A desert climate is a climate in which potential evaporation substantially exceeds precipitation over the long term, producing very dry conditions and limited naturally occurring vegetation. In the Köppen climate classification, desert climates are represented by the BWh category in hot deserts and BWk in cold deserts, with classification determined by temperature and precipitation thresholds. Desert climates can occur in tropical, subtropical, temperate and even high latitude environments. The defining characteristic is aridity rather than heat. A desert can therefore be extremely hot, relatively cool or cold depending on latitude, altitude, season and atmospheric circulation.
Desert aridity develops when atmospheric conditions limit precipitation and maintain a strong water deficit. Subtropical deserts are often associated with descending air within the Hadley circulation. Descending air warms and dries, suppressing cloud development. Other deserts form in the rain shadow of mountains, where moist air loses much of its water on the windward side before descending on the leeward side. Coastal deserts can develop where cold ocean currents stabilise the atmosphere and reduce rainfall. Continental interiors can become dry because moisture bearing air masses lose water before reaching the interior.
Temperature varies greatly among deserts. Hot deserts such as the Sahara experience very high daytime temperatures because intense solar radiation, clear skies and limited vegetation allow strong surface heating. Dry air and clear skies can also permit rapid radiative cooling after sunset, producing large daily temperature ranges. Cold deserts can experience freezing temperatures and snowfall, particularly at high elevations or latitudes. Even hot deserts can become cold at night or during winter. Consequently, desert climate should be defined by persistent aridity rather than by a fixed temperature threshold in ordinary geographical usage.
Rainfall in desert climates is usually low, irregular and highly variable in both space and time. Some deserts receive less than 100 millimetres of precipitation annually, while others classified as deserts receive somewhat more but remain strongly water limited. A year can be unusually wet followed by several very dry years. Rain may fall in intense short lived storms rather than gentle widespread precipitation. Because soils and surfaces are often dry and infiltration capacity can vary, intense storms can produce flash floods despite very low annual rainfall. Desert rivers may therefore experience dramatic changes in discharge.
Desert landscapes contain diverse landforms produced by wind, water, weathering, salt processes and gravity. Sand dunes form where wind transports and deposits loose sediment, but many deserts are dominated by gravel plains, rocky surfaces, alluvial fans, mesas, plateaus or salt flats rather than sand seas. Temporary streams can carve channels and wadis, while flash floods deposit sediment in alluvial fans and basins. Chemical weathering can be limited by dryness, but salt weathering and thermal stress can contribute to rock breakdown. Wind erosion can remove fine particles and produce deflation surfaces.
Desert ecosystems are adapted to limited and unpredictable water availability. Plants may have deep or extensive root systems, reduced leaves, water storage tissues or seasonal dormancy. Animals may be nocturnal or crepuscular to avoid extreme daytime heat, and many obtain much of their water from food or metabolic processes. Biological productivity is generally low compared with humid ecosystems, but desert biodiversity can still be substantial and highly specialised. Short periods of rainfall can trigger rapid plant growth and reproduction, producing temporary pulses of biological activity.
Human settlement in desert climates depends strongly on access to reliable water. Oases develop where groundwater reaches the surface or can be accessed relatively easily, while rivers such as the Nile support intensive agriculture across otherwise arid landscapes. Modern technologies permit irrigation, groundwater pumping, desalination and long distance water transfer, but these can create sustainability problems. Over irrigation can cause salinisation, while excessive groundwater extraction can lower water tables. Mining, energy production, tourism and transport are also important desert activities.
Desert climate differs from semiarid steppe climate because the latter receives more precipitation relative to potential evaporation and generally supports greater natural vegetation cover. Desertification is also not synonymous with desert climate. Desertification refers to land degradation and declining biological productivity in drylands and can occur in areas that are not naturally classified as deserts. The major controls of desert climate include atmospheric circulation, latitude, altitude, ocean currents, topography and continentality. Understanding those controls explains why major deserts occur in apparently very different parts of the world.
