Chapter 7
Atmospheric Circulations
Summary
A wide variety of types of air motion are examined in this chapter, ranging from shortlived microscale phenomena to the semi-permanent circulation patterns found in the earth's global
circulation.
The chapter can be divided roughly in half. The first portion begins with a quick
classification of the scales of atmospheric motion and looks at the formation of eddies. Wind shear
and the turbulent eddies that can form in clear air are of practical importance because they can
present a hazard to aviation. The formation of thermal circulations is then covered in some
detail. Sea, lake, and land breezes are common examples of thermal circulation and students
may have lived in or visited a region where these occur. The role that seasonal changes play in
the development of the Asian monsoon, which resembles a large thermal circulation, will also be
better appreciated. Several additional local scale winds including mountain and valley breezes,
katabatic winds, chinook, and the Santa Ana winds are described.
The earth's global scale wind and surface pressure patterns are treated in the second half of
the chapter. Single-cell and three-cell models have been developed in an effort to understand the
underlying cause of the general circulation pattern. Despite some unrealistic assumptions, the
three-cell model contains many of the surface features found in the real world. These features and
their seasonal movement can have an important effect on regional climate. Approximately 70%
of the earth's surface is covered with oceans; weather and climate are strongly affected by
interactions between the atmosphere and oceans. The major ocean currents are identified and
the chapter ends with discussions of the El Niño/Southern Oscillation phenomenon, as well as other
atmosphere-ocean interactions including the North Atlantic, Arctic, and Pacific Decadal
Oscillations.
Teaching Suggestions, Demonstrations and Visual Aids
1.
Clouds produced by some of the mesoscale and global scale wind circulation patterns
discussed in this chapter can be seen on satellite photographs. The ITCZ is often very clearly
defined on a full or half disk photograph from a geostationary satellite. Sea breeze
convergence zones may be visible along the eastern or western coastline of Florida. Ask students
why a strong convergence zone will form along the west coast one day and then along the east
coast another day. A book by T.S. Loebl has good examples of wave clouds produced by winds
blowing over mountain ranges (ref: T.S. Loebl, View From Low Orbit, Imaging Publications,
Hubbardston, Mass., 1991).
2.
During the winter, bring a surface weather map to class and compare temperatures to the
north and the south of the polar front. There is often a very sharp temperature gradient. Locate
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 7: Atmospheric Circulations
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the polar jet stream on an upper-level chart and determine the region where the maximum winds
are found.
3.
Construct a box approximately 12" wide x 12" high x 4" deep. The front side of the box
should be constructed out of clear plastic. Place two "cat food" cans at opposite ends of the bottom
of the box. Fill one can with dark soil, the other with water. Place a 150 or 200 Watt bulb so that
light shines equally onto the two cans in the box. After a few minutes, carefully introduce some
smoke into the chamber from a hole near the middle of the bottom edge of the plastic. With some
care, a circular circulation will be visible.
Student Projects
1.
Have students research and plot the path of some of the early voyages of discovery made
aboard sailing ships (Columbus or Magellan, for example). Do the routes appear reasonable in light
of what the students know about the earth's general circulation pattern?
2.
Have students collect climatological data for their location (or another region) during one
or two strong El Niño events. Select two or three periods when there was not a strong El Niño
to act as a control. What effects might the El Niño have on local weather or climate. Do the
students see any evidence of this in their climate data?
3.
Have students summarize the weather conditions prevailing during a period when strong
Santa Ana winds are being observed in southern California.
4.
Have students research and summarize the weather conditions that produce any local scale
winds that are unique to their area. Do these local winds have any important effects on the local
climate?
5.
Have students construct a windrose for their city or town. Can students find any evidence
that data of this type has been used by city planners?
6.
Using the Winds in Two Hemispheres activity on the ThomsonNow web site,
examine the 24-hour history of a U.S. weather map with wind vector overlays. Can you
find any evidence of small-scale circulations, such as thermal circulations or sea/land
breezes.
7.
Use the Winds in Two Hemispheres section of the ThomsonNow web site to try
to locate some of the major features of the global wind circulation. Which features can
you find? In which part(s) of the world did you find them?
9.
Use the Forecasting section of the ThomsonNow web site to look for jet
streams on an upper-level weather map. Name any jet streams that you find.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 7: Atmospheric Circulations
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Answers to Questions for Review
1.
Microscale: diameter of a few meters or less, time scale of seconds to a few minutes.
Example: turbulent eddies. Mesoscale: diameter of a few kilometers to hundreds of kilometers,
time scale of hours to a day. Example: thunderstorms. Synoptic scale: diameter of several hundred
to a thousand kilometers, time scale of days to weeks. Example: high and low pressure systems.
Global scale: spatial scale of entire earth, time scale of weeks to months. Example: longwaves in
the westerlies.
2.
Wind shear is the change in wind speed or direction with increasing altitude. When the
shear exceeds a critical value, waves break into large swirls with significant vertical movement.
When this happens in clear air it's called clear air turbulence.
