DEFINITION OF MONSOON DESERTS

In some places on the planet seasonal variations in wind systems

bring alternating dry and wet seasons. The Indian

Ocean is famous for its monsoonal rains in the summer as the

southeast trade winds bring moist air on shore. However, as

the moisture moves across India it loses moisture and must

rise to cross the Aravalli Mountain Range. The Thar Desert

of Pakistan and the Rajasthan Desert of India are located on

the lee side of these mountains and do not generally receive

this seasonal moisture.

A final class of deserts is the polar desert, found principally in

the Dry Valleys and other parts of Antarctica, parts of Greenland,

and northern Canada. Approximately 3 million square

miles (7.8 million km2) on Earth consist of polar desert environments.

In these places, cold downwelling air lacks moisture,

and the air is so dry that the evaporation potential is

much greater than the precipitation. Temperatures do not

exceed 50°F (10°C) in the warmest months, and precipitation

is less than one inch per year. There are places in the Dry Valleys

of Antarctica that have not been covered in ice for thousands

of years.

Polar deserts are generally not covered in sand dunes but

are marked by gravel plains or barren bedrock. Polar deserts

may also have landforms shaped by frost wedging, where

alternating freeze-thaw cycles allow small amounts of water

to seep into cracks and other openings in rocks. When the

water freezes it expands, pushing large blocks of rock away

from the main mountain mass. In polar deserts and other

regions affected by frost wedging large talus slopes may form

adjacent to mountain fronts, and these are prone to frequent

rock falls from frost wedging.

See also SAND DUNES; SAND SEA.

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