeast direction.
Both voyages relied on the power of the wind and so avoided the subtropical regions in the
vicinity of 30N and 30S. Mariners knew the winds in these regions where light or calm over large regions
of the subtropical oceans. Each of these regions is referred to as the
horse latitudes, so named because
horses where thrown overboard when vessels were becalmed in this region and the supply of drinking
water was low.
Centuries before Europeans were sailing to the “New World” the mid-Pacific islands (Polynesia,
Melanesia Micronesia) were being settled by seafarers from these islands. Capable of piloting much of the
Pacific Ocean, the skilled navigators were undoubtedly aware of steady northeasterly winds in the Pacific
between the Tropic of Cancer and the equator as well as the southeasterly winds between the equator and
Tropic of Capricorn.
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Using observations of global scale wind patterns, this chapter develops a conceptual model of
global scale winds, and explains the location of the world’s largest deserts, tropical rain patterns and the
existence of the jet stream.
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(Combine these two figures?)
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Why know features and conceptual models
A simple stick figure of a face is not very sophisticated (Figure 7.1). Yet it
symbolically demonstrates the major features of a human face: two eyes and ears, a nose,
a mouth, lips, hair, eyebrows, and chin. When you are describing a friend to someone
you give details about those features. “He has brown eyes, curly yellow hair, a big nose,
and small ears”, may not give a very complete picture of the person, but you might be
able to recognize the person at a crowded party. It is the same with describing the
atmosphere. With a general description of current atmospheric features you might be able
to recognize certain weather patterns.
Children draw simple faces based on observations. They observe that the nose
lies above the mouth and between the eyes. Eyebrows appear above the eyes and ears are
on the side. To make a simple, conceptual model of the global wind patterns requires
observations. We will rely on our own observations, historic records, and data from
satellites to identify the major features of a stick model that represents the atmospheric
circulation.
While all faces have the same basic features, few people look identical. It is the
same with the atmosphere. You can identify many of the basic features of the atmosphere
on most days, but you will not find two days with identical patterns. A web exercise is
designed to help you identify these atmospheric flow features on satellite images. Go
practice!
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Observations
A simple model of the global wind pattern must be able to explain the steady
winds that have long been observed by mariners.
A model of global atmospheric
circulation must also account for regions that consistently lack winds. A simple model of
global scale winds must also consistent with the position of deserts and regions of high
precipitation (Figure 7.2).
Global cloudiness is best achieved from the perspective of a satellite. Figure 7.3
is a general cloud pattern as observed from a series of satellites. There are several distinct
patterns in these images. Note the lack of clouds near the horse latitudes, consistent with
the precipitation maps. A band of thunderstorms extends around the world near the
equator. Tracking this cloud belt throughout the year, we find that when it is summer in
the Northern Hemisphere, this convective band is located at approximately 8N. The
average position of this cloud band is approximately 4S when the Southern Hemisphere
experiences summer. The average precipitation maps also depict these patterns in January
and July. The midlatitude cloud patterns have distinctly different characteristics from the
cloud systems we can observe in the tropics. The middle latitude cloud systems are more
transient and are further poleward in the summer hemisphere than the winter. In both
summer and winter, the middle latitude cloud systems tend to move from west to east.
You have probably observed this movement on TV weather reports.
Today’s weather reports often discuss the position of the jet stream. If you listen
to these reports carefully, and often enough, you will
realize there is more than one jet stream. Our conceptual
Jet stream is a narrow stream of
relatively strong winds (i.e. wind
speeds greater than 50 knots).
model will predict the existence of two of these jet streams. Both are fast moving
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currents of air that flow from west to east. The term jet stream is used to describe these
winds because of how they were discovered.
The existence of a jet stream moving from west to east, previously suspected
because of the movement of storm and cloud systems. But it remained unproven, until it
was confirmed during World War II, as the United State prepared for major air raids
against Japan.
