El clima árido o clima desértico, es una expresión utilizada para designar el clima de una región del planeta donde las precipitaciones anuales son menores a los 200 mm y donde el modelo climático estudiado se caracteriza por sus escasas precipitaciones, por debajo de la evapotranspiración. Sin embargo, debido al nivel de evapotranspiración, esta cifra puede llegar a 250 mm anuales o incluso superarla ligeramente. Se debe a distintas causas, como la disposición del relieve o la presencia de corrientes marinas frías que evitan o limitan la evaporación y, por consiguiente, la humedad, y dan origen a desiertos costeros.El medio natural desértico se localiza en las proximidades de los trópicos de cada hemisferio, entre los 15 y 35 grados de latitud aproximadamente.

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Desert climate (in the Köppen climate classification BWh and BWk, sometimes also BWn), also known as an arid climate, is a climate that does not meet the criteria to be classified as a polar climate, and in which precipitation is too low to sustain any vegetation at all, or at most a very scanty shrub. An area that features this climate usually experiences from 25 to 200 mm (7.87 inches) per year of precipitation and in some years may experience no precipitation at all. Averages may be even less such as in Arica, Chile, where precipitation normals annually stand at around 1 mm per year. In some instances, an area may experience more than 200 mm of precipitation annually, but is considered a desert climate because the region loses more water via evapotranspiration than falls as precipitation (Tucson, Arizona and Alice Springs, Northern Territory are examples of this). There are usually two or three variations of a desert climate: a hot desert climate (BWh), a cold desert climate (BWk) and, sometimes, a mild desert climate (BWh/BWn). Furthermore, to delineate "hot desert climates" from "cold desert climates", there are three widely used isotherms: either a mean annual temperature of 18 °C, or a mean temperature of 0 °C or −3 °C in the coldest month, so that a location with a "BW" type climate with the appropriate temperature above whichever isotherm is being used is classified as "hot arid" (BWh), and a location with the appropriate temperature below the given isotherm is classified as "cold arid" (BWk). To determine whether a location has an arid climate, the precipitation threshold is determined. The precipitation threshold (in millimetres) involves first multiplying the average annual temperature in °C by 20, then adding 280 if 70% or more of the total precipitation is in the high-sun half of the year (April through September in the Northern Hemisphere, or October through March in the Southern), or 140 if 30%–70% of the total precipitation is received during the applicable period, or 0 if less than 30% of the total precipitation is so received. If the area's annual precipitation is less than half the threshold, it is classified as a BW (desert climate).

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