|
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 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. |