The B-29s flew at an altitude of 30,000 to 35,000 feet where they
encountered a strong stream of westerly winds, the speeds of which were comparable to
the air speed of the jet planes of the time. This jet stream slowed the forward progress of
the planes as they flew towards the west. Our model must also be capable of explaining
the existence of these fast moving air currents. Observations of storm patterns in the
winter reveal that winter storms in the mid-latitudes move from west to east. These
storms are often accompanied by the jet stream. The simple model must describe this
observation as well. All of these observations are summarized in Figure 7.4
Simple Conceptual Model of Global Circulation Patterns
In this section we will develop a conceptual model of the global wind patterns.
Scientists often develop conceptual models to explain observations.
The beauty of
developing a model of the atmosphere's dynamics is that it allows us to identify some
important features of the general circulation of the atmosphere. Initially we will assume
Earth is covered with water. That is, we will assume there is no land. With no land
present we can assume that what happens in one hemisphere happens in the other. We
can neglect variations in circulation patterns that land masses cause. While this water
world is unrealistic, it will explain the basic observations we have discussed.
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Let’s begin our conceptual model by explaining the cloud band that nearly
encircles the tropics. These are convective clouds that require upward vertical motions.
Since this cloud band is nearly always present, there must be steady upward motions near
the equator. So, our conceptual model has upward vertical motions near the equator
(Figure7.5). This rising air produces cloud and precipitation patterns of the tropics.
Eventually, the rising air encounters the stable stratosphere and spreads northward and
southward along the tropopause. Let’s follow the northward branch of air.
As the air flows northward, the Coriolis force turns it to the right (in this case
eastward).
As you remember from Chapter 6, the magnitude of the Coriolis force
increases the further north the air flows. When this upper-air stream reaches 30N, the
Coriolis force has turned the flow causing the air to move from west to east, a westerly
wind. As the air move s poleward, it gets closer to the Earth’s axis of spin (Figure 7.7).
This brings us to a concept referred to as conservation of angular momentum. Before
going on we need to understand how conservation of angular momentum works.
A rotating body has angular momentum,
Conservation of Angular
Momentum requires a spinning body
defined as the product of mass times the rotation velocity to conserve its angular momentum.
times the perpendicular distance from the axis of rotation. An undisturbed rotating object
will conserve its angular momentum. To conserve angular momentum, skaters and divers
spin faster as their arms and legs are moved closer to the axis of rotation (Figure 7.6). As
a parcel of air moves north or south from the equator, its distance from the Earth’s spin
axis decreases. The parcel’s angular velocity must increase to conserve angular
momentum. This means that the air must travel faster as it moves poleward. Back to our
model.
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The ascending air flows poleward after reaching the tropopause. Deflected by the
Coriolis force on its poleward trek it becomes a
westerly wind in the upper troposphere. To conserve
Subtropical Jet is a jet stream
typically found between 20 and 30
latitude.
angular momentum the air is flowing rapidly from west to east, at approximately 70
knots. This stream of air is referred to as the subtropical jet stream (Figure 7.8). In our
model, the subtropical jet stream is flowing from west to east. The Coriolis force has
turned this upper tropospheric air at 30N so that it is circling the globe. As air continues
to flow into the subtropical jet stream region, some of the air must sink—air molecules
cannot continue to pile into this region forever. Since the air is near the tropopause, the
stable stratosphere suppresses upward motions, so in our model the air sinks toward the
surface. Sinking air suppresses cloud development and precipitation. It is therefore
logical that this sinking air is above our current desert locations and the horse latitudes
(see Figure 7.5), about 30 North and South latitude. Stratus clouds often appear over
oceans below this sinking air, as discussed in Box 8.1. Once the subsiding air reaches the
surface, it spreads north and south. Let’s follow the air near the surface that is flowing
towards the equator.
