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 the Earth cover nearly 5 million square
kilometers and are mostly 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.
[Illustration: The Dry Valleys of Antarctica have been ice-free for
thousands of years (courtesy of USGS Image Processing Facility,
Flagstaff, Arizona).]