What is the South Asia monsoon?

By Justine Parker
BBC News

The monsoon plays a vital role in South Asian economies

Severe floods have swamped northern India, Bangladesh and Nepal as South Asia endures one of its worst monsoons in years.

Around the world this month, heavy rains have caused flooding in central and northern China, the UK, Sudan and parts of the US. Japan has also suffered its worst typhoon in decades.

Meanwhile, South-Eastern Europe and southern China have been sweating through a heat wave and rare snowfall has hit Argentina, Bolivia and Chile in South America.

Omar Baddour, from the World Meteorological Organisation, says 2007 has been a very busy year for extreme weather around the world.

He says these climate patterns could be related - and linked to global warming - but it is too early to tell.

"We cannot link this year to global warming at this stage," he told the BBC News website.

"We need further investigation, studies and analysis to come up with the causal relationships."

What is clear is that since the monsoon season began in June, more than 1,000 people across India and Bangladesh have been killed in the floods and almost 20 million people displaced.

And it is not over yet, with rain still expected to sweep through central India.

Economic role

Monsoons occur every year in many countries around the world - northern Australia, Africa, South America and the US are also affected.

The flooding certainly is not over, but the heaviest may have already happened Rob McElwee BBC Weather

The term technically describes seasonal reversals of wind direction caused by temperature differences between the land and sea, but it is commonly used to refer to torrential rain linked to the winds.

The large land mass of South Asia (Bangladesh, India, Nepal, Pakistan and Sri Lanka) and a large body of water (the Arabian Sea and the Indian Ocean) combine to make the Asian Monsoon - which generally lasts from June to September - particularly strong.

From April, temperatures warm over the land, forming areas of low pressure around North India and the Himalayas.

The air is cooler over the oceans - sometimes by as much as 20C - creating areas of high pressure. Because of this difference, air starts flowing from the oceans to the land, bringing "wet" south-westerly winds across South Asia in late May.

THE ASIAN MONSOON Monsoon winds blow north-easterly for one half of the year, and from the south-west for the other half South-westerly winds bring the heavy rains from June to Sept Winds arrive in southern India six weeks before the north west Annual rainfall varies considerably BBC Weather: Monsoons

As temperatures cool towards the end of summer, land loses heat quicker than the ocean, shifting the winds to a "dry" north-easterly.

As these winds retreat, the rain will move over central India and should ease as it passes through, says the BBC's weather correspondent, Rob McElwee.

"The flooding certainly is not over, but the heaviest may have already happened," Mr McElwee said.

The region's economy is based largely upon agriculture, so changes in the strength and path of the monsoon can be disastrous.

If rain comes too late, farmers will sow few or no seeds, fearing drought. If rains are too hard, young plants can be washed away. And while floods can destroy crops, livestock and property, they also fertilise soil.

While this year's rains have been particularly severe, our correspondent says that in the last decade many South Asian monsoons have been weaker than average, making it hard to predict whether this year's devastation is a sign of more to come.