Polar deserts are areas with annual precipitation less than 250 millimeters and a mean temperature during the warmest month of less than 10Ã¸C. Cold dry air prevails and moisture available remains frozen throughout the entire year. Polar deserts on the Earth cover nearly 5 million square kilometers and are mostly bedrock or gravel plains. 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.