4.
During the day, the land heats more quickly than the adjacent water, and the intensive
heating of the air above produces a shallow thermal low. The air over the water remains cooler than
the air over the land; hence, a shallow thermal high exists above the water. The overall effect of
this pressure distribution is a sea breeze that blows from the sea toward the land. At night, the
land cools more quickly than the water. The air above the land becomes cooler than the air over the
water, producing a distribution of pressure, such as the one shown in Fig. 7.5b.With higher surface
pressure now over the land, the wind reverses itself and becomes a land breeze—a breeze that
flows from the land toward the water. Temperature contrasts between land and water are generally
much smaller at night, hence, land breezes are usually weaker than their daytime counterpart, the
sea breeze.
5.
During the winter, the air over the continent becomes much colder than the air over the
ocean. A large, shallow high-pressure area develops over continental Siberia, producing a
clockwise circulation of air that flows out over the Indian Ocean and South China Sea. Subsiding
air of the anticyclone and the downslope movement of northeasterly winds from the inland plateau
provide eastern and southern Asia with generally fair weather. Hence, the winter monsoon, which
lasts from about December through February, means clear skies (dry season), with winds that blow
from land to sea. In summer, the wind flow pattern reverses itself as air over the continents
becomes much warmer than air above the water. A shallow thermal low develops over the
continental interior. The heated air within the low rises, and the surrounding air responds by
flowing counterclockwise into the low center. This condition results in moisture-bearing winds
sweeping into the continent from the ocean. The humid air converges with a drier westerly flow,
causing it to rise; further lifting is provided by hills and mountains. Lifting cools the air to its
saturation point, resulting in heavy showers and thunderstorms. Thus, the summer monsoon of
southeastern Asia, which lasts from about June through September, means wet, rainy weather (wet
season) with winds that blow from sea to land.
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Instructor’s Manual
Chapter 7: Atmospheric Circulations
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6.
Valley breeze, because rising air will expand and cool adiabatically.
7.
The ideal setting for a katabatic wind is an elevated plateau surrounded by mountains, with
an opening that slopes rapidly downhill. When winter snows accumulate on the plateau, the
overlying air grows extremely cold and a shallow dome of high pressure forms near the surface.
Along the edge of the plateau, the horizontal pressure gradient force is usually strong enough to
cause the cold air to flow across the isobars through gaps and saddles in the hills. Along the slopes
of the plateau, the wind continues downhill as a gentle or moderate cold breeze. If the horizontal
pressure gradient increases substantially, such as when a storm approaches, or if the wind is
confined to a narrow canyon or channel, the flow of air can increase, often destructively, as cold air
rushes down-slope like water flowing over a waterfall.
8.
Chinook winds blow down mountain slopes. As air descends it warms adiabatically.
Chinook winds are dry because the air's moisture has been lost as the air ascended on the upwind
side of the mountain.
9.
As air descends from the elevated desert plateau, it funnels through mountain canyons in
the San Gabriel and San Bernardino Mountains, finally spreading over the Los Angeles basin and
San Fernando Valley. The wind often blows with exceptional speed in the Santa Ana Canyon (the
canyon from which it derives its name). These warm, dry winds develop as a region of high
pressure builds over the Great Basin. The clockwise circulation around the anticyclone forces air
downslope from the high plateau. Santa Ana winds are warm because of compressional heating of
the already warm, dry desert air.
10.
Dust devils form on clear, hot days over a dry surface where most of the sunlight goes into
heating the surface, rather than evaporating water from vegetation. The atmosphere directly above
the hot surface becomes unstable, convection sets in, and the heated air rises. Wind, often deflected
by small topographic barriers, flows into this region, rotating the rising air. Depending on the
nature of the topographic feature, the spin of a dust devil around its central eye may be cyclonic or
anticyclonic, and both directions occur with about equal frequency.
12.
The westerlies.
13.
When we compare the January and July maps, we can see several changes in the semipermanent pressure systems. The strong subpolar lows so well developed in January over the
Northern Hemisphere are hardly discernible on the July map. The subtropical highs, however,
remain dominant in both seasons. Because the sun is overhead in the Northern Hemisphere in July
and overhead in the Southern Hemisphere in January, the zone of maximum surface heating shifts
seasonally. In response to this shift, the major pressure systems, wind belts, and ITCZ shift toward
the north in July and toward the south in January.
14.
Since the polar front is a boundary separating the cold polar air to the north from the warm
subtropical air to the south, the greatest contrast in air temperature occurs along the frontal zone.
15.
The equator-to-pole temperature gradient is stronger in winter than in summer.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 7: Atmospheric Circulations
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16.
