Climatic Characteristics of South Asia

A distin­guishing characteristic of South Asia’s climate is the prevalence of monsoonal (from the Arabic word mausim meaning “season”), or seasonal wind systems, the al­ternating dry cold continental winds in winter and moisture-laden, tropical, oce­anic winds in summer. The term monsoon is applicable to most, but not all, parts of the subcontinent.

The idealized perception of the monsoon climate best applies to the areas of northern India and Pakistan. In southern India, in Kerala, and in south­western Sri Lanka, a tropical rain forest climate prevails. Along the southern parts of the eastern coast, the monsoon is not dry but crossing the Bay of Bengal brings rainfall.

In Pakistan and the Punjab plains, rainfall is brought by eastward moving cy­clonic disturbances originating over the Mediterranean basin during winters. These exceptions apart, the monsoonal winds have almost a universal effect in bringing summer rains and are of great human sig­nificance. Most wet season crops depend on rainfall associated with the monsoon winds.

A convenient division of the calendar in areas where monsoons have their great­est impact is:

(a) The rainy season of southwest summer monsoons,

(b) The dry winter season of northeast monsoons, and

(c) The intervening hot season between winter and arrival of the summer mon­soons.

The hot season in the subcontinent begins in March and lasts until June. From March to May, during the northward movement of the path of the sun’s vertical noon rays, the increasing length of day causes a belt of intense solar radiation to move progressively northward over the tropical areas of South Asia, advancing by June into the extra-tropical zone. By June the highest daytime temperatures 104°- 113°F (40°-45°C) are recorded in central Deccan and the Indus-Ganga plains. The highest temperatures are, however, re­corded in the Thar Desert and on the Indus Plain, and occasionally in the middle Ganga plains.

During summers winds move quickly into peninsular India and Bangladesh to­ward the low-pressure system of northwest India from the Arabian Sea and Bay of Bengal. The moisture carried by these southwesterly winds from the Arabian Sea and south and southeasterly winds from the Bay of Bengal bring rain to the subcon­tinent, leaving Tibet dry in the “rain shadow” of the Himalaya.

The rainy season sets in by the end of June and lasts until the end of August. This is the “sum­mer monsoon.” The amount of summer rainfall varies in the different parts of the subcontinent, depending on their distance from the coast or from the Himalaya, their altitudes, the direction of the monsoon winds and whether the locations lie on the windward or leeward side of mountains.

A part of the Bay of Bengal monsoon stream which strikes the Himalaya after crossing the Ganga delta, is uplifted, and sheds copi­ous precipitation. The lofty Himalaya also diverts the moist stream westward into the Ganga plains of north India. This diver­sion or channeling effect carries rainfall farther west.

As the monsoon stream moves westward in the Ganga plains, it continues to lose moisture, becoming in­creasingly dry. Most places in the middle Ganga plains receive between 25 and 50 inches (625-1,250 millimeters) of rainfall during the rainy season. In the west of pen­insular India most moisture is shed between the sea and the crest of the west­ern Ghats is reached, leaving the Deccan Plateau practically rainless in the “rain- shadow” of the mountains.

By September the systems change, re­sponding to southward movement of the sun toward the Southern Hemisphere. The development of a high-pressure system over the subcontinent sets in motion a stage for this retreat of the monsoon winds. Winds are now northeasterly and are dry and bring rain only to a southern section of the eastern coast, after picking up moisture while crossing the Bay of Ben­gal.

From October until March is the cool season when the entire subcontinent expe­riences mild and sunny weather. The mean January temperature is about 70°F (21°C) in southern India and 60°F (15.5°C) in Pakistan and north Indian plains. In the northwest occasional light frosts may also occur. In the Himalaya, temperatures de­crease with altitude.

Historical evidence indicates that an al­most continuous expanse of forest covered much of north India at the time of Alexan­der’s invasion in the 4th century B.C. Even during the 10th to 14th centuries, the Ganga plain contained vast patches of for­est. Human modification of interference with the “original” plant cover has re­sulted in its removal or degradation, creating serious problems of soil erosion and the growth of stunted “jungle” vegeta­tion (thickets, bushes and dwarf trees). At present, forests occupy only about 15 per­cent of the total land of the subcontinent.

Natural vegetation now exists only in a few areas at high altitudes or coastal swamps, or in the “protected” forest re­serves under government control. Of the several forest types identified, three major vegetation zones are: an evergreen broad- leaf forest, deciduous broadleaf forest and thorny scrub, with their distribution re­lated to climatic conditions and altitude.

Evergreen broadleaf trees are located mostly along the western coastal lowlands, Bengal and Sri Lanka, in areas of annual rainfall of more than 60 inches (1,500 milli­meters). The deciduous broadleaf zone is located in areas with annual rainfall of 750 to 2,000 millimeters falling mainly be­tween June and October. The thorny scrub zone lies in semi-arid areas in Paki­stan, peninsular India and parts of interior Sri Lanka. The Thar Desert supports some shrubs and grasses.

Soils follow the climatic and vegeta­tion zones. One-third of South Asia is covered with water and wind transported surface materials showing little profile de­velopment. About 20 percent is covered with alluvial (river-borne) sands, silts, clays, or gravels, deposited by the Indus, Ganga and other rivers. The Indus-Ganga deposits are among the thickest in the world, attaining thicknesses of several thousands of feet and are very fertile.

In the northwestern and north central part of peninsular India dark-colored regur soils are found. These are derived from the Dec- can Lavas. Rich in nutrient-content they are used widely for growing such crops as cotton and millets. In areas where annual rainfall is more than 60 inches (1,500 milli­meters), laterites are found. Laterite may be used as building material and for road surfacing.