2.4: Global Winds and Precipitation
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Precipitation
Besides their effect on the global wind belts, the high and low pressure areas created by the six atmospheric circulation cells determine in a general way the amount of precipitation a region receives.
Precipitation is any liquid or frozen water that forms in the atmosphere and falls back to the Earth.
In low pressure regions, where air is rising, rain is common. In high pressure areas, the sinking air causes evaporation and the region is usually dry.
Along the equator air is rising as it is warmed by solar radiation. Warm air contains more water vapor than cold air with the water content of air roughly doubling with every 10° C increase in temperature. So the air rising at the equator is warm and full of water vapor; as it rises into the upper atmosphere it cools. Cool air can no longer hold as much water vapor, so the water condenses and forms rain and we see wet habitats like tropical rainforests near the equator.
The amount of water vapor that air can hold __________ with temperature.
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increases
After rising and producing rain near the equator, the air masses move towards 30º latitude and sink back towards Earth as part of the Hadley convection cells. This air has lost most of its moisture after producing the equatorial rains, so the sinking air is dry, resulting in arid climates near 30° latitude in both hemispheres. Many of the major desert regions on Earth are located near 30° latitude, including much of Australia, the Middle East, and the Sahara Desert of Africa .
The air also becomes compressed and heats up as it sinks, absorbing any moisture from the clouds and creating clear skies. Thus high pressure systems are associated with dry weather and clear skies. This cycle of high and low pressure regions continues with the Ferrel and Polar convection cells, leading to rain and the boreal forests at 60º latitude in the Northern Hemisphere (there are no corresponding large land masses at these latitudes in the Southern Hemisphere). At the poles, descending, dry air produces little precipitation, leading to the polar desert climate.
Attribution
This article is a modified derivative of: