Vegetation found in deserts is often growing close to the limits of tolerance, so even slight changes in environment can lead to degradation.
[Tropical deserts]
have arid tropical climates, generally lying between 15ø and 30ø latitude where atmospheric circulation brings dry, subtropical air into mid-latitudes; examples are the Sahara and the Kalahari in Africa, the Sonoran in northern Mexico and the southwestern United States, the deserts of Australia, and those of Arabia, Iran, and Pakistan in southwestern Asia.
[Continental deserts]
are areas in the continental interiors, far from source of moisture where hot summers and cold winters prevail; examples are: Gobi, Takla Makan.
[Rainshadow deserts]
are areas where mountainous regions cause air to rise and condense, dropping its moisture as it passes over the mountains; examples are deserts east of the Sierra Nevada Mountains, California and Nevada, east of the Cascades of Oregon and Washington, and east of the Andes Mountains in South America.
[Coastal deserts]
result from the influence of the cool ocean currents and dry winds on the eastern sides of the oceanic subtropical high-pressure centers. The cold upwelling sea water cools the air and decreases its ability to hold moisture; examples are Atacama Desert of coastal Peru and the Namib Desert of coastal South Africa.
Deserts are areas where rainfall is less than 250 mm per year, or where evaporation exceeds precipitation. Deserts originate by several different mechanisms that result in different types of deserts. There are several classification of deserts: Subtropical deserts, Continental deserts, Rainshadow deserts, Coastal deserts and Polar deserts.