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| The word monsoon is derived from
the Arabic word 'mausim', meaning season. It
was first used by Arabic navigators to describe
the seasonal winds of the Arabian Sea. These
generally blow from the north-east for one half
of the year, and from the south-west for the
other half. Although the term monsoon actually
means a seasonal wind, it is often used to refer
to the torrential rainfall associated with these
winds. |
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Monsoons occur mainly in tropical regions - northern
Australia, Africa, South America and the
USA. However, the best known area affected
by monsoons is south-east Asia, particularly
India. During the winter, air over the Siberian
plateau becomes colder than air over the
surrounding seas, producing a large anticyclone
with winds circulating clockwise, thus causing
cool north-easterly winds to blow across
India and its neighbouring countries. This
brings dry, pleasant weather, and has a marked
drying effect on the land. During April and
May the winds abate, causing temperatures
to rise rapidly to over 35 °C.
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In the summer the process reverses. The Siberian
plateau is now warmer than the seas, and
low pressure develops over these seas. The
winds circulate anticlockwise and approach
India from the south-west, bringing very
moist air. These south-westerly winds bring
a drop in temperature and heavy downpours
of rain. In fact, during this monsoon, which
generally lasts from June to September, India
receives virtually all its rainfall for each
year.
The mountains of southern India split the
summer winds. The western arm of the monsoon
is deflected northwards, by the western Ghats,
to Bombay and then on to Pakistan. The eastern
arm travels up through the Bay of Bengal
to Calcutta and Assam, and is deflected north-westwards
by the Himalayas. On average, the winds arrive
in southern India about six weeks before
they arrive in north-west India. |
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Variation of rainfall by location |
The heaviest
monsoon rainfalls occur where the winds blow
side-on to the hills. The higher the hills
and more moist the air, then the greater
the amount of rainfall. These factors give
Cherrapunji, in Assam, one of the highest
rainfalls in the world; the western monsoon
winds having travelled an extra distance
over the warm seas of the Bay of Bengal,
then meeting the Himalayas. On average, Cherrapunji
has an annual rainfall total of nearly 11
metres, the maximum monthly amount occurring
in June. Bombay, in the eastern monsoon,
receives about 1.8 metres with the maximum
monthly total in July. In comparison, Delhi
registers only 64 cm of rainfall each year
(about the same as London), with the maximum
monthly total occurring in both July and
August. At Madras the pattern of rainfall
is different because the monsoon winds blow
along the coast. Here, the rainfall increases
gradually through the summer months with
larger amounts falling in October and November,
owing to tropical cyclones travelling westwards
across the Bay of Bengal. |
Monsoon
hazards
Monsoon rainfalls are unreliable in that
the amount varies considerably from year
to year. Low rainfalls cause great problems
for agriculture and water supplies in general.
On the other hand, even moderate rainfalls
can cause flood hazards. The eastern monsoon
releases most of its rainfall in the Ganges
plain, causing flooding to low-lying areas
where the river flows into the Bay of Bengal.
In the Indus river the flood problem is
often made worse because the monsoon rainfalls
can coincide with high river levels in
its tributaries, caused by water from the
melting mountain snows of the Himalayas. |
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