As the wind blows over the oceans, it causes the surface water to drift along with it. The
moving water gradually piles up, creating pressure differences within the water itself. This leads to
further motion several hundreds of meters down into the water. In this manner, the general wind
flow around the globe starts the major surface ocean currents moving. Major ocean currents do not
follow the wind pattern exactly; rather, they spiral in semi-closed circular whirls called gyres. In
the North Atlantic, the prevailing winds blow clockwise and outward from the subtropical highs,
while the ocean currents move in a more or less circular, but still clockwise, pattern. As the water
moves beneath the wind, the Coriolis force de.ects the water to the right in the Northern
Hemisphere and to the left in the Southern Hemisphere. This deflection causes the surface water to
move at an angle between 20° and 45° to the direction of the wind. Hence, surface water tends to
move in a circular pattern as winds blow outward, away from the center of the subtropical high.
17.
Summer winds tend to parallel the coastline of California. As the wind blows over the
ocean, the surface water beneath it is set in motion. As the surface water moves, it bends slightly to
its right due to the Coriolis effect. The water beneath the surface also moves, and it too bends
slightly to its right. The net effect of this phenomenon is that a rather shallow layer of surface water
moves at right angles to the surface wind and heads seaward. As the surface water drifts away from
the coast, cold, nutrient-rich water from below rises (upwells) to replace it. Upwelling is strongest
and surface water is coolest where the wind parallels the coast, such as it does in summer along the
coast of northern California.
18.
a. A situation in which El Niño conditions last for many months, and a more extensive
ocean warming occurs. This extremely warm episode occurs at irregular intervals of two to seven
years and covers a large area of the tropical Pacific Ocean. b. They reverse in a seesaw pattern:
high at one end and low at the other. c. In both hemispheres, the air is warmer over low latitudes
and colder over high latitudes. This horizontal temperature gradient establishes a horizontal
pressure (contour) gradient that causes the winds to blow from the west, especially in middle and
high latitudes.
19.
Unusually cold surface temperatures.
20.
Over the Atlantic there is a reversal of pressure (called the North Atlantic Oscillation, or
NAO) that has an effect on the weather in Europe and along the east coast of North America. For
example, in winter if the atmospheric pressure in the vicinity of the Icelandic low drops, and the
pressure in the region of the Bermuda-Azores high rises, there is a corresponding large difference
in atmospheric pressure between these two regions that strengthen the westerlies. The strong
weterlies in turn direct strong storms on a more northerly track into northern Europe, where winters
tend to be wet and mild. During this positive phase of the NAO, winters in the eastern United
States tend to be wet and relatively mild, while northern Canada and Greenland are usually cold
and dry. The negative phase of the NAO occurs when the atmospheric pressure in the vicinity of
the Icelandic low rises, while the pressure drops in the region of the Bermuda high. This pressure
change results in a reduced pressure gradient and weaker westerlies that steer fewer and weaker
winter storms across the Atlantic in a more westerly path. These storms bring wet weather to
southern Europe and to the region around the Mediterranean Sea. Meanwhile, winters in Northern
Europe are usually cold and dry, as are the winters along the east coast of North America.
21.
Cold.
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Instructor’s Manual
Chapter 7: Atmospheric Circulations
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22.
During the warm (or positive) phase, unusually warm surface water exists along the west
coast of North America, while over the central North Pacific, cooler than normal surface water
prevails. At the same time, the Aleutian low in the Gulf of Alaska strengthens, which causes more
Pacific storms to move into Alaska and California. This situation causes winters, as a whole, to be
warmer and drier over northwestern North America. Elsewhere, winters tend to be drier over the
Great Lakes, and cooler and wetter in the southern United States. The cool (or negative) phase
finds cooler-than-average surface water along the west coast of North America and an area of
warmer-than-normal surface water extending from Japan into the central North Pacific. Winters in
the cool phase tend to be cooler and wetter than average over northwestern North America, wetter
over the Great Lakes, and warmer and drier in the southern United States.
Answers to Questions for Thought and Exploration
1.
In early morning you would likely experience the mountain breeze, with winds blowing
down the mountain slope.
2.
The very cold Antarctic air is quite dense, and it tends to 'drain' down the valleys, creating
strong katabatic winds.
3.
The equator-to-pole temperature gradient must reverse directions.
5.
Strongest sea breezes: west coast. Strongest land breezes: east coast.
6.
Because of the Ekman Spiral, the average movements of surface water down to a depth of
about 100 m is at right angles to the surface wind direction. Icebergs, which may extend downward
to depths greater than 100 m, move with this surface water at nearly right angles to the surface
wind direction.
7.
The fastest winds are found in the jet stream core. Clear air turbulence is found above and
below the jet stream core.
8.
This is due mainly to the reversal of winds associated with the summer and winter
monsoon.
9.
Upwelling is strongest in summer and when the winds blow parallel to the coast. Strong
upwelling conditions bring cold water to the surface. In winter when upwelling is not as strong, the
surface water is not as cold.
10.
Large subtropical highs tend to be centered over the oceans off the western margin of
continents in both hemispheres. Winds around the highs blow clockwise (from the north along the
western margin of the continents) in the Northern Hemisphere, and counterclockwise (from the
south along the western margin of continents) in the Southern Hemisphere.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 7: Atmospheric Circulations
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