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.