BioEd Online

Typhoon Durian is not finished yet, according to warnings from meterologists. After slamming into the northern Philippines yesterday, bringing winds of almost 250 kilometres per hour and leaving at least 388 dead in the region surrounding the capital Manila, the storm is now on course for the Asian mainland.

Vietnam has now been put on typhoon alert, with weather forecasts predicting that a resurgent storm will hit the south of the country over the weekend. But meteorologists say that the strength of storm on landfall is difficult to predict.

"Intensity forecasts are much more difficult than track forecasts," says Julian Heming, a tropical prediction scientist at the UK Meteorological Office in Exeter.

Tropical storms, known as typhoons in the Pacific and hurricanes in the Atlantic, are fed by water evaporating by warm seas. So Durian could grow in force as it heads westward over the South China Sea. The storm was weakened only modestly by passing relatively quickly over the island terrain of the Philippines, and now has a chance to recoup its strength.

But the intensity of a storm depends not only on the surface temperature of the sea, but also on how far the warm water extends below the surface, Heming explains. Last summer, the Gulf of Mexico developed several deep, stable belts of warm water. Hurricanes Katrina and Rita both happened to pass over one of these bands, feeding the storms with moisture and whipping up their deadly force.

It is unclear whether the same will happen to Durian as it tracks west towards Vietnam, Heming admits. "Some storms in the South China Sea just wind down to nothing," he says.

This may be due to the presence of high-altitude 'shear' winds that drag the swirling cylinder of a tropical storm out of shape and suck away its strength. "What the storm needs to fuel it is for air to rush up vertically, or as vertically as possible," Heming explains. "This leads to a greater inflow of air and energy at the bottom."

Where next?

The Japanese Meteorological Agency, which tracks Pacific typhoons, estimates that Durian is currently carrying winds of around 130 kilometres per hour. It forecasts that the storm is most likely to make landfall on Sunday, with Ho Chi Minh City likely to experience winds exceeding 100 kilometres per hour.

This summer has been a torrid one for the Pacific. "The western Pacific has actually been below average in terms of number of storms, but quite a lot of them have been major ones that have hit land," Heming says. In May, both the Philippines and China were battered by Supertyphoon Chanchu.

In contrast, the tropical Atlantic is breathing a sigh of relief after a relatively uneventful storm season (see ' The calm instead of the storm') .

The typhoon season has no official end date, but is usually over by the end of November.

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