IT would be of little consolation to the homeless thousands in Bangladesh to know that the cyclone which caused their troubles earlier this week was meteorologically identical to a hurricane or a typhoon. These are all local names for the same phenomenon, technically described as a "tropical revolving storm".
One feature, however, which distinguished this most recent event from the cyclone which killed 140,000 in the same region in 1991, and the even worse disaster that left one million dead in November 1970, was that this one struck mercifully at low tide. Apart from the obvious effects of the very strong winds, many deaths from a cyclone are caused by what is often referred to as a tidal wave", but which is more properly called a storm surge.
Hurricane force winds over the open ocean bring with them windgenerated waves of quite spectacular heights. The swell produced in this way propagates outwards from the storm in all directions; in the case of a typical cyclone in the Bay of Bengal heading north towards Bangladesh, the waves initiated on its eastern flank bearing in mind that they blow anticlockwise around the disturbance move rapidly in the same direction as the storm itself. But whereas a cyclone typically advances some 500 miles in a 24 hour period, the storm generated waves propagate much faster and may cover 800 miles in a single day. Before satellites, aero-planes or radar, the arrival of this massive swell was often the only warning that coastal dwellers had that a cyclone might be on the way.
But the worst effect comes with a large and sudden rise in sea level "a surge" which occurs as the "eye" of the storm approaches land. The surge is produced by a combination of the sudden reduction in atmospheric pressure and the very strong winds which tend to "pile up" sea water against the coastline. The ultimate height of the surge depends on several factors, including tides, local currents, the shape of the sea bed and the coastline in the vicinity, the intensity of the cyclone itself, and the speed and angle at which it approaches the shore. Typically, however, a raised dome of water 10 to 15 ft above the normal tide level, and some 40 to 50 miles across, might be expected.
This rise in water level, coupled with heavy rains, can cause flooding on a massive scale. Evacuation to higher ground not an option in the case of most of Bangladesh - may well be the only hope of survival for the local populace.