Ferocious winds drove Tacloban surge

[n2doc]

The destruction in Tacloban City on the Philippines' east coast has been described in terms that are reminiscent of recent great tsunamis - of buildings and people being swept away by a huge wall of water.

The cause is very different, of course. For tsunamis, the wall arises from an earthquake's displacement of the sea floor. In the "storm surge" that Tacloban and surrounding towns experienced, it is primarily the ferocious winds that create the unstoppable mass of water.

Several factors, though, have to come together to make the kind of phenomenon witnessed in the Philippines.

One is an extreme zone of low air-pressure, which allows the ocean to rise up.

For Typhoon Haiyan, the meteorologists were talking of 895 millibars - 100 millibars or so below what one would normally expect.

But this in itself would probably have raised the water by no more than a metre.

The winds, on the other hand, had a huge impact. They were estimated to have reached an average of 310km/h (195mph) as the storm centre made landfall.

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http://www.bbc.co.uk/news/science-environment-24903698