Regional climate depends on several factors, of which latitude (distance from the Equator) is among the most important. The Sun's varying effect on air temperature sets the world's air masses in motion.
The Earth's spherical shape means sunshine strikes the Equator at almost 90 degrees, while its angle becomes increasingly shallow towards the poles.
Globally, the decreasing solar energy absorbed at higher latitudes, combined with the rotation of the Earth, creates patterns of large-scale 'cells' of circulation. The largest cells (named Hadley cells after English meteorologist George Hadley) extend from the Equator to 30-40° latitude. Here warm, water-laden air rises, condensing to form a broken line of thunderstorms, sustaining the world's tropical rainforests.
From the tops of these storms, air flows north and south, where it sinks to produce high-pressure regions with hot, dry air: the deserts. Out-flowing air from these higher latitudes forms the trade winds that blow in an easterly direction across the oceans.
At the opposite extreme are the smaller, weaker polar cells (from 60-70° latitude to the poles). Here, the air is very dry and stable - Antarctica is the driest continent on Earth. The cold air sinks and flows away from the poles.
In between, in the mid-latitudes, warm, moist air from the subtropics meets cold, dry air from high latitudes, bringing unsettled wet weather typical of the temperate zones.
There are many other regional climate influences, but these cells explain in broad strokes the different climate zones across the world.
Last updated: 17 December 2013
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