There is simply no natural phenomenon more terrifying than a tornado. ‘Twisters’ as they’ve become known, appear almost without warning at any time day or night. They can also occur any month during the year providing the conditions are right.

Around 1,000 tornadoes touch down in the United States of America annually. Tornadoes frequently cause devastation to farms, towns, and even cities whenever and wherever they may strike. Fifty-two people were killed by tornadoes in 2002 alone and many hundreds more have been injured.

Eye witnesses who’ve had close brushes with these furious spiralling winds describe their approach like the roar of a jet engine. Tornadoes blast debris in every direction – often at speeds of hundreds of miles per hour. Most houses are made to withstand wind speeds of up to 70mph, but a tornado doesn’t even register unless it reaches 73mph.

How and Why
In order for a vortex – a spiralling funnel of wind – to be classified as a tornado, it must be in contact with the ground and the storm cloud above it. Where the funnel finally does touch down, it creates a path of concentrated destruction. A tornado’s path is rarely more than 250m across, but can be as much as a mile wide.

Tornadoes are generated by a large variety of weather patterns. However, most meteorologists agree that tornadoes happen when warm and cool airstreams collide to create a rotating area of low atmospheric pressure. Air within a low pressure front has a natural tendency to rise and creates a strong upward draught. This draws in surrounding warm air from ground level, causing it to spin faster and faster sucking surrounding air up like a vacuum cleaner. In extreme cases these powerful air currents can reach speeds of 300mph.

The most powerful tornadoes occur when ‘super-cell’ storms occur. These rotating thunderstorm clouds can be clearly seen on weather radars as a well-defined circulation that meteorologists call a mesocyclone. The giant super-cell thunderstorm clouds which rise to heights greater than Mount Everest.

Fujita Scale
The intensity of a tornado is determined by the Fujita scale, named after the late great tornado scientist, Dr. Fujita of Chicago University. Twisters are measured by the amount of damage they cause, and not their physical size. It’s also important to remember that the size of a tornado is not necessarily an indication of its ferocity. Large tornadoes can be weak, and small tornadoes can be violent.

F1 Tornado - wind speeds between 73 and 112mph. Even these tornadoes can peel slates from roofs and drive moving cars off the road. Mobile homes may be overturned and sheds collapsed.
F2 Tornado - wind speeds between 113 and 157mph. Roofs of some houses will begin to lift off and mobile homes in the tornado’s path would be demolished. This twister could also blow railway carriages off their tracks.
F3 Tornado - wind speeds between 158 and 207 mph. Heavy trees will be uprooted and walls and roofs of solid buildings will be torn away like matchsticks. This is a severe tornado.
F4 Tornado - wind speeds between 208 and 260mph. Locomotive engines and 40 tonne articulated trucks will be flung around like toys. There will be widespread devastation left in the tornado’s wake.
F5 Tornado - wind speeds between 261 and 318mph. Tornadoes this intense obliterate everything in their path. Cars are flung like stones for hundreds of metres, and even entire buildings can be ripped from the ground. The force is similar to that of an atomic bomb.