Deserts are areas where the rainfall is too low to sustain any vegetation at all, or only very scanty scrub. The rainfall in desert areas is less than 250 mm or 10 inches per year, and some years may experience no rainfall at all. The hot deserts are situated in the subtropical climate zone where there is unbroken sunshine for the whole year due to the stable descending air and high pressure. Such areas include the Sahara, Saudi Arabia, large parts of Iran and Iraq, northwest India, California, South Africa and much of Australia. Here, maximum temperatures of 40 to 45C are common, although during colder periods of the year, night-time temperatures can drop to freezing or below due to the exceptional radiation loss under the clear skies.
The Gobi desert in Mongolia is an example of a cool desert. Though hot in summer, it shares the very cold winters of central Asia. The Arctic and Antarctic regions, too, receive very little precipitation during the year, owing to the exceptionally cold dry air, but are more usually classified as types of polar climate. Semi-desert areas include the Steppes of southern Russia and central Asia, and the Parries of Canada.