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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 |
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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 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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