All About Monsoons

More Than Just the Rainy Season

Asian monsoon monks
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Derived from mauism, the Arabic word for "season," a monsoon often refers to a rainy season—but this only describes the weather a monsoon brings, not what a monsoon is. A monsoon is actually a seasonal shift in wind direction and pressure distribution that causes a change in precipitation.

A Change in the Wind

All winds blow as a result of pressure imbalances between two locations. In the case of monsoons, this pressure imbalance is created when temperatures across vast landmasses such as India and Asia, are significantly warmer or cooler than those over neighboring oceans.

(Once the temperature conditions on the land and oceans change, the resultant pressure changes cause the winds to change.) These temperature imbalances happen because oceans and land absorb heat in different ways: bodies of water are more slow to heat up and cool down, while land both heats and cools quickly.

Summer Monsoonal Winds are Rain-Bearing

During the summer months, sunlight heats the surfaces of both lands and oceans, but land temperatures rise more quickly due to a lower heat capacity. As the land's surface becomes warmer, the air above it expands and an area of low pressure develops. Meanwhile, the ocean remains at a lower temperature than the land and so the air above it retains a higher pressure. Since winds flow from areas of low to high pressure (due to the pressure gradient force), this deficit in pressure over the continent causes winds to blow in an ocean-to-land circulation (a sea breeze).

As winds blow from the ocean to the land, moist air is brought inland. This is why summer monsoons cause so much rain.

Monsoon season does not end as abruptly as it begins. While it takes time for the land to heat up, it also takes time for that land to cool in the fall. This makes monsoon season a time of rainfall that diminishes rather than stops.

A Monsoon's "Dry" Phase Occurs in Winter

In the colder months, winds reverse and blow in a land-to-ocean circulation. As the land masses cool faster than the oceans, an excess in pressure builds over the continents causing the air over land to have higher pressure than that over the ocean. As a result, air over the land flows to the ocean.

Even though monsoons have both rainy and dry phases, the word is rarely used when referring to the dry season.

Beneficial, But Potentially Deadly

Billions of people around the globe depend on monsoon rains for their yearly rainfall. In dry climates, monsoons are an important replenishment for life as water is brought back into drought-stricken zones of the world. But the monsoon cycle is a delicate balance. If rains start late, are too heavy, or not heavy enough, they can spell disaster for people's livestock, crops, and lives.

If rains don't start when they're supposed to, it can lead to growing rainfall deficits, poor ground, and an increased risk of drought which reducing crop yields and famine. On the other hand, intense rainfall in these regions can cause massive flooding and mudslides, destruction of crops, and kill hundreds of people in floods.

A History of Monsoon Studies

The earliest explanation for monsoon development came in 1686 from the English astronomer and mathematician Edmond Halley. Halley is the man who first conceived the idea that differential heating of land and ocean caused these giant sea-breeze circulations. As with all scientific theories, these ideas have been expanded upon.

Monsoon seasons can actually fail, bringing intense drought and famines to many parts of the world. From 1876-1879, India experienced such a monsoon failure. To study these droughts, the Indian Meteorological Service (IMS) was created. Later, Gilbert Walker, a British mathematician, began to study the effects of monsoons in India looking for patterns in climate data. He became convinced that there was a seasonal and directional reason for monsoon changes.

According to the Climate Prediction Center, Sir Walker used the term ‘Southern Oscillation’ to describe the east-west seesaw effect of pressure changes in climate data. In the review of the climate records, Walker noticed that when pressure rises in the east, it usually falls in the west, and vice versa. Walker also found that Asian monsoon seasons were often linked to drought in Australia, Indonesia, India, and parts of Africa.

Jacob Bjerknes, a Norwegian meteorologist, would later recognize that the circulation of winds, rain, and weather were part of a Pacific-wide air circulation pattern he called Walker circulation.

To view real-time monsoon data and maps, visit the NOAA Climate Prediction Center's global monsoons page. For the latest in monsoon weather news, visit NOAA's monsoon page.

Edited by Tiffany Means