general_circulation

general_circulation - Chapter 8 Chapter Atmospheric...

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Chapter 8 Chapter 8 Atmospheric Circulation Atmospheric Circulation and and Pressure Distributions Pressure Distributions
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George Hadley (1685-1768) proposed a simple circulation pattern called the single-cell model to describe the general movement of the atmosphere. In the single-cell model, air expands upward, diverges toward the poles, descends, and flows back toward the equator near the surface. Winds blowing east-to-west or west-to-east are zonal winds ; those moving north-to-south or south-to-north are meridional .
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The three-cell model divides the circulation of each hemisphere into three distinct cells: the heat-driven Hadley cell that circulates air between the Tropics and subtropics, a Ferrel cell in the middle latitudes, and a polar cell .
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Along the equator, strong solar heating causes air to expand upward and diverge toward the poles, creating a zone of low pressure called the equatorial low or the Intertropical Convergence Zone. The ITCZ is the rainiest latitude zone in the world and is observable as the band of convective clouds and showers extending from northern South America into the Pacific on this satellite image.
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Global precipitation from satellite SACZ South Atlantic Convergence Zone South Pacific convergence Zone
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Organization of the SACZ
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At about 20° to 30° latitude, air in the Hadley cell sinks toward the surface to form the subtropical highs , large bands of high surface pressure. Cloud formation is greatly suppressed and desert conditions are common in the subtropics. In the Northern Hemisphere, as the pressure gradient force directs surface air from the subtropical highs to the ITCZ, the weak Coriolis force deflects the air slightly to the right to form the northeast trade winds . In the Southern Hemisphere, the northward-moving air from the subtropical high is deflected to the left to create the southeast trade winds .
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Flanking the Hadley cell in each hemisphere is the Ferrel cell , which circulates air between the subtropical highs and the subpolar lows , or areas of low pressure. On the equatorial side of the Ferrel cell, air flowing poleward from the Northern Hemisphere subtropical high undergoes a substantial deflection to the right, creating a wind belt called the westerlies . In the Southern Hemisphere, the pressure gradient force propels air southward, but the Coriolis force deflects it to the left, producing a zone of westerlies in that hemisphere as well.
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surface air moves from the polar highs to the subpolar lows . Compared to the poles, air at subpolar locations is slightly warmer, resulting in low surface pressure and rising air. Very cold conditions at the poles create high surface pressure and low-level motion toward the equator. In both hemispheres, the Coriolis force
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This note was uploaded on 12/27/2011 for the course GEOG 110 taught by Professor Leila during the Fall '09 term at UCSB.

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general_circulation - Chapter 8 Chapter Atmospheric...

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