The Arctic and Antarctic regions are classified as deserts due to their extremely low levels of precipitation. Although many people associate deserts with hot, sandy environments like the Sahara, the technical definition of a desert is based on aridity, or dryness, rather than temperature.
In polar areas, the climate is cold and dry, leading to very little moisture in the air and, consequently, very little precipitation—often less than 10 inches (25 cm) annually. Most of this falls as snow. The frozen ground in these regions retains very little moisture, which means that even though the landscape may seem vast and populated with ice, it does not receive enough precipitation to meet the definition of a non-desert environment.
Additionally, the cold temperatures mean that the air is often too dense to hold much moisture, resulting in clear skies and further limited precipitation. This combination of low precipitation and cold conditions is what categorizes both the Arctic and Antarctic as deserts.