A monsoon is a regional wind that reverses directions seasonally. In southern Asia, wet, hot monsoon winds blow from the southwest during the summer months and bring heavy rains to a large area that includes India, Bangladesh, Sri Lanka, Pakistan, and Nepal. Northeasterly winds (winds are named by the direction from which they blow) that blow down from the Himalaya Mountains in the winter are cool and dry.

Monsoon winds occur in many regions around the world, in Africa, Australia, and in North America, where the Mexican monsoon brings over half of the year’s total rain to Northern Mexico, Arizona, and New Mexico each June through August. The Mexican monsoon is a smaller version of the classic and well-known southern Asian monsoon.

The word monsoon comes from the Arabic word mausim which means “season.” In southern Asia, the monsoon controls the seasons: hot and wet in the summer, cool and dry in the winter.

Plants and animals of southern Asia have adapted ways to survive the annual cycles of flood and drought (prolonged period of dry weather). Humans depend on the rains to fill storage reservoirs and to water crops, especially water-intensive staples like rice and cotton.

How monsoons work

The Asian monsoon works like a large version of reversing land-sea breezes along coastlines. Water heats and cools more slowly than dry land. On a sunny day at the beach, the air over land heats more quickly than air over the water. Warmer air expands and rises.

Cool air moves in from the ocean to replace the rising warm air, and this movement creates a nonshore breeze. When the sun goes down in the evening, the land cools more quickly than the sea, and the wind changes direction.

The Indian peninsula is a piece of low land surrounded on three sides by the waters of the northern Indian Ocean; it separates the Arabian Sea from the Bay of Bengal. The massive Himalaya Mountains to the north isolate it from winds and weather in the rest of Asia.

India lies just north of the equator, so it receives very intense sunlight and heats up beginning in April. When the hot air over the peninsula rises, wet air from the tropical Indian Ocean flows onshore to take its place.

The wind flows from the southeast, across India and into the Himalayan foothills where it is forced upward. Warm air holds more moisture than cool air. The moist ocean air blown in by these winds cools and condenses (changes into liquid from a gas) as it rises, resulting in heavy rains. Monsoon rains are often hard, sustaining rains.

The rainy season

The rainy season
The rainy season

The “season of the peacock” begins in mid-May as the monsoon wet phase reaches southern India. Peacocks are the symbol of life-giving rains in India. Male peacocks begin courting females by flashing their brilliant tail feathers a few weeks before clouds form and the arrival of downpours that transform the parched, brown landscape to a lush, green paradise.

As the rainy season progresses, two arms of wet weather extend across the region: one reaches up from the tear-drop-shaped island of Sri Lanka (formerly Ceylon), to the tip of India, and toward Pakistan; the other comes in from the Bay of Bengal and Bangladesh, and crosses central and northern India.

By early July, the two arms have merged, and the torrential monsoon rains extend throughout south Asia. Animals mate and seeds germinate. Rivers, lakes, reservoirs, and wells fill with water. Humans plant and water their crops.

Monsoon rains continue through the summer months and begin to let up by the end of September. During many years, the rains that were so welcome in spring have become “ too much of a good thing” by late summer, when flooding threatens crops, buildings, and lives throughout southern Asia.

Soil and rock layers that hold water are saturated (completely full of water), and rainwater runs directly off the land surface. Small streams in the Himalayan foothills flow over their banks and flood crops and towns. Small floods run downhill to join others, and then flow together as a very large pulse of water into the mighty Brahmaputra and Ganges Rivers.

The Ganges-Brahmaputra system collects water from most of South Asia; the Indian province of West Bengal and the country of Bangladesh cover its massive delta (the fan-shaped area of land at the river’s mouth).

The residents of the Ganges Delta have developed some strategies for surviving the annual deluge, but there are years when particularly strong monsoon rains and heavy snows in the Himalayas create huge, uncontrollable floods that devastate the area.

During floods in September 1998, almost 70% of Bangladesh, an area about the size of the state of Tennessee, was underwater. The 1998 floods in Bangladesh killed hundreds of people and caused millions to lose their homes. Poor sanitation and ruined crops led to widespread disease and starvation.

The dry season

In the fall, cold air flows down from the high peaks of the Himalayas as the continent begins to cool. A dry northwest wind blows across India and the rain moves offshore into the ocean.

Floodwaters recede and leave behind new layers of silt (fine soil particles) and nutrients to fertilize the flooded agricultural lands. By the following July, the land is parched and dry, and south Asians look forward to another drenching rainy season.

Asian monsoon is a blessing and a threat

a blessing and a threat
a blessing and a threat

Plants and animals of South Asia depend on the summer rains and have evolved (changed over time) to survive the floods and droughts of the monsoon. Plentiful rain gives rise to lush forests and grasslands that provide food and shelter for some of Earth’s most exotic animal species, including Bengal tigers and Indian elephants.

South Asian plants and animals have adapted strategies to reproduce and thrive during the rainy season and then lie dormant (inactive) or survive on stored water during the dry months.

Approximately one quarter of the world’s population depends on the monsoon rains and is threatened by related floods and droughts. The monsoon counties are very heavily populated.

The land area of India is about the same size as the United States but its population is more than three times as large. Bangladesh, a relatively poor country, is one of the most densely settled places on Earth. (Imagine the entire population of the United States living in Oregon.)

Scientists have discovered links between the Asian monsoon and global climate. They worry that natural or human-induced global warming or cooling could affect the monsoon pattern and lead to increased drought or flooding.

More than one billion people face starvation, illness, and the loss of their homes during exceptionally rainy or dry monsoon years. Natural ecosystems (interaction of living organisms and their environment in a community) of plants and animals that are suffer as well.

The governments of monsoon countries, the United Nations, scientists, and non-profit organizations are working to better understand the monsoon and to develop solutions to economic and environmental problems of South Asia.