As the air flows south towards the equator, the
Coriolis force pulls it to the right (west) (to the left in the
Southern Hemisphere). The Coriolis force weakens as
the air approaches the equator, resulting in a northeast
wind in the Northern Hemisphere and a southeast wind
Trade winds are steady winds that
occupy most of the tropics and blow
from the subtropical highs. They are
northeasterly in the Northern
Hemisphere and southeasterly in the
Southern Hemisphere. The dividing
line between the two trade winds is
the Intertropical Convergence Zone
(ITCZ).
in the Southern Hemisphere. This is the wind the traders
sought in traveling from Europe to the Americas. These steady winds are called the trade
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winds. Notice how the trade winds from the two hemispheres converge, supplying moist
air for cloud development. The region where the two trade winds converge is called the
Intertropical Convergence Zone (ITCZ) (see Figure 7.9). In this conceptual model there
is a circulation cycle extending from the equatorial to subtropic regions. Air flows
upward at the ITCZ, spreads poleward at the tropopause to form a jet stream, in the
vicinity of which the air subsides. When the air reaches the surface, some of the air flows
equatorwards and converges into the ITCZ (Figure 7.5). This circulation cell is called the
Hadley Cell. George Hadley was the first person to propose a reasonable explanation of
the trade winds, though he assumed the descending branch occurred over each pole and
not the subtropical deserts!
Our conceptual model predicts regions of
relatively high atmospheric pressure near the descending
branch of the Hadley Cell. Here's why. Imagine a ring of
Hadley Cell is comprised of rising
winds near the equator, poleward flow
in the upper troposphere, subsidence
over the deserts, and the trade winds.
It is an important feature of the global
winds.
air moving poleward from the equator. If this ring stays
at the same altitude from the Earth’s surface, it gets smaller as it goes poleward (Figure
7.10). Air moving poleward is therefore ‘squeezed’ or is converging. We anticipate,
since air is converging above, to find high pressure at the surface in the vicinity of 30N
and 30S. Figure 7.11 is the average sea level pressure observed over the globe for
January and July. Notice the high pressures at the surface near 30N and 30S, particularly
over the oceans, confirming our model. These semi-permanent pressure systems are
referred to as Subtropical Highs. These subtropical highs, or anticyclones, have a major
influence on the weather and climate of the subtropics and mid-latitudes. The pressure
gradients are weak around the subtropical highs and hence the surface winds are light or
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calm over large regions of the subtropical oceans, the
horse latitudes, consistent with the observations of
Subtropical Highs are high surface
pressures observed over the ocean.
They are semi-permanent in their
location in that they shift with the
seasons.
mariners!
Air near the surface flows outward from the subtropical highs (Chapter 6). The
trade winds are thus equatorward of the subtropical high. Trade winds exist in both
hemispheres and converge into the ITCZ. In equatorial regions where neither trade wind
dominates, the wind is calm. These wind regions are know as the doldrums, crews of
sailing ships dreaded the doldrums because of the light winds, hot temperatures, and high
relative humidity.
Let's turn our attention to the Polar Regions, where more energy is emitted to
space then is gained from solar radiation. Subsidence over the poles warms the polar air
and suppresses precipitation. Subsidence is in agreement with the small amount of
precipitation observed near the poles (Figure 7.2), and provides a means of offsetting the
radiation energy losses. When the sinking air over the poles reaches the ground it flows
equatorward. People living in the midlatitude regions have probably experienced air
flowing out of the arctic during winter! These cold arctic outbreaks are accompanied by
cold and sometimes hazardous frigid weather.
Since this air is flowing southward from northern polar regions, the Coriolis force
acts to turn these winds forming polar easterlies at the surface poleward of 60 (Figure
7.5). Eventually, air must rise and flow poleward to replace the sinking air over the
poles, completing the air circulating through the polar cell (Figure 7.5). The sea level
pressure maps show that the polar easterlies are well developed in the winter and over
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continents. Eventually, as these winds flow equatorward, they are met by subtropical air
flowing poleward.
On the poleward sides of the descending branch of the Hadley Cell, surface air
moves poleward (see Figure 7.5). The Coriolis force, which increases in magnitude as
the poles are approached, moves the surface wind into a westerly direction in both
hemispheres. Our conceptual model produces midlatitude westerlies at the surface,
which on average blow from the southwest in the Northern Hemisphere. The midlatitude
westerlies are consistent with the sea level pressure maps (Figure 7.11), validating our
conceptual model. These southwesterly winds encounter the polar easterlies around
60N. The confrontation location separates warm tropical air from cold polar air and is
referred to as the polar front. Along the polar front region, warm air rises over the
colder air, producing clouds and precipitation consistent with the weather associated with
fronts.
