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Making of a Monsoon
Weaver bird

Weaver birds build their nests during monsoon.

For centuries, people have tried to understand why the mighty Indian monsoon arrives each June and dies away each September. But before the modern era of weather satellites and sophisticated computers, climate scientists had only a basic understanding of how titanic atmospheric forces collide to make a monsoon. Now that is changing, and the age-old dream of predicting a monsoon's exact schedule and rainfall seems tantalizingly close.


Scientists say that the two key ingredients needed to make a monsoon are a hot land mass and a cooler ocean. In India, for instance, the land absorbs more heat from the sun than the surrounding Indian Ocean does. This causes air masses over the land to heat up, expand, and rise. As the air rises, it's replaced with cooler, moister, and heavier air from over the ocean. Over India, this damp, chilly layer can be three miles thick! As the cool air arrives, the winds also shift. During the dry season, the winds blow offshore, from land to sea. Then, as the monsoon begins, the winds blow onshore, from sea to land. This phenomenon probably explains why early Arabs named the monsoon "Mausin," or "the season of winds."

"Since the Indian Ocean is bounded to the north by the largest land mass on the planet, the effects of differential heating" are especially intense in India, notes David Stephenson, a climatologist with Meteo-France, a research institute in Toulouse, France. Together with K. Rupa Kumar, an Indian colleague, Stephenson maintains an extensive Web site on the Indian monsoon, which he calls "the most intense in the world." In monsoon season, for instance, some parts of India will receive up to 40 feet of rain in less than four months. One town, Cherrapunji, was drenched by over three feet in a single day.

"Because of the intensity of the weather, [monsoons] are a natural laboratory for scientists to observe the way the land, sea, and atmosphere regimes interact with each other and influence weather," Stephenson says. Eventually, he and other climate scientists hope to use their studies of the monsoon to predict where and when the rains will occur with better accuracy.

"For an economy based on agriculture, such as India's, the importance of accurate prediction and modeling cannot be overstated," Stephenson says. "An important feature of the monsoon is the timing of the beginning of the wet season. The ability to model and predict this onset date would allow farmers to pick the best time to plant crops in order to take advantage of the rains."

Key ingredients for a monsoon: a hot land mass and a cooler ocean.

Already, an international scientific effort is producing forecasts that are increasingly accurate. This year, for instance, the India Meteorological Department (IMD) is predicting that India will have a relatively normal monsoon for the 11th consecutive year, one that will deliver an average of 3 feet of rain to wide swaths of the country.

How much confidence can people put in the forecast? Judging from past experience, a fair amount. The prediction is based on a complex computer model that takes into account 16 different factors, from air temperatures to wind patterns, and has proven increasingly accurate. But the software is far from perfect: over the last five years, for instance, it has tended to underestimate average monsoon rainfall by about 4 percent. As a result, Indian scientists are continually working to create improved forecasting models.

They are not alone. Throughout Asia, countries are increasingly cooperating in an effort to understand one of Earth's largest weather patterns. The effort has even united uneasy neighbors: China and Taiwan recently agreed to join forces to study the South China Sea's monsoon pattern. Explaining the unusual alliance of long-time enemies, C.Y. Lam of the Hong Kong Observatory told SCIENCE magazine: "Meteorologists are born collaborators, because the weather affects everybody."

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