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Circulation of the Atmosphere
Chapter 7
© 2011 Pearson Education, Inc.

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7.1 Scales of Atmospheric Motion
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Scales of Atmospheric Motion
Small- and large-scale circulation:
 Microscale
 Mesoscale
 Macroscale

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Scales of Atmospheric Motion
Microscale winds:
 The circulation is small and chaotic.
 They can last from seconds to minutes.
 They can be simple gusts, downdrafts, and
small vortices, such as dust devils.

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Scales of Atmospheric Motion
Mesoscale winds:
 They can last from minutes to hours.
 They are usually less than 100 km across.
 Some mesoscale winds (thunderstorms and
tornadoes) also have a strong vertical
component.

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Scales of Atmospheric Motion
 Macroscale winds:
 These winds are the largest wind patterns.
 These planetary-scale patterns can remain
unchanged for weeks at a time.
 Smaller macroscale circulation is called synoptic
scale.
 These wind systems are about 1000 km in
diameter.
 Smaller macroscale systems are tropical storms and
hurricanes.

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Scales of Atmospheric Motion

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Scales of Atmospheric Motion
Structure of wind patterns:
 Global winds are a composite of motion on all
scales.
 Hurricanes appear as a large cloud moving
slowly across the ocean.
 The large cloud contains many mesoscale
thunderstorms.
 The thunderstorms consist of numerous
microscale bursts.

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7.2 Local Winds
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Local Winds
Land and sea breezes

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Local Winds
Mountain and valley breezes

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Local Winds
Valley Breeze:
 Air along Mt. slopes is heated more intensely
than air at the same elevation over the valley
floor. This warm air glides up the Mt. slope and
generates a valley breeze.
Mt. Breeze:
 After sunset process is reversed. Rapid heat loss
along the mountain slopes cool the air. Which
drain into the valley.
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7.3 Global Circulation
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Global Circulation
Single-cell circulation
model
Hadley model
 Hadley proposed that the
contrast in temperatures
between the poles and
the equator creates a
large convection cell in
both the Northern and
Southern hemispheres.

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Global Circulation
Hadley Model
 Warm equatorial air rises until it reaches the
tropopause. Where it spreads towards the poles.
Eventually this uplift level flow reaches the
poles, where cooling causes it to sink and
spread out at the surface as equatorward moving
winds.
 It reaches the equator warms and they cycle
repeats.
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Global Circulation
Three-cell model

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Global Circulation
 A three-cell circulation model was proposed in the
1920s.
 Warm air rises at the equator (Hadley cell).
 As the flow moves poleward, it begins to cool
and sinks at 20°–35° latitude.
 Trade winds meet at the equator, in a region with
a weak pressure gradient, called the doldrums.

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Global Circulation
The westerly circulation of surface winds
(prevailing westerlies) between 30°–60°
latitude is called the Ferrel cell.
 Not a good model for winds aloft because it
predicts winds that flow from east to west. Just
opposite of what is observed.
Circulation (at 60°–90°) within a polar cell
produces polar easterlies; surface flows that
move toward the equator.
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7.4 Pressure Zones Drive the Wind
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Pressure Zones Drive Winds
Idealized zonal pressure belts:
1. The equatorial low is an intertropical convergence
zone (ITCZ).
 Low pressure and trade winds converge
1. Subtropical highs (STH) are high-pressure zones in
the belts about 20°–35° latitude on either side of
the equator.
 Where westerlies and trade winds originate.

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Pressure Zones Drive Winds
3. Subpolar low is a low pressure region about
50°-60° latitude. Polar easterlies and westerlies
clash in low pressure convergence zone; this is
the reason for bad weather in our area in the
winter
4. Polar highs near the Earth’s poles are where the
polar easterlies originate.
 Higher than average pressure.
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Pressure Zones Drive Winds

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Pressure Zones Drive Winds
Semi permanent pressure systems: The
real world

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Pressure Zones Drive Winds
January pressure and wind patterns
 Siberian high- a very strong high pressure
center positioned over frozen area in northern
Asia.
 As highs over the continents grow the lows over
oceans grow weak.
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Pressure Zones Drive Winds
In Jan but not in July; 2 intense
semipermenant low pressure centers
 Aleutian low- by Alaska
 Icelandic Low
Areas are frequently clouds and get lots of
winter precipitation.
© 2011 Pearson Education, Inc.

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Pressure Zones Drive Winds
Semi permanent pressure systems: The
real world

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July pressure and wind pattern
July pressure and wind pattern
 In the summer months, the subtropical high in
the northern hemisphere migrate west and
become stronger than during the winter.
© 2011 Pearson Education, Inc.

