Last week, Oman was hit by Cyclone Keila which left 14 people dead and Oman under nearly 2 meters of water in some places. Now, the country is about to be hit again with Cyclone 4- the fourth such cyclone to form in the Arabian Sea this year. Scientists say airborne pollution from South Asia is helping to brew monster storms in the Arabian Sea that have claimed thousands of lives and cost billions of dollars, say environmental scientists.

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The scientists, led by Amato Evan of the University of Virginia, point the finger at a haze known as the 'Asian brown cloud', which hangs over parts of the northern Indian Ocean, India and Pakistan. Several kilometres thick, the cloud comprises brownish particles of carbon soot and sulphates spewed by factories, diesel exhaust and poorly-burnt biomass. "In addition to the multitude of known health impacts associated with aerosols that comprise the 'Asian brown cloud', we suggest that the increasing intensity of landfalling tropical cyclones is a consequence of regional emissions of pollution aerosols," they write in today's issue of Nature.

The scientists looked at patterns in cyclones in the Arabian Sea from 1979 to 2010. They found the region historically only averaged two or three cyclones a year and these typically were weak - even though the sea was clearly hot enough to fuel very powerful storms. The reason for the weakness and infrequency lies in a phenomenon called vertical wind shear which occurs in July and August during the hot months of the monsoon season, the scientists found.

Vertical wind shear occurs when strong winds flow in the upper and lower atmosphere in opposite directions. In the lower levels, it blows from the southwest, and in the upper atmosphere, from the east. The shear rips the top off a would-be cyclone, preventing it from developing the circular winds that are its muscular hallmark. As a result, the few cyclones that occurred in the Arabian Sea typically happened before or after the monsoon season - usually one in May/June and a couple more in August to December - when the wind shear was far less.