Deserts it is accepted to call geographical areas in which within a year less than 200 mm of precipitation drop out. Also in deserts extremely dry air and high average monthly temperature. These are facts of common knowledge. But very few people know how there was a formation of deserts.
Formation of deserts happened because of uneven distribution of moisture and heat. Over the equator, air heats up stronger and rises up. In process there is its cooling that leads to loss of a large amount of moisture. Just moisture drops out on the earth in the form of precipitation – tropical heavy rains. It turns out that in an upper atmosphere the equatorial air is distributed to the North and the South. After a while air masses fall by the Earth's surface which is very strongly heated. But there is no moisture in this masses any more. Similar circulation of air masses happens during the whole year.
Because of such circulation air is very strongly heated. That is why average temperature in the desert reaches forty degrees in a shadow in the summer. Sometimes it rises almost up to 60 wasps. As for a soil surface, it can heat up to 80 wasps and keep this temperature throughout long time. Precipitation in the desert happens extremely seldom and that generally the strongest heavy rains. Just small precipitation cannot reach the Earth's surface. Because of high temperature water evaporates in air.
The driest areas of our planet can be considered the deserts of South America. For example, on the Pacific coast only one millimeter of precipitation a year drops out. It catastrophically is not enough. And in the valley of the Nile River in four last years there was no rain. There are such natural anomalies. Most often precipitation drops out in deserts in the spring and in the winter. But in some precipitation happens also in summertime of year.
In the evening the sun leaves for the horizon and air temperature in deserts on average falls by thirty degrees. If to speak about the soil, then in a day it heats up much stronger than air. But also cooling of the soil happens more quickly. In the morning on a surface dew can act. And become covered winter of the desert by rather thick frost deposit. Deserts can arise not only in subtropics, but also in a moderate belt in especially droughty territories. Central Asia means. Here about 200 millimeters of precipitation a year drop out. Though the amount of precipitation can be less. Continuous air circulation and specific geographical conditions led to formation of a zone of deserts to the South and the North from the equator. The most part of deserts is surrounded by the massif. By the way, mountains supply deserts with water. The rivers flow down from slopes and irrigate foothill plains. Then they completely vanish in sands.