What Are Deserts?

Deserts are arid regions where precipitation is scarce enough that evaporation and transpiration would exceed it over the course of a typical year, supporting only sparse vegetation and limited populations of animals and people. The most widely used quantitative threshold treats land receiving less than about 250 millimeters of annual rainfall as arid, following the classification Peveril Meigs published in 1953, which separated extremely arid, arid, and semiarid lands. By the U.S. Geological Survey's account of what a desert is, roughly one-third of the Earth's land surface meets the definition.

Deserts are studied within physical geography and geomorphology, and they are of direct interest to geoscience and remote sensing because their sparse vegetation and low cloud cover make them the most consistently observable land surfaces from orbit. That same transparency makes them useful calibration targets for optical satellite sensors.

Classification and Distribution

Aridity has several distinct causes, and the resulting deserts differ in character. Subtropical deserts, including the Sahara and the Arabian, sit under the descending branches of the Hadley circulation near 30 degrees latitude, where sinking air warms and dries. Rain shadow deserts such as the Great Basin form on the lee side of mountain ranges that strip moisture from prevailing winds. Coastal deserts including the Atacama and the Namib lie next to cold upwelling currents that suppress convection. Polar deserts in Antarctica and the high Arctic are arid because cold air holds little moisture. The subtropical, rain shadow, and coastal cases appear in the Köppen scheme as the BW classes, subdivided into hot and cold variants, while the polar cases fall under the E climates, and the distinction matters for both ecology and engineering design temperature ranges.

Surfaces and Landforms

The popular image of an unbroken sand sea describes only a minority of desert terrain. Sand covers about 20 percent of desert surface, mostly concentrated in sand sheets and sand seas, while close to half of desert surfaces are plains where wind deflation has removed fines and left a lag of pebbles and cobbles known as desert pavement, as described in the USGS survey of desert features. Other characteristic landforms include playas that flood briefly and evaporate to leave evaporite crusts, alluvial fans and bajadas at mountain fronts, wadis and arroyos cut by infrequent flash floods, and inselbergs left standing above eroded plains. Wind-driven sediment transport, saltation, and creep organize sand into ripples and into dune forms whose geometry records the prevailing wind regime.

Dust, Climate, and Observation

Deserts are the dominant source of atmospheric mineral dust, which affects radiative transfer, cloud microphysics, ocean fertilization, and air quality far from the source region. A NASA analysis of CALIPSO lidar data collected between 2007 and 2013 put the average at 182 million tons of dust carried past the western edge of the Sahara each year, a large part of it transported over the North Atlantic. Satellite instruments such as MODIS and VIIRS track individual plumes in near real time, as shown in Earth Observatory coverage of Saharan dust storms. Desert surfaces also present high albedo and extreme diurnal temperature swings, both of which enter regional climate models and complicate the thermal design of equipment deployed in these environments.

Applications

Study of deserts supports work in a range of fields, including:

• Satellite radiometric calibration using stable bright desert targets
• Solar energy siting, where high direct normal irradiance meets dust soiling losses
• Mineral exploration, since arid erosion exposes economically valuable deposits
• Groundwater resource assessment and managed aquifer recharge
• Dust storm forecasting for aviation and air quality management
• Planetary geology, where terrestrial dune fields serve as analogues for Mars and Titan
• Infrastructure and materials engineering for high-temperature, abrasive environments