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The Himalayas isolate South Asia from
the rest of Asia. South of these mountains, the climate,
like the terrain, is highly diverse, but some
geographers give it an overall, one word
characterization violent. What geographers have in mind
is the abruptness of change and the intensity of effect
when change occurs the onset of the monsoon rains,
sudden flooding, rapid erosion, extremes of temperature,
tropical storms, and unpredictable fluctuations in
rainfall. Broadly speaking, agriculture in India is
constantly challenged by weather uncertainty.
It is possible to identify seasons, although these do
not occur uniformly throughout South Asia. The Indian
Meteorological Service divides the year into four
seasons: the relatively dry, cool winter from December
through February; the dry, hot summer from March through
May; the southwest monsoon from June through September
when the predominating southwest maritime winds bring
rains to most of the country; and the northeast, or |
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retreating, monsoon of October
and November.
The southwest monsoon blows in from sea to land. The
southwest monsoon usually breaks on the west coast early
in June and reaches most of South Asia by the first week
in July (see fig. 6). Because of the critical importance
of monsoon rainfall to agricultural production,
predictions of the monsoon's arrival date are eagerly
watched by government planners and agronomists who need
to determine the optimal dates for plantings.
Theories about why monsoons occur vary. Conventionally,
scientists have attributed monsoons to thermal changes
in the Asian landmass. Contemporary theory cites other
factors the barrier of the Himalayas and the sun's
northward tilt (which shifts the jet stream north). The
hot air that rises over South Asia during April and May
creates low-pressure areas into which the cooler,
moisture-bearing winds from the Indian Ocean flow. These
circumstances set off a rush of moisture-rich air from
the southern seas over South Asia.
The southwest monsoon occurs in two branches. After
breaking on the southern part of the Peninsula in early
June, the branch known as the Arabian Sea monsoon
reaches Bombay around June 10, and it has settled over
most of South Asia by late June, bringing cooler but
more humid weather. The other branch, known as the Bay
of Bengal monsoon, moves northward in the Bay of Bengal
and spreads over most of Assam by the first week of
June. On encountering the barrier of the Great Himalayan
Range, it is deflected westward along the Indo-Gangetic
Plain toward New Delhi. Thereafter the two branches
merge as a single current bringing rains to the
remaining parts of North India in July. |
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