The approximate location of the polar front is
indicated in Figure 7.12. Air pressure decreases more
Polar Front Jet is a jet stream found
in the middle and upper latitudes in
association with the polar front.
rapidly with altitude in the cold polar air mass than in the warmer tropical air. A pressure
gradient force is thus generated between cold and warm masses of air, with relative lower
pressure over the cold air. The difference between the air pressure in the warm and cold
air increases with altitude above the surface. The horizontal pressure difference generates
a pressure gradient force that accelerates the air from the region of higher pressure
towards the lower pressure.
The Coriolis force eventually balances the horizontal
pressure gradient (Chapter 7) force resulting in a strong stream of air that flows from the
west towards the east in the upper troposphere in the vicinity of the polar front (Figure
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7.12). This jet stream of air is referred to as the polar front jet. The polar front jet
stream is displaced further poleward than the subtropical jet stream (Figure 7.13).
Our conceptual model explains important circulation features of the atmosphere:
the ITCZ, the subtropical jet stream, the subtropical deserts, the trade winds, midlatitude
fronts and the polar front jet! This model is a simplification of actual global circulation.
In reality there are landforms, which tend to disrupt our simple model. In addition,
neither the polar front jet nor the subtropical jet flows directly west to east. Both meander
like a river producing a wavelike pattern of troughs and ridges.
These upper-air
westerlies are important for midlatitude weather. The next section gives a description of
these meandering winds. They are also discussed in more detail in Chapter 9.
The conceptual model includes of westerly winds in the midlatitude regions of the
Northern and Southern Hemisphere. These winds are referred to as the midlatitude
westerlies, or sometimes the westerlies. At the Earth's surface the west-to-east flow of
air occurs between latitudes of approximately 35 and 65. In the upper atmosphere the
midlatitude westerlies extend further poleward.
Upper-Air Midlatitude Westerlies
The winds in the upper troposphere, above approximately 500 mb, flow in wavelike patterns with troughs and ridges (Figure 7.14). The airflow through these upper level
waves results in storms and moves warm air poleward, and cold air towards the equator.
The winds have a clockwise curvature in the ridges and a counter-clockwise curvature in
the troughs. The Northern Hemisphere is typically encircled by two to five (Figure 7.14
show three) of these waves at any particular time. These long waves are called Rossby
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waves, named after Carl-Gustav Rossby, the meteorologist who discovered them. Rossby
waves typically drift slowly eastward. As discussed in Chapter 6, rising motions tend to
occur downstream of the wave troughs, with sinking motions upstream. The movement
of these waves is very important in determining the development of surface weather
systems, so understanding these waves is central to weather prediction as
Waves are described by their wavelength (distance between troughs or ridges) and
amplitude (north-south extent). Their amplitude and wavelength determine the type of
weather associated with these waves.
In winter the waves tend to have a larger
wavelength and larger amplitude because of the large temperature difference between the
equator and pole. In the summer, the temperature difference between the pole and
equator is reduced, weakening the north-south pressure gradient and the amplitude of the
westerly waves.
North-south air flow is referred to as meridional
wind while the west-east flow is called a zonal wind. A
zonal flow pattern indicates the winds are flowing
Index cycle describes the Rossby
wave patterns. A low index is a
meridional type pattern and indicates a
slow moving pattern while a high
index has zonal flow with small northsouth movements of weather systems.
primarily from west to east, nearly parallel to the lines of
constant latitude (Figure 7.15a). In this type of small amplitude flow pattern, cold air
masses tend to stay towards the polar regions and the warm air remains to the south. A
meridional flow pattern occurs when the waves have large amplitude, with deep troughs
and peaked ridges (Figure 7.15b). In a meridional flow pattern cold air flows towards the
subtropics and warm air flows poleward.
While these are but two types of flow patterns, different configurations are also
possible. A split flow pattern can occur where zonal flow exists near the pole with a
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meridional flow pattern further to the south. Sometimes the meridional pattern becomes
so strong that masses of air separate and become cutoff from the main westerly air flow.
