What Causes Monsoons to Occur?

Wind Of Change

When people talk of a monsoon, three words come to mind; rain, flooding and India. However, this is not the only vocabulary involved, neither from a weather system perspective nor from the derivation of the word. In this article which forms part of a series on weather and weather systems, we will look at what causes monsoons and explain where the word originates from.

A Common Weather Pattern

Although it is notoriously difficult to predict what the weather will be with a high and reliable level of certainty, due to the complexity of the interactions between the atmosphere and the surface of the Earth, there are some reliable patterns of weather which are likely to have foreseeable consequences. Probably, the first humans to appreciate patterns of weather were sailors and it is from these people that the term monsoon derives.

The word monsoon comes from the Arabic language and the word mausim which means season. The ancient sailors plying trade in the Indian and Arabian seas knew that the wind would blow persistently from the southwest during the summer in the northern hemisphere and that it would reverse in the winter to blow from the northeast. So in reality, the word monsoon originally was used to refer to a seasonal wind. To ancient mariners, a reliable wind was a Godsend, of course.

The Indian Monsoon Season

In modern popular usage, the term monsoon is taken to refer to the seasonal rain which affects the Indian sub-continent.

There are three processes to be considered which underlie the monsoon: (a) a pressure differential caused by uneven heating of the sea and land which generates winds which move from high pressure to low pressure zones; (b) the Coriolis force which is caused by the Earth’s rotation and sheers winds to the right in the northern hemisphere and the left in the southern hemisphere; and (c) evaporation of water into water vapor, the extent of which governs the strength and location of monsoon rainfall.

In India, the main monsoon season brings significant rainfall with it. It occurs between the months of June and September. The precipitation develops from moist sea air which is carried over the subcontinent on the monsoon wind. The moisture laden cloud that comes off the Indian Ocean to deliver rain to the subcontinent can be as thick as three miles! It has been estimated that up to 90% of the nation’s rainfall occurs during the monsoon season.

Heavy Rain, Unreliable Rain or No Rain

Rainfall during the monsoon can be intense and may result in severe flooding. For instance, the town of Cherrapunji was inundated by three feet of rain which fell in the course of a single day. In some regions, as much as 40 feet of rain may fall in the course of the monsoon season.

India is the world’s most populated democracy. With a population of almost 1.2 billion people, agricultural output is critical. A good knowledge of when, where and how intense the monsoon will be is critical to farmers to help them know when to sow their crops. If the monsoonal rains are late (or fails to come), farmers delay planting crops, fearing drought. If the monsoon sputters, seedlings may wither and die during the drier period. If the monsoon is very intense, the seedlings may drown or be washed away – agriculture in monsoonal India is a precarious affair! The Indian Institute of Tropical Meteorology is actively researching the monsoon and adjusting the “hindsight” forecasting of its models to gain better predictions for future monsoon seasons.

If the monsoon rains fail, food will be in short supply. In the modern world, India would be able to call upon its own strategic food reserves or import food from elsewhere, but this was not always the case. In 1770, a failure of the monsoon lead to a famine which claimed 10 million lives in the state of Bengal. On the other hand, too much rain also claims lives. It is estimated that, on average, 1000 Indians will lose their lives in flooding (often flash floods) each year.

U.S. Monsoon

Having established that what causes monsoons is the differential heating between land and sea, it will come as no surprise that India is not the only country to experience a monsoon. Indeed, in the U.S. there is a monsoon in Arizona. Over the summer, the prevailing winds shift from a west or northwest direction to a south or southeasterly direction. The transition allows moisture to be be picked up from the Gulf of California and the Gulf of Mexico and stream into Arizona, causing a radical, seasonal change in moisture conditions across the state.

Learning about monsoons and more intense weather patterns will help you be prepared in case of a catastrophic event such as this.


This post is part of the series: Weather Systems

The weather has a dramatic effect on life on planet earth. This series of article looks at various weather systems and offers an explanation of them.
  1. The Monsoon – A Critical Event For Millions
  2. The Nature of Thunderstorms
  3. The Making of a Tropical Storm
  4. The Impact of a Tsunami