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.
The withdrawal of the monsoon is a far more gradual process than its onset. It usually withdraws from northwest India by the beginning of October and from the remaining parts of the country by the end of November. During this period, the northeast winds contribute to the formation of the northeast monsoon over the southern half of the Peninsula in October. It is also known as the retreating monsoon because it follows in the wake of the southwest monsoon. The states of Tamil Nadu, Karnataka, and Kerala receive most of their rainfall from the northeast monsoon during November and December. However, 80 percent of the country receives most of its rainfall from the southwest monsoon from June to September.
South Asia is subject to a wide range of climates--from the subfreezing Himalayan winters to the tropical climate of the Coromandel Coast and from the damp, rainy climate in the states of Assam and West Bengal to the arid Great Indian Desert. Based on precipitation and temperature, experts define seven climatic regions: the Himalayas, Assam and West Bengal, the Indo-Gangetic Plain, the Western Ghats and coast, the Deccan (the interior of the Peninsula south of the Narmada River), and the Eastern Ghats and coast (see fig. 7).
In the Himalayan region, climate varies with altitude. At about 2,000 meters, the average summer temperature is near 18°C; at 4,500 meters, it is rarely above 0°C. In the valleys, summer temperatures reach between 32°C and 38°C. The eastern Himalayas receive as much as 1,000 to 2,000 millimeters more precipitation than do the Western Himalayas, and floods are common.
Assam and West Bengal are extremely wet and humid. The southeastern part of the state of Meghalaya has the world's highest average annual rainfall, some 10,900 millimeters.
The Indo-Gangetic Plain has a varied climatic pattern. Rainfall and temperature ranges vary significantly between the eastern and western extremes (see table 2, Appendix). In the Peninsula region, the Western Ghats and the adjoining coast receive heavy rains during the southwest monsoon. Rainfall in the peninsular interior averages about 650 millimeters a year, although there is considerable variation in different localities and from year to year. The Eastern Ghats receive less rainfall than the western coast. Rainfall there ranges between 900 and 1,300 millimeters annually.
The northern Deccan region, bounded by the Western Ghats, the Vindhya Range and the Narmada River to the north, and the Eastern Ghats, receives most of its annual rainfall during the summer monsoon season. The southern Deccan area is in a "rain shadow" and receives only fifty to 1,000 millimeters of rainfall a year. Temperature ranges are wide--from some 15°C to 38°C--making this one of India's most comfortable climatic areas.
Throughout most of non-Himalayan India, the heat can be oppressive and sometimes, such as was experienced in 1994 and 1995, literally can be a killer. Hot, relatively dry weather is the norm before the southwest monsoons, which, along with heavy rains and high humidity, bring cloud cover that lowers temperatures slightly. Temperatures reach the upper 30s°C and can reach as high as 48°C during the day in the premonsoon months.
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