Cut-off lows or cut-off highs (Figure 7.15c) represent large pools of cold or warm air that
block the eastward progression of weather systems. These blocking patterns can persist
for extended periods of time and can result in extreme weather events such as floods and
droughts.
A blocking pattern contributed to the summer flooding of the Midwest and
drought in the southeast United States in 1993. This blocking pattern had a cold pool of
air stalled over the Northern Rocky Mountains and Pacific Northwest. In another year, a
different blocking pattern had warm, high pressure system over the central US with
troughs over the east and west coasts. This particular blocking pattern contributed to the
spring and summer drought in the Midwest, the Northeast, and the Great Plains in 1988
(Figure 7.16).
Super-imposed on the Rossby long-waves are ripples referred to as short-waves
(Figure 7.17). Short-waves travel rapidly through the longer Rossby waves.
The
shortwaves travel along with the Rossby waves and are sometimes difficult to observe
and it is therefore difficult to predict their position and the associated weather. Both
short-waves and long-waves are needed for the development of storms.
Careful
observations of the upper-air pattern and the location of fronts at the surface indicate that
regions to the east of upper-level troughs appear favorable for the development of winter
storms. Clear sky conditions and surface high-pressures are often observed to the west of
the upper-level troughs (Figure 7.18). Chapter 6 discussed the importance of divergence
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in the upper atmosphere in storm development. Divergence must exist to the east of a
trough in the upper atmosphere.
A common flow pattern that results in divergence to the east of a trough is shown
in the 200 mb chart of Figure 7.18. The contours are lines of constant height of the
altitude at which the atmospheric pressure equals 200 mb. Air flowing towards the
trough converges, just like the two constant height lines converge. Air flow to the east of
the trough diverges. You can envision this divergence the next time you are in a pool.
Put your forearms together with your palms touching each other, as well as your elbows.
Now submerse your forearms, elbows and hands half way into the pool water. Keeping
your elbows together, move your palms away from one another. The surface water will
spread-out, or diverge, and water from below will rush up to replace the removed water.
By causing a divergence near the top of the water you created a circulation.
The Poleward Transport of Energy
In Chapter 2 we learned that radiative energy gains exceed the radiative losses at
the top of the atmosphere in the tropical regions of the globe. In the polar regions the
radiative losses exceed the gains. Circulations in the atmosphere and ocean transfer heat
poleward to compensate for these regional differences in energy budgets. Models of
global air patterns should reflect this. Moving warm air poleward and cold polar air
equatorward is one way this heat transfer occurs. In addition, air rising at the moist
adiabatic lapse rate at one latitude, and then sinking at the dry adiabatic lapse rate at a
more northern latitude will also transport heat poleward (Chapter 2). The air flow of our
conceptual model shows these processes. Thus, in addition to explaining the main
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weather features of the globe, our description of the atmospheric circulation shows a heat
transport from the tropical regions to the poles.
If there were no north-to-south, heat transfer the poles would be much colder and
the tropics much warmer.
Figure 7.19 shows the annual average poleward energy
transport accomplished by the atmosphere. The region of maximum energy transport by
the atmosphere occurs between 35 and 60 latitude, in both hemispheres. This is
northward of the subtropical high pressure belts and is accomplished by the mid-latitude
cyclones. Chapter 10 discusses these storms in detail.
Seasonal Variations
There are seasonal variations in the amount of solar energy received by the planet,
which results in seasonal changes in weather. How does our simple model handle these
seasonal differences? The global circulation pattern made up of the ITCZ, the polar front,
the subtropical highs, and the jet streams, shift with the sun, moving poleward in spring
and equatorward in autumn. This is reflected in the mean sea level pressure maps for
January and July (Figure 7.11). The subtropical highs are shifted further poleward in
each of the summer hemispheres.
The position of the ITCZ also shows a seasonal variation. The average position of
the ITCZ is approximately 4N. Seasonal averages show that the ITCZ is shifted toward
the summer pole (Figure 7.20). North-south variations in the position of the ITCZ are
larger over land than over ocean. This results from the different thermal properties of
continents and oceans. The large thermal inertia of the oceans anchors the ITCZ to within
approximately 4S and 8N.
