The Asian Monsoon
The word 'monsoon' appears to have originated from the Arabic word 'mausim', which means season.
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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.
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?
The temperature difference between the land and sea can be as much as 20°C...Meanwhile, over the oceans the air is cooler and denser so it is linked to areas of high pressure. The temperature difference between the land and sea can be as much as 20°C - land temperatures in India can even exceed 45°C, while the surrounding water in the Bay of Bengal and the Arabian Sea remains in the low 20s.
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.
From the diagram we can see the movement of this monsoon across southern Asia and then its withdrawal, usually completed by late December.
Forecasting the arrival of the monsoon
Accurate forecasting of the timing of the onset is ... very important...Accurate forecasting of the timing of the onset is therefore very important, as it then allows farmers to pick the best time to plant crops in order to take advantage of the rains. Too much or too little rain can have disastrous effects on the people and the economy.
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.