Tornadoes

Tornadoes: the most violent of all atmospheric phenomena

Tornadoes: the most violent of all atmospheric phenomena

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 llaid 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 ppressure(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- wwould 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, iit 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 tthe 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 ...