Despite their geographical and biological differences, the Polar Regions have a lot in common, namely the vast amounts of snow and ice.
Snow is precipitation in the form of ice crystals. Snow is formed in clouds when it is cold enough for water vapor to condense directly into ice without passing through the liquid phase. Formation of snow requires two conditions: low temperatures and sufficient moisture in the air. A humid environment like the rainforest is moist enough, but not cold enough. A desert may get very cold, but is generally too dry to produce snow.
