A monsoon is a seasonal change in the direction of the prevailing wind. This wind shift typically brings about a marked change in local weather. Monsoons are often associated with rainy seasons in the tropics (the areas of Earth within 23.5 degrees latitude of the equator) and the subtropics (areas between 23.5 and about 35 degrees latitude, both north and south). In these areas, life is critically dependent on the monsoon rains. A weak monsoon rainy season may cause drought, crop failures, and hardship for people and wildlife. However, heavy monsoon rains have caused massive floods that have killed thousands of people.
Many parts of the world experience monsoons to some extent. Probably the most famous are the Asian monsoons, which affect India, China, Japan, and Southeast Asia. Monsoons also impact portions of central Africa, where their rain is critical to supporting life in the area south of the Sahara Desert. Lesser monsoon circulations affect parts of the southwestern United States. These summer rainy periods bring much needed rain to the dry plateaus of Arizona and New Mexico.
Monsoons, like most other winds, occur in response to the Sun heating the atmosphere. In their simplest form, monsoons are caused by differences in temperatures between the oceans and continents. They are most likely to form where a large continental landmass meets a major ocean basin. During the early summer, the landmasses heat up more quickly than ocean waters. The relatively warm land surface then heats the air over it, causing the air to convect, or rise. The convection of warm air produces an area of low pressure near the land surface. Meanwhile, air over the cooler ocean waters is humid, more dense, and under higher pressure.
The atmosphere always tries to maintain a balance by having air move into areas of low pressure from surrounding areas of high pressure. This movement is known as wind. Thus during the summer, oceanic air flows toward the low pressure over land. This flow is continually supplied by cooler oceanic air sinking from higher levels in the atmosphere. In the upper atmosphere, the rising continental (landmass) 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.
Circulation cell: A circular path of air, in which warm air rises from the surface, moves to cooler areas, sinks back down to the surface, then moves back to near where it began. The air circulation sets up constant winds at the surface and aloft.
Convection: The rising of warm air from the surface of Earth.
Jet stream: High-speed winds that circulate around Earth at altitudes of 7 to 12 miles (12 to 20 kilometers) and affect weather patterns at the surface.
Subtropics: Regions between 23.5 and about 35 degrees latitude, in both the northern and southern hemispheres, which surround the tropics.
Tropics: Regions of Earth's surface lying within 23.5 degrees latitude of the equator.
As it flows onto shore, the moist ocean air is pulled upward as part of the convecting half of the circulation cell. The rising air cools and soon can no longer contain moisture. Eventually rain clouds form. Rain clouds are especially likely to occur when the continental areas have higher elevations (mountains, plateaus, etc.) because the humid ocean 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 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 (high pressure) 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.
The world's most dramatic monsoon occurs in India. During the early summer months, increased solar heating begins to heat the Indian subcontinent, which would tend to set up a monsoon circulation cell between southern Asia and the Indian Ocean. However, the development of the summer monsoon is delayed by the subtropical jet stream.
Jet streams are great rivers of air that ring Earth at levels in the atmosphere ranging from 7 to 8 miles (11 to 13 kilometers) above the surface. The subtropical jet stream is a permanent feature, flowing westerly (from west to east). It migrates over the year in response to the seasons, moving northward to higher latitudes in the summer and southward in the winter.
As summer progresses, the subtropical jet slides northward. The extremely high Himalayan mountains present an obstacle for the jet; it must "jump over" the mountains and reform over central Asia. When it finally does so, a summer monsoon cell develops. The transition can be very fast: the Indian monsoon has a reputation for appearing suddenly as soon as the subtropical jet stream is out of the way. As the air is forced to rise over the foothills of the Himalayas, it causes constant, heavy rains, often resulting in destructive flooding. The town of Cherrapunji, India, located on the Himalayan slopes, receives an annual rainfall of over 36 feet (11 meters), making it one of the wettest places on Earth.
The importance of monsoons is demonstrated by the experience of the Sahel, a band of land on the southern fringe of Africa's Sahara Desert. The rains of the seasonal monsoon normally transform this arid (dry) area to a grassland suitable for grazing livestock. The wetter southern Sahel can support farming, and many residents migrated to the area during the years of strong monsoons. Beginning in the late 1960s, however, the annual monsoons began to fail. The pasture areas in the northern Sahel dried up, forcing wandering herders and their livestock southward in search of pasture and water. The monsoon rains did not return until 1974. In the intervening six years, the area suffered devastating famines and loss of life, both human and animal.