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 windswinds 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 20S 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 30N and 30S. 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.

1

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.

2

(Combine these two figures?)

3

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 8N. The average position of this cloud band is approximately 4S 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).

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model will predict the existence of two of these jet streams. Both are fast moving

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 30N, 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,

defined as the product of mass times the rotation velocity

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.

Conservation of Angular Momentum requires a spinning body to conserve its angular momentum.

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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

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 30N 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

in the Southern Hemisphere. This is the wind the traders

sought in traveling from Europe to the Americas. These steady winds are called the

Subtropical Jet is a jet stream typically found between 20 and 30 latitude.

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).

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trade 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 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

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.

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winds are light or calm over large regions of the subtropical oceans, the horse latitudes,

consistent with the observations of 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

Subtropical Highs are high surface pressures observed over the ocean. They are semi-permanent in their location in that they shift with the seasons.

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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

60N. 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

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

Polar Front Jet is a jet stream found in the middle and upper latitudes in association with the polar front.

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front (Figure 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 wave-

like 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

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called Rossby 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

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.

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 north-south movements of weather systems.

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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

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

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the upper-level troughs (Figure 7.18). Chapter 6 discussed the importance of divergence

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 4N. 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 4S and 8N.

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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 8S 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 planetary-

scale circulations. Our original model of atmospheric

circulation assumed that the Earth was covered with

water. To examine an important seasonal variation in

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

rain. Orographic lifting by the Himalayas generates additional rain. Outflow from the

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.

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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 windssought

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

Conservation of Angular Momentum

Doldrums

Hadley Cell

Horse Latitudes

Index Cycle

Intertropical Convergence Zone

Midlatitude Westerlies

Meridional Flow

Monsoon

Polar Easterlies

Polar Front Jet Stream

Rossby Wave

Shortwave

Subtropical High Pressure

Subtropical Jet Stream

Thermal Low

Trade Winds

Zonal Flow

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Review Questions1. 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?

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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

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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

accompanying figure, the upper region of the layer sinks farther than the lower region

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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.

-30 -25 -20 -15 -10 -5 0 5 10 15 200

1000

2000

3000

4000

5000

6000

Temperature (C)

Ele

vatio

n (m

eter

s)

Original location and temperature of the layer

Altitude andtemperature of the layer after subsidence

Dotted lines represent the sinking air as it warms at the dry adiabatic lapse rate.

The descending branch of theHadley cell often resuts ininversions near the surface

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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 20C with a dew point of 12C. As the air is orographically

lifted on the windward side it cools because of expansion at the dry adiabatic lapse rate,

approximately 10C 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

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temperature decreases. The dew point decreases approximately 2C 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 6C 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 8C per kilometer faster than the de point increases.

When the air returns to sea level, its temperature is 28C with a dew point temperature of

4C! The air has warmed and has a much lower relative humidity.

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(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)

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