When air masses are forced over mountains and downslope, they warm and their capacity for holding water vapor increases. Evaporation exceeds precipitation and an arid environment or rainshadow is created on the leeward side. Interiors of continents. Usually in combination with the rainshadow effect, distance from a major source of moist air results in dry climates in the interior of a land mass. The Great Basin desert of the US, the Australian desert, and the Gobi desert of Mongolia can all be explained in large part to their interior positions.
Potential evaporation exceeds precipitation in the annual water budget.
Furthermore, rainfall is highly localized and relatively unpredictable in terms of when it will occur, although usually there are seasons of highest probability for precipitation. Annual variation in total precipitation may also be great. Temperatures are also variable. Shrubs are the dominant growthform of deserts. Shallow but extensive root systems procure rainwater from well beyond the canopy of the shrub whenever it does rain.