Tornadoes are an excellent example of an intense, rapid-onset small-scale hazard of short duration. Because windspeeds of up to 500 km h-1 can develop just above the base of the narrow column of helically rising air of the tornado, they are the most violent of all atmospheric phenomena. In the most extreme cases, swathes up to 8 km in width and 350km in length have been laid waste by tornadoes with a forward motion (translatory velocity) of up 110 km h-1. more typically, a tornado will have winds of 150- 200km h-1, create a damage track 50- 100m wide and will travel 2-5 km with a translatory velocity of 30-60 km h-1.
The high damage potential of this hazard is a consequence of the high wind velocities directly affecting structures, objects being entrained in the circulation and then ejected, the intense windshear and the extremely low central pressure(perhaps up to some 200mb lower than the surrounding air). It was once believed that this great pressure deficit would cause a building to explode if a tornado moved directly over it, as the air inside- still at atmospheric pressure- would blow out the walls in an attempt to equalize with the external low pressure of the tornado core, but it is now accepted that much of the damage will have already been done by flying objects and the high winds, before the core arrives. Furthermore, any such explosive effects are likely to have been created by an overpressure- by the hydraulic ramming of air through open/broken windows and doors which is capable of blowing buildings apart.
The global distribution of tornadoes is extremely difficult- it not impossible- to map with accuracy, as most counties do not specifically document their occurrence. In fact, good quality data exist only for UK and the USA, and even then differing documentation procedures are used. However, it is known that tornadoes are characteristic of areas where warm and cold air masses of particularly contrasting natures clash often, namely the mid- latitudes- and tornado reports are most frequent from north- western Europe and the Mid-Western plains of the USA. In global terms, the majority of tornadoes occur in Europe, North America, Australasia, India, Bangladesh, China and Japan. They are uncommon in equatorial regions, and are virtually unheard of in the Artic and Antarctic.
With a poor understanding of the global distribution, it is very difficult to assess the changing significance of the tornado hazard. However, two questions can be addressed: (1) is the frequency of tornado occurrence changing, and (2) are anthropogenic changes altering the potential for tornadic damage?
Statistic from both the UK and the USA clearly show that the number of reported tornadoes is increasing- in the UK, for instance, from 1950 to 1964 only 1954 and 1961 saw yearly totals reach double figures, while since 1965, only 1969 and 1992 failed to reach double figures. There is strong evidence that this is purely reflecting improved monitoring, reporting and data collection, as a breakdown of the American statistics into weak, strong and violent tornadoes shown that more than 77 percent of the average annual increase is accounted for by weak tornadoes, while the reported number of violent events is increasing by only 0.004 per cent per year. The devastation that violent tornadoes (typically long-track and duration) bring is unlikely to go unreported, while weak tornadoes (typically short-track and duration) which were most likely to have been missed in the past are now being reported more frequently.
With many hazards, it is possible at least modify the hazard or even affect the cause, in order to ameliorate its effects. However, as nothing short of a nuclear explosion will affect or modify the tornado hazard, it is essential that efforts be directed to modifying loss potential through adjustments.
In the USA alone the average number of death per year is about 100 and several tornadoes have each caused over/ 100 million of property damage. One response has been the establishment of the government- funded National Severe Storms Forecast Center (NSSFC) in Kansas City, Missouri, to predict the likely conditions for the development of tornadoes and severe local storms, using ground and upper air abservations, satellites and both Dopper and Rainfall radar systems. A tornado watch is issued and remains valid for as long as atmospheric conditions are conducive to tornado formation. When a tornado is suspected or detected, whether by eye- witness reports, chase teams, Doppler radar and/or by monitoring powerline breaks, a tornado warning is then issued by the local office of the National Weather Service (NWS) to alert the public and emergency services. Warnings are broadcast via television and radio, and when a tornado is imminent, civil defence sirens are used, if available to warn the public to seek shelter immediately- usually in the interior of a dwelling, or better still in a basement, cellar or a purpose- built ‘tornado shelter’. These forecast and detection systems have contributed to a significant decline in total death in the USA from these storms, despite population increase in the affected areas.