Polar deserts are areas with annual
precipitation less than 250 millimeters and a mean
temperature during the warmest month of less than 10 °C. Polar
deserts on
Earth cover nearly 5 million square kilometers and are mostly hard
bedrock or
gravel plains.
Sand dunes are not prominent features in these deserts, but snow dunes occur commonly in areas where precipitation is locally more abundant. Temperature changes in polar deserts frequently cross the freezing point of water. This "freeze-thaw" alternation forms patterned textures on the ground, as much as 5 meters in diameter.