“Monsoon,” derived from an Arabic word for “season,” refers to a wind
system with pronounced seasonal reversal. Monsoons develop in response
to temperature variations between continents and oceans. The southeast
trade winds of the Indian Ocean, for example, provide heavy summer rains
in India as they move onshore. As the monsoon crosses India, it loses
moisture on the eastern slopes of the Aravalli Range. The Rajasthan
Desert of India and the Thar Desert of Pakistan are parts of a monsoon
desert region west of the range.
[Illustration: The Indus River floodplain, lower left, is the western
border of the Thar Desert. This Landsat image of the monsoon desert
shows small patches of sand sheets in the upper right, with three types
of dunes; some dunes are almost 3 kilometers long.]