Weather wise

ONE OF last week's worst natural disasters was caused by an unusual phenomenon. In the Philippines last Thursday, at least 17 people were killed when a waterspout ripped a ferry in two on a lake.

Waterspouts begin with warm air at sea level and a rapid decrease in temperature with height, which is most likely around noon. They begin when air just above the surface of the water is warmed and rises. The resulting decrease in pressure causes cooler surrounding air to spiral in to replace the rising warm air. This is turn is warmed by the water and rises, to be replaced by more cooler air. The wind spiralling in during the process causes an eddy to form in the water.

Meanwhile, the water vapour in the warm rising air soon reaches its condensation point and the spiralling system is imbued with more energy thanks to the latent heat of condensation. The effect becomes like that of water rushing down a plughole, except that in this case it is the energy of the air spiralling upwards which sucks the water upwards with it. And the constant supply of energy makes it spiral faster and faster.

Soon a dense cascade of sea spray forms in a rising ring around the original eddy. The rising vortex looks just like a tornado. Only this one grew from the bottom up. It will collapse only when the influx of warm air into the vortex finally weakens, which is usually between 10 and 15 minutes after it began. In those few minutes, a waterspout can do a great deal of damage, as the Filipino ferry passengers learnt to their cost.