Weather: Twisting the night away

The tornado that hit Selsey was of rare ferocity for Britain, though smaller ones are not uncommon. Despite the widespread damage caused, this was only a minor tornado by international standards.

Chambers Dictionary dates the word "tornado" back to the 16th century, "altered from Spanish tronada `thunderstorm', influenced by tornar to turn". A convenient piece of linguistic confusion, as a turning thunderstorm is about as good a definition of a tornado one could hope for. Yet accounts of tornadoes in Britain are far older than the word itself. There are even six accounts of different tornadoes recorded between the years 1000 and 1200.

Estimates for the number of tornadoes hitting Britain vary from about 30 a year to 60 a year, but most are of relatively minor intensity. As with other forms of natural disaster, there is a scale - called the Torro Scale - to grade the intensity of tornadoes. Working its way up from Torro Force FC (Funnel Cloud or incipient tornado, when the spout of the tornado is seen aloft, but has not yet touched the ground) and Force 0 (Light Tornado), which would not seriously threaten any structure more secure than a marquee or rickety chimney, it then moves into the more serious types:

Force 1 (Mild): light garden furniture lifted; sheds and outhouses damaged; shrubs uprooted.

Force 2 (Moderate): light caravans moved; detached garages damaged; big branches torn off trees.

Force 3 (Strong): caravans badly damaged; outbuildings torn from foundations; severe roof damage; some strong trees uprooted.

Force 4 (Severe): caravans and other mobile homes destroyed; entire roofs torn off; well-rooted trees uprooted or torn apart.

Force 5 (Intense): roofs blown away, but strong walls remain intact; trees fly through the air.

Force 6 (Moderately Devastating): Large motor vehicles lifted off the ground; most roofs destroyed, some walls too; swathes cut through forests.

Force 7 (Strongly Devastating): Walls of wooden buildings blown away; some stone walls beaten down; steel-framed buildings buckled; trains derailed.

Force 8 (Severely Devastating): Most houses collapse; steel structures badly damaged; motor vehicles hurled great distances.

On this scale, the Selsey tornado seems to fall somewhere between force 3 and 4.

In his book The Weather of Britain, from which the above information on tornado intensities is taken, Robin Stirling wrote, with characteristic understatement: "Fortunately, tornadoes of force 9 and 10 have not occurred in Britain". Fortunate indeed.

We can easily comprehend the destructive power of an earthquake, flood or volcanic eruption, but these vicious twisting winds are right up there with them among nature's most potent pieces of armoury. In the United States an estimated 200 people each year are killed by tornadoes.