A seasonal wind, especially in south
Asia. In summer, which is the monsoon season, the winds normally blow from
the sea to the land and bring rain, but in winter there is a complete change
of direction and the winds blow out from the land, giving dry weather. Some
monsoon regions are very wet. Cherrapunji in India, for example, receives
over 11,000 mm (433 in) per year, but others can be dry, such as the Thar
Desert between India and Pakistan where the annual rainfall is less than
250 mm (10 in). The major monsoon areas are in Asia, where the seasonal
reversal of wind is greatest. This is because the largest continent, Asia,
is adjacent to the largest ocean, the Pacific. In the small continents of
South America, Africa, Australia, and North America, the monsoonal effects
are less marked. These small continents do not have such wet summers or
such dry winters, and are sometimes referred to as "eastern marginal" rather
than true monsoon.
Caused by low pressure areas over land masses in the summer (see diagram),
monsoons bring wet winds from the sea, and high pressure areas in the winter
causing dry winds to blow from the land. During the summer (top) large areas
of mainland Asia are heated by the Sun. The air over these regions expands
and rises forming regions of low pressure. Wet winds from the sea then blow
into these areas giving the summer monsoons. In the winter (bottom), the
situation is reversed and regions of high pressure are formed over the land.
Dry winter monsoon winds then blow out to sea. The paths of these winds
are deflected due to the Coriolis effect.
PHENOMENA AND STRUCTURES