The foyer of the Dublin headquarters of Met Eireann is dominated a large, black marble mural whose inscription reads: Is mairg a baitear in am an anfa, mar tagann an ghrian i ndiaidh na fearthainne. Loosely translated, it says: "Pity him who perishes at the time of the flood, because the sunshine reappears when the heavy rain has stopped." Mutatis mutandis, the sentiment is apposite in the context of the recent devastation in Oklahoma and nearby; despite its legacy of almost unbelievable destruction, a tornado at any particular spot lasts only a few minutes, and the atmosphere quickly returns to its normal and unthreatening state.
Unlike a hurricane, a tornado is a very localised phenomenon, so small, indeed, that it cannot be represented by a system of isobars on a normal weather chart. The violent whirlwind is typically a few hundred yards in diameter, and it moves forward at between 10 and 20 m.p.h., more or less in a straight line. The winds raging around a tornado can reach speeds of 250 m.p.h. or more, but it was thought for many years that most of the damage left behind was a consequence, not of the wind, but of the explosive power of the low pressure at the centre of the vortex.
It is well known that a 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 therefore 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.
A revisionist view, however, is based on 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. So current 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; then the winds attack the roofless walls. The combination of rising roofs and falling walls may look explosive - but it's not.
The advice now is that the pressure differential is more or less irrelevant, and that rushing to open windows at a critical time may just increase the risk of people being cut by broken glass.
