These dunes look like motionless waves . The shapes of these dunes depend on the strength of the wind and the amount of sand it carries. Dunes containing less sand tend to curve, while dunes containing more sand tend to be long and rippling with deep troughs cut between them. While the lighter parts of the sediments are blown away by the wind, causing dunes, the heavier materials are left behind and form a rubbly surface known as a desert pavement , or a heavy mass. This mass causes the remaining sand and dust to be sealed off from the wind.
Winds also form thick clouds of sand.
These clouds of sand look like huge dark waves and block out the sun, acting like a thick fog. The winds swirl about catching sand, and form columns called dust devils . Over time, when all the sand is blown away from the area, it leaves behind only rocks. In polar deserts , snow dunes occur commonly in areas where precipitation is locally more abundant. Temperatures change frequently, crossing the freezing point of water.