The Asian Monsoon

Monsoon is used to describe seasonal reversals of wind direction, caused by temperature differences between the land and sea. They occur in a number of countries around the world. However, the most well-known of these where the term is most often applied, is the Asian Monsoon.

The cause
In some respects it is a large version of the 'land-sea breeze', where on a sunny day at the beach, the land warms more quickly than the ocean. As the hot air rises over the land, it is replaced by the cooler air over the water.

At night, however, the land cools at a quicker rate than the water, so the wind shifts, blowing from the land to the warmer water.

So our two key ingredients for the Asian Monsoon are a large land mass and a large ocean - namely southern Asia (Bangladesh, India, Nepal, Pakistan and Sri Lanka) and the surrounding Arabian Sea and the Indian Ocean.

When does it start?
From April, the pre-monsoon heat builds over the land and with time will result in continuous rising of less dense air (as the land warms faster) and form areas of low pressure, most commonly over North India and the Himalayas.

To maintain the energy balance the air starts flowing from the oceans (high pressure) to the land (low pressure) bringing in the moisture rich southwest winds across southern Asia - the 'wet' phase.

When the 'wet' phase starts, around 25 May, it has two arms. One coming into Sri Lanka and the other one moving up from the Bay of Bengal into parts of NE India and Bangladesh.

The retreat
As the land and ocean begin to cool in late summer and into autumn, the land loses heat quicker than the ocean. The wind reverses during this 'dry' phase, becoming a north-easterly.

Forecasting the arrival of the monsoon
Every year the distribution and the pattern of precipitation in this monsoon is different. The monsoon is very important to southern Asia, as it's economy is based on agriculture.

BBC Broadcast Meteorologist Everton Fox told us what meteorologists look for when forecasting the monsoon.

"The main precursor is the onset of south-westerly winds. During late April and early May we start looking for south-westerly winds, which take a while to properly set in. More and more cloud starts building up over southern India and Sri Lanka and then rolls in from the Indian Ocean. We then see showers becoming more and more frequent."

The importance of monsoon forecasting can therefore not be underestimated. Computer models of the monsoon are becoming more complex and increasingly accurate. However, the impact that global warming may have on the monsoon is not yet fully understood.

Scientific research will have to be increased in this area to determine these effects, which may have important implications for the economy, health and agriculture, not only in southern Asia, but across the whole planet.