Monsoon season

Monsoon season is a welcome relief to drought conditions in many areas of the world. Monsoons can also bring about widespread famine and enough rain to kill hundreds of people in floods. While the Asia and India monsoons are famous, there are even monsoon season in the United States. So, what is a monsoon?
Monsoons, or rainy seasons, are a shift in wind direction which causes excessive rainfall in many parts of the world including Asia, North America, South America, and Africa. The primary mechanism behind a monsoon is a shift in global wind patterns.

During most of the year, winds blow from land to ocean making the air dry. Winds originating from land are called continental. During certain months of the year, the winds begin to blow from the ocean to the land making the air moist. Winds originating over a body of water are called maritime. This moist ocean air is what causes monsoonal rains over many countries.

Differential heating occurs when the sun heats the land and oceans. Incoming solar radiation heats landmasses faster than large bodies of water. In tropical and sub-tropical climates, solar heating is most intense in the summer months. As the land heats throughout the summer, a large low pressure system builds over the land. The heat from the sun also warms the surrounding ocean waters, but the effect happens much more slowly due to the high heat capacity of water. Therefore, the ocean temperatures as well as the layer of air above the oceans stays cooler longer. The cooler air above the oceans is moist and more dense creating a high pressure zone relative to the pressure above the landmass.

Winds flow from high pressure areas to low pressure areas due to the pressure gradient. Once the temperature conditions on the land and oceans change, the resultant pressure changes cause the winds to change from a land-to-ocean direction to an ocean-to-landdirection. 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 ends.

History of Monsoon Studies

The word monsoon is derived from the Arabic word mausim meaning season. The most famous monsoon is the Indian monsoon. The intense rainfall in these regions can cause massive flooding and destruction of crops. In dry climates, monsoons are an important replenishment for life as water is brought back into drought-stricken zones of the world. Part of the reason India gets such an intense monsoon season is due to its elevation. The higher the land mass, the higher the likelihood of the development of a low pressure zone. The Tibetan Plateau to the north of India is one of the largest and highest plateaus on Earth.

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.