Changes

Evaporation contributes greatly to a desert's aridity. In some deserts, the amount of water evaporating exceeds the amount of rainfall making that desert very dry. Rising air cools and can hold less moisture, producing clouds and precipitation; falling air warms, absorbing moisture. Areas with few clouds, bodies of water and little vegetation absorb most of the sun's radiation, thus heating the air at the soil surface. More humid areas deflect heat in clouds, water and vegetation, remaining cooler. High wind in open country also contributes to evaporation.

Evaporation contributes greatly to a desert's aridity. In some deserts, the amount of water evaporating exceeds the amount of rainfall making that desert very dry. Rising air cools and can hold less moisture, producing clouds and precipitation; falling air warms, absorbing moisture. Areas with few clouds, bodies of water and little vegetation absorb most of the sun's radiation, thus heating the air at the soil surface. More humid areas deflect heat in clouds, water and vegetation, remaining cooler. High wind in open country also contributes to evaporation.

Locations of deserts have changed throughout the years. Modern desert regions are centered in the [[horse latitudes]], typically straddling the [[Tropic of Cancer]] and the [[Tropic of Capricorn]], between 15 and 30 degrees north and south of the [[equator]]. Some deserts, such as the [[Kalahari]] in central [[Africa]], are very old. The [[Sahara Desert]] in northern Africa quite old, as is the [[Sonoran Desert]] of North America.

Locations of deserts have changed throughout the years. Modern desert regions are centered in the [[horse latitudes]], typically straddling the [[Tropic of Cancer]] and the [[Tropic of Capricorn]], between 15 and 30 degrees north and south of the [[equator]]. Some deserts, such as the [[Kalahari]] in central [[Africa]], are very old. The [[Sahara Desert]] in northern Africa quite old, as is the [[Sonoran Desert]] of North America. Deserts can also occur at near-polar latitudes, for example the interior of [[Iceland]].

Because they are poised in such harsh extremes of heat and aridity, deserts are among the most fragile [[ecosystem]]s on the planet.

Because they are poised in such harsh extremes of heat and aridity, deserts are among the most fragile [[ecosystem]]s on the planet.