Deserts, like forests and grasslands, occur all around the world. Symmetrical clusters of deserts are found around the Tropic of Cancer and the Tropic of Capricorn - the two latitudes that define the area where the sun's angle at solar noon is closest to ninety degrees overhead at the Equinox.
Some famous deserts are the Gobi and Taklamakan deserts in Asia, the Sahara in northern Africa, and the Great Sandy and Simpson deserts in Australia. Deserts can come and go with changes in climate. The Sahara was once green and filled with animals, as portrayed in wall paintings in its sandstone monuments. But when the ice glaciers Sahara is passed over by strong winds from the north that pick up any available moisture on their way to the equator.
There are both cold and hot deserts. Both kinds of deserts receive very little rainfall and both get very cold at night, but hot deserts bake during the day. The vegetation typically is ground cover, stunted bushes, or occasional trees and is specially adapted for life in such difficult circumstances.
Two key factors in the creation of deserts are the rain shadows of mountains and the big circulation pattern of global wind. In the case of mountains, as water-filled air is forced up a mountain slope, it cools and dumps its water (precipitation) on that side of the mountain. If the mountain range is large enough, little water gets to the other side. Global wind patterns are complicated, but worth understanding. The winds that circle the globe are caused by the difference between warm equatorial temperatures and cooler polar temperatures. When air is warmed at the equator, it rises. It then moves north and south towards each of the poles, where it cools, loses moisture, sinks, and returns toward the equator. On its return, the air can hold more water, so it picks up any available moisture over the desert areas near the equator.
In a desert, water is all-important. Adaptations to the scarce rainfall typical of deserts include:- Annual plants have seeds that are able to stay dormant until there is sufficient rainfall to support the young plant.