"THE great tragedy of science," was how Thomas Huxley described it. "The slaying of a beautiful hypothesis by an ugly fact." And it happens sometimes even in meteorology. A recent example has been the debunking of the long-standing theory that the greatest destruction caused by a tornado is that of the explosive power of the low pressure at the centre of the vortex.
Unlike a hurricane, a tornado is a very localised phenomenon, so small that it cannot be represented by a system of isobars on a normal weather chart. It is a violent whirlwind, anything from a few yards to half a mile in diameter, and it moves forward at between 10 and 20 m.p.h., more or less in a straight line. But despite this somewhat leisurely advance, the winds raging around a tornado can reach speeds of 250 m.p.h. or more.
It was thought for many years, however, that much of the damage left behind by a tornado was not a consequence of wind at all. A very sharp drop in atmospheric pressure occurs at the centre of the wind-spiral, with falls, of up to 200 hectopascals having been recorded; the theory was that this fall in pressure occurs so suddenly outside buildings in the path of the vortex, that the internal pressure then causes the structures, quite literally, to explode.
Vulnerable residents were advised to open windows on the lee side of their houses if they saw a tornado coming in their direction, in the hope that this would reduce the deadly pressure differential.
Huxley's ugly facts in this case were provided by more careful examination of tornado damage. Such studies have revealed that windward walls nearly always fall inwards, implying that the structures are damaged mainly by the brute force of the wind itself, and not by any sudden drop in atmospheric pressure.
Now the theory is that the tornadic winds act on over-hanging eaves like a head-wind on an aircraft's wing, providing sufficient lift to raise the roof, and then the winds attack the roofless walls. The combination of rising roofs and falling walls may look explosive, but this does not, in fact, appear to be the case.
Following this realisation, the US National Weather Service has abandoned its advice to open windows. The assumption is that the pressure differential, such as it is, is more or less irrelevant, and rushing to open windows at a critical time just increases the risk of people being cut by broken glass. Residents are urged to shelter, if they can, in a "central" closet, availing of the added protection provided by interior walls.