Most of the world's deserts are situated in between the latitudes of 15° and 35° on each side of our Earth's equator respectively. These zones are characterized by substantial atmospheric pressure which is those areas where cool air falls and becomes warmer. These zones characterized by high pressure are established by the way in which the air circulates over the world. Air will become warm and continuously rise at the equator in which temperatures are the highest. When the air ascends it will become cool thus emitting moisture over areas close to the equator. In time, the air will fall over the areas that extend between 15° and 35° both south and north of our Earth's equator.
As the air begins to fall it will become drier and warmer. It is this warm air that formulates our desert conditions. For example, the Sahara desert and several other large deserts will tend to lay within this area. Those areas that are separated by an ocean and by mountains will also tend to be much drier. Coming from the ocean, this air is blown towards in-land which loses its moisture as it goes over mountains and thus becomes much cooler. When this wind begins to fall on the side of the mountain face the wind will become dry and warm. This warmer air establishes a rain shadow or otherwise known as a dry area.
The deserts found in North America have been known to have formed largely in part due to the rain shadow principle. Further, oceans that tend to be flowing cold water beside a continent can lead to deserts being formed in areas alongside this continents coast. These deserts will be established because this cold wind will blow across the cool water and make its way onto the land which will carry very miniscule amounts of moisture, for example, the Atacama Desert situated in South America.
When there is a substantial fluctuation in climate this can lead to a change in the location of the desert, as well the conditions found within a desert. For instance, over the last few thousand years there've been varied amounts of deserts that have been created as our Earth's climate has changed from cold and wet, to dry and warmer. Scientists would also argue that our activities as humans have also led to the change and eventual expansion of desert regions throughout the world; often when expansion happens leading to the impactful loss of fertile regions along the outside of desert regions.
This loss mainly occurs when overgrazing takes place, which means simply that livestock induced grazing as well those lives of plants are abolished. Without the sheath of plant life, water, and wind, will lead to increases in soil erosion. The switch from fertile to dry land is the process known as desertification.