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Our transcription: To develop a simple definition of a desert, we can find a common denominator in each of these harsh desert landscapes. From the polar regions of Antarctica and Northern Alaska to the vegetated sands of the Mojave Desert -- "deserts are regions with infrequent precipitation averaging less than 25 centimeters per year." In most deserts, evaporation rates are high enough to quickly remove this moisture. To understand the origin of the Earth's hot subtropical deserts, the largest on the planet, we must first consider the Equator. Like the deserts themselves, the Equator is a hot place, but it is also very humid with torrential rain any time of year and steamy tropical rainforests. Why are the tropics wet and the subtropics dry?
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