Desert Climate

A desert climate (in the Köppen climate classification BWh, BWkl, 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 scrub.

An area that features this climate usually experiences less than 250 mm (10 inches) per year of precipitation and in some years may experience no precipitation at all. In some instances, an area may experience more than 250 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 (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 "BS" type climate with the appropriate temperature above whichever isotherm is being used is classified as "hot semi-arid" (BWh), and a location with the appropriate temperature below the given isotherm is classified as "cold semi-arid" (BWk).

To determine whether a location has an arid climate, the precipitation threshold must first be determined. Finding the precipitation threshold (in millimeters) 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).