General Monsoon Circulation
Monsoons, like most other winds, occur in response to the sun heating the atmosphere. In their simplest form, monsoons are caused by differences in solar heating between the oceans and continents, and they are most likely to form where a large continental land mass meets a major ocean basin. During the early summer, the increasing solar energy heats up the land surfaces fairly quickly. Water, on the other hand, heats much more slowly in response to the sun. This is one reason why we cool off by swimming in lakes during the summer—the water is still chilled from the recent winter and takes much of the summer to warm up. The enormous quantity of water in the oceans guarantees they will remain cooler than the nearby continents during the early summer. The relatively warm land surface will heat the air over it, causing it to rise, or convect. The convection of warm air produces an area of low pressure near the land surface. Meanwhile, the air over the cooler ocean will be more dense and tend to stay at the surface or sink downwards from aloft. Thus during the summer, oceanic air flows onshore toward the low pressure over land. This onshore flow is continually supplied by cooler oceanic air sinking from higher levels in the atmosphere. In the upper atmosphere, the rising continental air is drawn outward over the oceans to replace the sinking oceanic air, thus completing the cycle. In this way a large vertical circulation cell is set up, driven by solar heating. At the surface, the result is a constant wind flowing from sea to land.
The oceanic air moving onto the land is usually quite humid, due to its prolonged contact with the sea surface. As it flows on shore the moist marine air is pulled upward as part of the convecting half of the circulation cell. The rising air cools and eventually forms rain clouds. Rain clouds are especially likely when the continental areas have higher elevations (mountains, plateaus, etc.), because the humid sea air is forced upward over these barriers, causing widespread cloud formation and heavy rains. This is the reason why the summer monsoon forms the rainy season in many tropical areas.
In the late fall and early winter, the situation is reversed. Land surfaces cool off quickly in response to cooler weather, but the same property of water that makes it slow to absorb heat, called heat capacity, also causes it to cool slowly. As a result, continents are usually cooler than the oceans surrounding them during the winter. This sets up a new circulation in the reverse direction: air over the sea, now warmer than that over the land, rises and is replaced by winds flowing off the continent. The continental winds are supplied by cooler air sinking from aloft. At upper atmospheric levels the rising oceanic air moves over the land to replace the sinking continental air. Sinking air prevents the development of clouds and rain, so during the winter monsoon continental areas are typically very dry. This winter circulation causes a prevailing land to sea wind until it collapses with the coming of spring.
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