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In each hemisphere, the position of the jet is displaced further poleward in
summer than in winter. During the summer, the positions of the subtropical highs sift
poleward, so the boundary between the surface air flowing poleward from the subtropical
highs is displaced poleward. This boundary indicates the position of the jet stream. The
poles are warmer in summer than winter. Since the polar air is warmer the temperature
difference between the polar air and the subtropical air is less, so the average strength of
the jet is expected to be less during the summer. The transport of energy from the
midlatitudes to poles should be smaller during the summer than the winter, as the summer
poles are illuminated and thus gain solar energy. The polar jet stream plays an important
role in the poleward heat transport by the atmosphere.
The global maps of mean pressure indicate regions of low and of high pressures.
The semi-permanent low pressure systems also have a seasonal variation in their position
(Figure 7.11). In January there is a series of relative low pressures around 8S latitude
over South America, Africa and Indonesia. This corresponds to the average position of
the ITCZ. During July there is a low pressure region west of Central America that is
associated with the mean position of the ITCZ in this region. During January there are
also low pressure regions near Iceland and near the Aleutian Islands. These are the
subpolar lows and are regions where midlatitude winter storms are often spawned.
During July there is one polar low in the Northern Hemisphere and it is weaker than
during winter. The low pressure region around the south pole is associated with the cold
Antarctica continent and its topography.
There are also low pressure regions in the vicinity of Iraq and the Southwest
United States in July. These lows are associated with deserts and are referred to as heat
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lows or thermal lows. These heat lows develop because of intense surface heating due to
absorption of solar radiation that occurs in the absence of clouds. This surface heating
warms the air above causing it to expand, both vertically and horizontally, throughout a
deep layer. This expansion reduces the air density, making the pressure near the surface
lower than if the air were cooler.
Monsoons
A monsoon is a weather feature driven by seasonal differences in the heating of
land and ocean along with seasonal shifts in planetaryscale circulations. Our original model of atmospheric
circulation assumed that the Earth was covered with
water. To examine an important seasonal variation in
A monsoon (from the Arabic word
mausam, meaning season) is a
circulation pattern characterized by a
seasonal reversal in the prevailing
winds that result in a wet and dry
season.
precipitation, we will have to add land masses to our conceptual model. The distribution
of land and especially large mountain ranges has a strong influence on weather patterns
(Box 8.2). One good example of where this effect is prominent is in the weather of the
Indian sub-continent.
The Indian monsoon is a complex interaction of seasonal shifts in the global
circulation patterns, thermal heating differences between land and sea, and the interaction
of winds with the Himalayan mountains.
In spring and summer, the sun heats the land and the air above it. With cooler air
over the surrounding water, a horizontal pressure gradient, directed from ocean to land, is
established near the surface, bringing humid air inland. The solar heating inland triggers
convection, causing hot humid air to rise, expand and cool, leading to condensation and
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rain. Orographic lifting by the Himalayas generates additional rain. Outflow from the
convective storms flows out over the Arabian Sea and the Bay of Bengal where it
subsides, completing the monsoon circulation (Figure 7.21). The air flowing over the
water remains over the ocean a long time, which causes it to gain moisture.
During autumn and winter the air above the land cools faster than over the water,
establishing a pressure gradient force from land to sea. The winds are reversed from the
summer monsoon flow—land to sea (Figure 7.22). Subsidence above the land suppresses
cloud development and precipitation. As a result, the winter monsoon is a dry season and
the summer monsoon a wet season.
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Summary
In this chapter we have developed a conceptual model to explain patterns of
global circulation and their associated weather and climate. Upward vertical motions in
the vicinity of the equator explain the Inter-Tropical Convergence Zone (ITCZ), which
can be identified in global satellite imagery and precipitation maps. When the rising air
reaches the tropopause and moves poleward, it becomes closer to the axis for rotation and
increases speed to conserve angular momentum. The Coriolis force turns this air to
produce the subtropical jet stream. The descending air of the Hadley Cell is compressed
and warms, lowering the relative humidity. This sinking air explains the large deserts of
Africa, Saudi Arabia, and Australia. The sinking air of the Hadley Cell results in calm
winds at the surface, producing the Horse Latitudes dreaded by ancient mariners who had
to use wind power to travel. Some of the descending air that reaches the surface moves
equatorward to supply moisture to the ITCZ. As the air moves equatorward it, the
Coriolis Force acts to produce the steady Northeast and Southwest trade windssought
after by mariners in sailing ships.
