Explained

Simply explained, a tornado is a very rapidly rotating column of air which is pendular from a parent cumulonimbus cloud and comes into contact with a land surface. Tornadoes bring the strongest winds on earth, speeds can reach in excess of 300mph.The highest recorded wind velocity within a tornado is 318mph which was recorded by scientists with mobile radar in May 1999.

They are most common in North America where they can see between 800 and 100 each year. A huge percentage of these occur in a region called 'Tornado Alley', an area which stretches right through the middle of thel country from Nebraska down to Texas. Of course tornadoes can develop anywhere in the USA and indeed anywhere in the world. With the exception of the two Poles, any land mass on earth is prone to them.

Meteorologists are not 100% certain as to exactly how and why tornadoes actually form. The most respected hypothesis is that strongly sheared environments, that is wind moving at different speeds and directions at different levels of the atmosphere, cause substantial horizontal vorticity or 'spin'. To get an idea imagine rolling a pencil between your flattened out hand and a hard surface. This spin can be tilted upright by the strong updrafts of thunderstorms and then contained within the storm. When thunderstorms develop in these conditions their whole updraft can begin to rotate. The rotating section of a thunderstorm is called a mesocyclone. At the same time as the thunderstorm is spinning above, the air being drawn into the storms updraft from the ground also begins to rotate. This rotation can continue to tighten until it all becomes focused into one small vortex and a tornado is born.

Tornades in the UK

We do experience tornadoes here in the UK, in fact we actually experience the highest frequency of tornadoes per unit area than any other country in the world when land mass is taken into account. We can see up 45 on average every year. Although the majority of UK tornadoes are weak, we can and have experienced very strong tornadoes. In the past we have had tornadoes reach T8 on the T-Scale (winds up to 240mph) which have leveled buildings. In the UK we also hold the record for the largest European tornado outbrake. On November 21, 1981, 105 tornadoes touched down in less than 5 and a half hours. Research into severe local storms such as tornadoes is undertaken by TORRO, the Tornado and Storm Research Organisation. TORRO have been researching and investigating tornado events since 1972 and hold records dating back hundreds of years.One of thr aims of TORRO is to be able to forecast severe thunderstorms and advise the public of the likelihood of tornado development.

A Waterspout is very similar to a tornado but comes into contact with a water surface. Usually they are not as strong, but can be highly dangerous to mariners and coastal towns and buildings. If a waterspout comes ashore on to the land, then it becomes a tornado and vice versa. Damaging waterspouts which come ashore as tornadoes can happen in Britiain. A classic example was the Selsey, West Sussex tornado/waterspout in January 1998.

Typically waterspouts do not develop in the same way. It is thought they develop due to vertical rotation already present in the atmosphere in coastal areas due to converging winds i.e. sea and land breezes. This rotation is then consumed by updraft of thunderstorms and even strong, developing towering cumulus. As a result a fast rotating but often weak spout develops.

 

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Revised: December, 2006.