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Strengths of the Three-Celled Model
The three-celled model does give a fairly accurate image of the general
patterns of surface winds and pressures outside the mid-latitudes. There
actually is a region of low pressure near the equator, there are often highpressure regions near 30° latitude, and there are often high-pressure
regions near the poles. The three-celled model also gives a picture of wind
patterns and pressure systems that is useful for climate studies, because
understanding climates involves averaging patterns over long periods.
Description of Wind and Pressure Belts
Because the sun heats the tropics more than other parts of Earth, the
warm, rising air creates a low-pressure zone at the surface in the tropics.
This zone is called the intertropical convergence zone (ITCZ) because
surface winds from the two hemispheres come together there. In the ITCZ,
the air is hot and humid, there is little or no wind, and rain is common.
This region has historically been called the doldrums. Due to the lack of
wind, sailing vessels were often stuck in the doldrums for days at a time.
Between the latitudes of 20° and 35°, air generally sinks, forming the
so-called subtropical highs. Because the sinking air yields very little
precipitation, these regions contain many of Earth’s deserts. They have
historically been called the horse latitudes. The story goes that because
sailing ships were likely to be becalmed in this area, with little or no wind,
horses being transported would be thrown overboard to lighten the load.
Between the doldrums and the horse latitudes, the easterly trade
winds blow. The trade winds are warm and relatively steady in both
direction and speed. They are known as trade winds because merchants
used them, whenever possible, as trade routes for their sailing ships. The
predictable southeast winds of northern Australia are trade winds.
The high-pressure regions where cold air sinks at the poles are called
the polar highs. Surface winds in the polar regions are usually easterly.
Winds that usually blow from the same direction, such as the trade winds
and polar easterlies, are described as prevailing winds.
Image not available.
Please refer to the
image in the textbook
or in the eEdition CD.
ITCZ The band of
clouds near the equator
marks the warm, moist, rising
air of the ITCZ. Notice also the
spiraling clouds in the mid-latitudes
and the clear skies of the subtropical
highs and polar highs.
19.3 Section Review
1
How is the direction of wind flow changed at the equator? at the
poles? Why?
2
What determines the direction of surface winds in the mid-latitudes?
3 At which latitudes would you expect to find high surface pressures?
4
CRITICAL THINKING Why are the trade winds in the Southern
Hemisphere southeasterly?
5 APPLICATION Modern sailboats have motors so that they do not
get stranded when there is no wind. According to the three-celled
model, would a sailor be more likely to need a motor when sailing
from Georgia to southern Florida, or when sailing from
Massachusetts to Maine? Explain.
Chapter 19 The Atmosphere in Motion
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