Some of the air associated with the descending branch of the Hadley Cell moves
poleward and meets with cold polar air masses that are moving equatorward. Fronts exist
where the cold air meets the warm subtropical air moving poleward, producing
midlatitude storms that tend to move from west to the east. These storms are embedded in
the midlatitude westerlies causing them to move eastward. The polar jet stream exists in
the vicinity of fronts.
In our simple conceptual model we assumed the globe to be covered with water.
A good approximation, considering the Earth is 70% water. However, land versus water
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is very important in weather and climate. The Indian Monsoon provided an example of
how ocean, land, and topography can interact to form clouds and precipitation or a dry
season. A monsoon is a seasonal reversal in wind patterns associated with changing
regional energy budgets. The wind direction results in a dry and a wet season. Over the
Indian subcontinent, the wet season occurs in summer and the dry season in winter.
The general circulation of the atmosphere transports heat from the equator
towards the poles. The world's oceans also transport heat. How ocean circulations
accomplish this heat transport is a topic of the next chapter.
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Terminology
You should understand all of the following terms. Use the glossary and this Chapter to
improve your understanding of these terms.
Angular Momentum
Polar Easterlies
Conservation of Angular Momentum
Polar Front Jet Stream
Doldrums
Rossby Wave
Hadley Cell
Shortwave
Horse Latitudes
Subtropical High Pressure
Index Cycle
Subtropical Jet Stream
Intertropical Convergence Zone
Thermal Low
Midlatitude Westerlies
Trade Winds
Meridional Flow
Zonal Flow
Monsoon
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Review Questions
1. What might the global circulation of the atmosphere be if the Earth did not rotate and
was covered with water? (Hint: start with an ITCZ).
2. Explain the seasonal shift in the polar front jet over the continental US for January
and July.
3. Why are the subtropical highs referred to as semi-permanent highs as opposed to
permanent high pressure systems?
4. Draw a picture of the global air flow near the surface of the Earth.
5. Explain the existence of the Intratropical Convergence Zone.
6. Can the subtropical highs over Bermuda be considered deserts?
7. Discuss the differences and similarities between a land/sea breeze and a monsoon.
8. Locate the major features of the atmospheric circulation pattern in today’s global
satellite image.
9. Describe how the Coriolis force affects a circulation feature of the atmosphere.
10. Explain why the Coriolis force does not influence how your sink drains.
11. Just south of the Tropic of Cancer lies Mount Waialeale (5,148 feet elevation), which
receives approximately 460 inches of rain a year. It is on the island of Kauai.
Explain why this region receives so much rain.
12. Explain why many ski-slopes are located on the windward side of mountains.
13. Why are the Southern Hemisphere Westerlies slighted faster than the northern
Hemisphere Westerlies?
7-3
Web Activities
Web activities related to subjects in the book are marked with subscript . Activities
include:
Satellite Observations of Global Circulation Patterns
Identifying Global Circulation feature in today’s weather
Tropical Easterly Jet
Tropical Easterly waves
Regions of ocean stratus
Practice multiple choice exam
Practice true/false exam
7-4
Box 8.1 Marine stratocumulus cloud regions
It is common to find stratocumulus clouds in the vicinity of the descending branch of the
Hadley Cell that lies over cold ocean regions. This is evident in the accompanying
satellite image of the Western Hemisphere. The marine air near the surface is cool and
humid. The upward and downward air movements generated by turbulent mixing create
this marine air layer. As the air in the upper troposphere descends it warms adiabatically,
and its relative humidity lowers. When a large area of the atmosphere sinks, a
temperature inversion can develop due to adiabatic compression. As illustrated in the
7-5
accompanying figure, the upper region of the layer sinks farther than the lower region and
thus warms more. After subsiding, the depth of the atmosphere the top of the layer is
warmer than the bottom. This is called a subsidence inversion because it results from
descending, or subsiding, air. In meteorology, subsidence denotes sinking motions. This
particular subsidence inversion is also called the trade wind inversion because it occurs
in the tradewinds. The temperature increases sharply with altitude in the trade wind
inversion. The stratus clouds observed in the satellite image lie just below the trade wind
inversion. The stratus clouds form when the air approaches saturation due to mixing of
the dry subsiding air with the moist air near the ocean surface. Because of the tradewind
inversion, which inhibits vertical motion, the clouds cannot develop vertically.
