Global Weather Patterns

Deserts tend to occur in two belts that circle the globe. Both the Northern and Southern Hemisphere have this belt located between 15 and 35 degrees latitude, roughly centered over the Tropic of Cancer and the Tropic of Capricorn. This is no accident. The sun is more directly overhead the equatorial region so it receives the most intense sunlight, and this solar energy heats the air. Hot air has two important qualities: it can hold enormous quantities of moisture, and it rises up into the atmosphere. So hot tropical air tends to be moist and rise into the atmosphere. As this air rises it cools, condensing the moisture and converting it to water where it falls as rain. This is why rain forests tend to occur near the equator. What goes up must come down, and gravity pulls this mass of rising air back to the ground. Tropical air typically falls at about 30 degrees latitude on either side of the equator and along the desert belt, but robbed of its moisture it is now hot and dry. The result are often persistent high pressure systems that tend to block incoming storms, or push them into other regions. These patterns make deserts possible, but typically other factor must also intrude to make deserts a reality.

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