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7.5 Monsoons
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Monsoons
 Monsoon refers to a seasonal reversal of winds.
 The Asian monsoon, which affects India and its
surrounding areas, China, Korea, and Japan.
 The monsoon is driven by pressure differences.
 The North American monsoon occurs in the
southwestern U.S. and northwestern Mexico.
 This monsoon is driven by the extreme temperatures,
which generate a low-pressure center over Arizona and
results in a circulation pattern that brings moist air
from the Gulf of California and from the Gulf of
Mexico, to a lesser degree.

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Monsoons

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Monsoons

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7.6 The Westerlies
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The Westerlies
Why Westerlies?
 Difference between pole and equator drive these
winds
 Pressure gradient from equator to pole and
Coriolis force deflects winds and a balance is
reached.

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The Westerlies
Waves in the westerlies:
 Westerliesflow in wavy paths that have long
wavelengths.
 The longest wave patterns are known as
Rossby waves, which usually consist of 4–6
waves that encircle the globe.
 Rossby waves can have a large impact on
weather.

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7.7 Jet Streams
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Jet Streams
Jet streams:
 Embedded in westerlies
 Widths vary from less than 100 km to more than
500 km.
 Speeds can attain 100–400 kph. (60-240 mph)
 Polar and subtropical

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Jet Streams
The polar jet stream is the most prevalent.
 It occurs along a major frontal zone, the polar
front.
 The jet stream moves faster in winter.
 During the winter, occasionally it moves north–
south.
 If the jet stream is more equatorward weather
will be colder and drier than normal. More
poleward, weather will be warmer and more
humid.

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© 2011 Pearson Education, Inc.

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Jet Streams
The subtropical jet stream is a semipermanent
jet stream over the subtropics.
 It is a west-to-east current, centered at 25° N and S.
 It is mainly a winter phenomenon.
 The subtropical jet stream is slower than the polar.

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Jet Streams
Jetstreams and
Earth’s heat budget
Relatively mild
temperature occur
south of jet stream
and cold temperature
north of jet stream the
waves begin to
meander.

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7.8 Global Winds and Ocean Currents
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Global Winds and Ocean Currents
Energy is passed from moving air to the
surface of the ocean through friction.
Resulting in water being dragged by the
wind.
The Coriolis force deflects surface currents
poleward, which form nearly circular
patterns of ocean currents called gyres.
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Global Winds and Ocean Currents
Gyres are found in each major ocean basin
centered around subtropical high-pressure
systems
The Gulf stream is strengthened by
westerly winds and continues
northeastward.

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Global Winds and Ocean Currents
© 2011 Pearson Education, Inc.

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Global Winds and Ocean Currents
Importance of ocean currents:
 Ocean currents have an important on climate,
which helps maintain the Earth’s heat balance.
 Cold currents offshore result in a dry climate.
 Warm offshore current produce a warm moist
climate.
 Ocean currents account for ¼ of total heat
transport. Wind accounts for the other ¾.

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Global Winds and Ocean Currents
Ocean currents and upwelling:
 Upwelling is the rising of cold water from
deeper layers to replace warmer surface water.
 A wind-induced vertical movement
 It occurs where winds blow parallel to the
coast toward the equator.

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7.9 El Nino and La Nina and the Southern
Oscillation
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El Niño and
La Niña and
the
Southern
Ocean

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El Niño and La Niña and
the Southern Ocean
ElNiño is a gradual warming of eastern Pacific
waters in December or January.
 Periodof abnormal warming happen at irregular
intervals of 2-7 years and persist for spans of 9
months to 2 years.
La Niña is the opposite of El Niño and refers to
colder-than-normal ocean temperatures along
the coast of Ecuador and Peru

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El Niño and La Niña and
the Southern Ocean
Impact of El Niño:
 It is noted for its potentially catastrophic impact
on weather and economies of Chile, Peru,
Australia, and other countries.
 Arid areas can receive a lot of precipitation.
 A change in surface water temperature can kill
fish.
 El Niño has been recognized as part of the
global atmospheric circulation pattern.

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El Niño and La Niña and
the Southern Ocean
Impact of La Niña:
 La Niña is also an important atmospheric
phenomenon.
 In the western Pacific, wetter than normal
conditions occur.
 There are also more frequent hurricanes in
Atlantic.

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El Niño and La Niña and
the Southern Ocean
Southern oscillation:
 This is the seesaw pattern of atmospheric
pressure between the eastern and western
Pacific.
 Winds are the link between pressure changes
and the ocean warming and cooling associated
with El Niño and La Niña.
© 2011 Pearson Education, Inc.

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7.10 Global Distribution of Precipitation
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Global Distribution
of Precipitation
Zonal Distribution of
precipitation

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Global Distribution of Precipitation
Distribution of precipitation over the
continents