6000
Original location and temperature of the layer
Elevation (meters)
5000
Dotted lines represent the sinking air as it
warms at the dry adiabatic lapse rate.
4000
3000
2000
The descending branch of the
Hadley cell often resuts in
inversions near the surface
1000
0
-30
Altitude and
temperature of the
layer after subsidence
-25
-20
-15
-10
-5
0
Temperature (C)
5
10
15
20
7-6
Box 8.2 Precipitation Patterns and Topography
As discussed in Chapter 4, orographic lifting (lifting of air as it travels over a
mountain) is one mechanism of lifting air to reach saturation, cloud formation, and thus
precipitation. Careful examination of global precipitation patterns indicates a correlation
between total precipitation and topography. Precipitation is increased on the windward
side of the mountain. The correlation is not perfect. Precipitation is also related to the
temperature and humidity of the rising air, the speed and the direction of the winds
relative to the mountain topography, and the stability of the air. The Hawaiian Islands are
mountainous. The Hawaiian Islands are a good example of this relationship between
topography and precipitation. Because of their location in the trade winds, the upwind
side (the side the wind strikes) of a mountain receives a lot of rain while the downwind
side can resemble the grass lands of the central North America. To explain this
observation let's consider a simplified model of the weather situation.
Our simple conceptual model consists of a mountain ridge 4 kilometers high on an
island located in the trade winds.
Suppose that the air temperature of the wind
approaching the mountain is 20C with a dew point of 12C. As the air is orographically
lifted on the windward side it cools because of expansion at the dry adiabatic lapse rate,
approximately 10C for each 1 km it is lifted. As the parcel cools, the relative humidity
increases. As the parcel is forced to rise the mixing ratio remains the same until a cloud
forms. As the parcel rises, it expands, thus decreasing the vapor pressure. So, the air must
be cooled even further to reach the point of saturation. In other words, the dew point
7-7
temperature decreases. The dew point decreases approximately 2C per kilometer it is
lifted. So, after being lifted 1 km, the temperature and the dewpoint temperature are the
same, so the air is saturated. Then the mixing ratio decreases because water vapor
molecules are removed from the air to form cloud droplets. Further lifting causes the air
to cool at the moist adiabatic lapse rate, approximate 6C per kilometer. Collision and
coalescence results in large droplets and precipitation as the air ascends the mountain. At
the summit, assumed here to be 3 kilometers, the air has cooled to approximately -2C.
As long as the air remains cloudy, the dew point and the temperature are equal. As the
cool air begins its descent on the downwind side, the mountain the cloud droplets
evaporate as the air warms because of compression heating which lowers the relative
humidity. The cloud quickly dissipates after the summit. The now heated rising air is
depleted of moisture resulting in a local minimum in rainfall on the downwind or leeward
side. This local minimum is called a rain shadow.
In addition, as the air moves down the mountain side, the temperature increases at
the dry adiabatic lapse rate and the dew point increases at about 2 C per kilometer. So,
the air temperature warms about 8C per kilometer faster than the de point increases.
When the air returns to sea level, its temperature is 28C with a dew point temperature of
4C! The air has warmed and has a much lower relative humidity.
7-8
T= -2C Td= -2C
T= 8C Td= 0C
T= 18C, Td= 2C
T= 28C Td= 4C
T= 4C Td= 4C
T= 10C Td= 10C
T= 20C Td= 12C
(Include clouds over the region where the temperature (T) and dew point (Td) are
the same. See Ahrens hard cover, Figure 7.19 for another example)