The world's broad "desert belts'' north and south of the equator result from the global circulation pattern (Chapter 1): equatorial air rises up into the atmosphere, heated by intense sunlight and loses its water vapor as sudden rainstorms. Eventually, this air comes back down hundreds of kilometers from the equator, and heats up as it is compressed, holding even more tightly onto what little water vapor remains within it. In such a situation, with dry air nearly always moving in from above, there is inevitably a more or less arid climate on the outer fringes of the tropics. The deserts of the Sahara, Arabia, central Asia, Australia and the USA owe their existence mainly to this process of dry air moving in from above. There are also other geographical factors that can help make a desert. Mountains can also block moist winds from the sea, forcing the air to rise, cool and drop its rain on their slopes—so that there is hardly any water vapor left to form rain on the inland side. This rain shadow effect helps to reinforce the dryness of the North American deserts, and the deserts of central Asia and Australia, combining with the descending equatorial circulation in a sort of "double whammy'' of aridity.
Some other deserts are the result of cold upwelling seawater just off the coast; the cold water does not evaporate much water, and when the wind off the sea moves over hot land it holds more tightly on to what little water vapor it has. This has produced the coastal strip deserts of Peru and Namibia (Chapter 1).
Was this article helpful?