Chapter 7
CIRCULATION OF THE ATMOSPHERE
Learning Objectives
After reading, studying, and discussing this chapter, students should be able to do the
following:
1. Distinguish among macroscale, mesoscale, and microscale atmospheric motions.
2. List four different mesoscale winds, other than land and sea breezes, and describe
the details of their formation.
3. Describe the idealized pattern of global circulation as proposed by George Hadley.
4. Sketch and label a diagram showing the idealized three-cell model of global
circulation.
5. Discuss how the locations of continents and oceans influence the global pressure
distribution.
6. Discuss what monsoons are and list two monsoonal circulations.
7. Describe what jet streams are, why they exist, and where the midlatitude jet stream
can be found.
8. Describe what Rossby waves are and how they influence cyclonic activity.
9. Describe the relationship between oceanic circulation and general atmospheric
circulation.
10. Discuss how ocean currents influence climatic conditions of adjacent land areas.
11. Describe the relationship between the Southern Oscillation and El Niño and La
Niña.
Also, describe some impacts associated with both El Niño and La Niña.
12. Describe and explain how global precipitation distribution is related to global winds
and global pressure patterns.
13. Describe precipitation patterns on a uniform Earth and compare those patterns with
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actual patterns that exist on Earth.
14. Define the terms listed in the vocabulary review.
Answers to the Chapter Review
1.
Macroscale winds, the largest scale wind pattern, involve timescales of from days
(example, hurricanes) to years (example, mean global airflow). Mesoscale winds
involve a timescale of from minutes to days and include local winds such as the sea
breeze as well as phenomena such as tornadoes. Microscale winds are very short-lived
(from seconds to minutes) and involve such things as turbulence.
2.
This would be unlikely because the global wind pattern is a statistical average of
wind observations over many years. On any given day, the pattern may be quite
unlike the overall average.
3.
The “norther” is produced by the circulation around an anticyclone, which is a
synoptic scale (macroscale) wind system. Since the wind is not generated locally, it is
not a true local (mesoscale) wind.
4.
At these locations, the temperature differential between land and sea is large.
Consequently, the sea-to-land pressure gradient will also be large.
5.
These winds originate when cold air, situated over higher terrain, falls or drains to
nearby lower elevations. The greater density of this cold air is the cause of this flow.
Examples include the mistral, which blows from the Alps over France, and the bora,
which originates in the mountains of the former Yugoslavia and blows to the Adriatic
Sea.
6.
Cities can create their own local winds called a country breeze because of the
urban heat island effect. This usually happens on clear, calm nights when the
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relatively warm, less dense air over the city rises, which in turn initiates the countryto-city flow.
7.
Hadley proposed that on a nonrotating Earth, the more intensely heated air near
the equator would rise and move poleward. Upon reaching the poles, it would sink
and flow back along the surface toward the equator. When the Coriolis force is added
to this model, the flow aloft becomes westerly, while the flow at the surface is
easterly. Subsequent pressure-distribution observations did not support this model of
global circulation.
8.
The two factors that contribute to the general subsidence in this zone are as
follows: a) radiation cooling of the air aloft, making it denser, and b) the Coriolis
force deflects this poleward-moving air into a nearly west-to-east flow, which results
in the convergence of air aloft.
9.
A stronger Coriolis force would create convergence aloft at lower latitudes, so the
location of the Earth’s belts of subtropical high pressure would be nearer to the
equator than they presently are.
10.
Most of the United States is situated in the zone of prevailing westerlies.
11.
a. Since the southern ocean in this zone is continuous, there is no differential
heating of land and water to complicate the pattern.
b. During winter, high pressure exists over the cold continents. During summer,
however, the heat of the land creates low pressure over the continents. This difference
in pressure is balanced by an increase in pressure over the oceans. Thus, the
subtropical highs in the North Atlantic are stronger in July than in January.
c. Latitudinal temperature contrast in the Northern Hemisphere is greatest during
the winter and is also greatest further to the south in winter than in summer. This
influences the pattern of the jet stream as it moves further south in winter than in
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summer. The jet stream influences the path of cyclonic storms, so cyclonic storms are
more common in the winter.
d. This anticyclone is chiefly the result of the very frigid temperatures
experienced in Siberia in the winter. The greater density of the frigid air produces the
cell of high pressure.
12.
The Asian monsoon is driven by pressure differences that are generated by
unequal heating of Earth’s surface. In the summer, intense solar heating in India and
surrounding Southeast Asia causes temperatures to soar, generating a low-pressure
area over southern Asia. This in turn generates outflow aloft and encourages inward
flow at the surface, which brings in moisture-laden air from the Indian Ocean
landward. In winter, the Siberian high dominates circulation over Asia and produces
predominantly offshore winds. To a lesser extent, the seasonal migration of the ITCZ
also affects the Asian monsoon. Summer is the wet season of the Asian monsoon.
13.
Large areas of the southwestern United States and northwestern Mexico.
14.
When one examines the pressure gradient aloft, it is directed from the tropics
toward the poles. That is, pressures aloft near the equator are higher than pressures at
similar altitudes farther poleward. When the effect of the Coriolis force is added to
this, the flow aloft becomes westerly. When one examines the pressure gradient aloft,
it is directed from the tropics toward the poles. That is, pressures aloft near the
equator are higher than pressures at similar altitudes farther poleward. When the
effect of the Coriolis force is added to this, the flow aloft becomes westerly.
15.
In the winter. Because the large wintertime temperature gradients (especially in
the midlatitudes) create stronger pressure gradients and thus stronger winds.
16.
The discovery of the existence of jet streams was first made by bombers flying
toward Japan during World War II. They encountered extremely strong winds that did
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not allow them to make much headway. Upon abandoning their missions, the planes
experienced strong tailwinds during their flight home.
17.
Pressure distribution aloft is shown by height contours that show the height of a
given pressure level, rather than giving the pressure at a fixed height as surface maps
do. Areas of high pressure appear on these maps as having higher heights and areas of
low pressure are indicated by lower heights for the pressure value that the map
depicts.
18.
The dishpan experiments used rotating fluids to duplicate Earth’s circulation. A
large circular pan was heated around the outer edge to represent the equator and the
center was cooled to duplicate the poles. Colored particles were added to the fluid so
that the flow patterns could be observed. These experiments added greatly to our
understanding of global circulation, for when the pan was rotated, the simple
convection cell pattern broke down and was replaced by a wavy pattern with eddies
embedded between meanders. The experiments indicated that changes in either the
rate of rotation or in the temperature gradient largely controlled the nature of the flow
pattern.
19.
Major El Niño events generally cause the following: winters in the northern
United States and Canada to be warmer than normal; generally dry portions of Peru
and Ecuador, as well as the eastern United States, to experience wet conditions;
drought conditions in Indonesia, Australia, and the Philippines; and suppress the
number of Atlantic hurricanes.
20.
La Niña and El Niño are climatic opposites. La Niña events are characterized by
colder-than-average surface temperatures in the eastern Pacific, while El Niño events
are characterized by warmer-than-average surface temperatures in the eastern Pacific.
21.
Factors other than wind and pressure that influence the distribution of
precipitation include mountain barriers, degree of continentality, and ocean currents.
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