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India Climate
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.Indian seasons : 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 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.