Page 192 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

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arth’s atmospheric circulation transfers thermal energy from the equator toward the poles. The overall pattern of the atmospheric circulation
(GIA 6.1) arises from the distribution of high- and low- pressure regions, which determines patterns of precipitation (GIA 6.2) as well as winds.
GENERAL ATMOSPHERIC CIRCULATION MODEL
In both the Northern and Southern Hemispheres, zones of unstable, rising air (lows) and stable, sinking air (highs) divide the troposphere into circulation cells, which are symmetrical on both sides of the equator.
Polar jet stream
H
Midlatitude circulation
The westerlies are the prevailing surface winds, formed where air sinks and diverges along the poleward
border of the Hadley cells.
L.
Polar front
L
H
Subtropical jet stream
Subtropical high
Hadley cell
Hadley cell
Subtropical high
Subtropical jet stream
Surface trade winds
Surface trade winds
H
H
ITCZ
In the
Hadley cells,
winds rise along
the ITCZ and sweep
poleward at high altitude,
then sink to the surface in the subtropics, and circulate back
toward the equator as the trade winds.
Infer: In what directions would the westerlies and northeast trade winds blow if there were no Coriolis force?
H L
H
H
L
H
South pole
Subtropical High-Pressure Cells
Persistent highs (anticyclones) produce regions where air is mechanically pushed downward, compressed, and warmed. Earth’s major deserts form beneath these cells. Cause: Dynamic
Polar jet stream
6.1a
Visit the Study Area in MasteringGeographyTM to explore atmospheric circulation.
Visualize: Study geoscience animations of Assess: Demonstrate understanding of Earth’s atmospheric circulation patterns. atmospheric circulation (if assigned by instructor).
Polar High-Pressure Cells A small atmospheric polar mass is cold and dry, with weak anticyclonic high pressure. Limited solar energy results in weak, variable winds called the polar easterlies. Cause: Thermal
Subpolar Low-Pressure Cells Persistent lows (cyclones) over the North Pacific and North Atlantic cause cool, moist conditions. Cold, northern air masses clash with warmer air masses to the south, forming the polar front. Cause: Dynamic
Intertropical Convergence Zone (ITCZ)
Lying along the equator, the ITCZ is a trough of low pressure and light or calm winds—the doldrums. Moist, unstable air rises in the ITCZ, causing heavy precipitation year-round. Cause: Thermal
North pole
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Surface westerlies
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C
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geosystems in action 6 ATMoSPHERIC CIRCuLATIoN
Equator
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r
o
c
o
T
Westerlies
p
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