Hot deserts cover about 14 percent of Earth's land, the biggest climate zone
The hot desert is the most widespread climate on land, covering roughly 14.2% of Earth's land area and found on every continent except Antarctica. What makes a desert isn't heat, though, but evaporation outrunning rain, which is why the Gobi, with its freezing winters, counts too.
Climatologists using the Köppen system draw the line with a formula. Multiply the average yearly temperature in Celsius by 20, then add 280 if at least 70% of the rain falls in the summer half of the year, 140 if between 30% and 70% does, or nothing otherwise. A place getting less than half that figure in annual rainfall is a desert; between half and the full amount makes it semi-arid. A mean annual temperature of 18 °C then splits hot deserts from cold ones.
Most arid places receive between 25 and 200 mm of rain a year, though extreme evaporation lets parts of Central Australia and the Sahel qualify with as much as 430 mm. At the other extreme, Chile's Atacama averaged just 5 mm over 17 years, and Kufra in Libya records 0.86 mm. Death Valley officially gets 60 mm, yet over 40 months from 1931 to 1934 it collected only 16 mm in total.
Hot deserts sit mostly between about 20 and 33 degrees north and south, under a belt of sinking air and high pressure that sweeps away clouds. Summer months usually average 29 to 35 °C, with midday readings of 43 to 46 °C common, and Death Valley holds the highest temperature on record, 56.7 °C. Dallol in Ethiopia averages close to 35 °C across the whole year. Clear skies let heat escape at night, so winter nights can dip to freezing, though rarely much below.
Cold deserts usually hide in the rain shadow of tall mountains at higher altitudes, in the 30s and 40s of latitude. Patagonia lies in the lee of the Andes, and Central Asia's Taklamakan and Kyzyl Kum are cut off from monsoon rains. Summers can still be hot, but winters are cold and dry, and snow is rare. Even Antarctica's McMurdo Dry Valleys show desert features, though their chill puts them in the polar category.
Source: Desert climate
