A dry place
What is a desert?
USGS describes desert as arid land with meager rainfall that supports only sparse vegetation and a limited population of people and animals. USGS Britannica describes a desert as any large, extremely dry area of land with sparse vegetation. Britannica
Desert environments are so dry that they support only extremely sparse vegetation; trees are usually absent. Britannica Deserts are always dry. USGS Deserts are natural laboratories in which to study the interactions of wind and sometimes water on the arid surfaces of planets. USGS
A useful rainfall system
In 1953, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. USGS In Meigs's widely accepted system, extremely arid lands have at least 12 consecutive months without rainfall. USGS Arid lands have less than 250 millimeters of annual rainfall. USGS Semiarid lands have a mean annual precipitation of between 250 and 500 millimeters. USGS Arid and extremely arid land are deserts, and semiarid grasslands generally are referred to as steppes. USGS
How dry is "desert"?
USGS, following Peveril Meigs, treats arid lands as those with less than 250 millimeters of annual rainfall, and extremely arid lands as those with at least 12 consecutive months without rain. The Britannica account we checked defines a desert by extreme dryness and sparse vegetation, and does not use that millimetre cutoff in the extract. We keep the USGS figures, and we do not invent a different numerical cutoff from elsewhere.
What about cold deserts?
According to some definitions, any environment almost completely free of plants is considered desert, including regions too cold to support vegetation, called "frigid deserts." Britannica Other definitions apply the term only to hot and temperate deserts, a restriction followed in the Britannica account checked here. Britannica Polar deserts are given a mean annual precipitation less than 250 millimeters and a mean temperature during the warmest month of less than 10 degrees C. USGS Polar deserts on the Earth cover nearly 5 million square kilometers and are mostly bedrock or gravel plains. USGS
Are polar deserts "deserts"?
USGS includes polar deserts, giving them a mean annual precipitation less than 250 millimeters and a mean temperature during the warmest month of less than 10 degrees C, and says they cover nearly 5 million square kilometers. Britannica notes that some definitions include "frigid deserts," but the account we checked restricts the term to hot and temperate deserts. We report both approaches.
Read on for why deserts form, or skip to the types.