BBC News Online: World

Nature at its most powerful

Tornadoes, sometimes called twisters, are tight whirlwinds with wind speeds of between 50 to more than 300 mph ( 75 - 450 kmph).

They usually arise out of massive thunderstorms - known as supercells - which occur when rising swells of warm, moist air push through an overlying stable layer of cool, dry air.

As the air rises, its cools and the moisture in it begins to form a cloud.

As it falls back to earth - pulled down by rain and hail - the rising air can be forced into powerful vortexes, travelling vertically at up to 100 mph.

Air rushing in to fill the void left by the tornado creates the additional fierce winds, which are responsible for most of the damage.

The strength of a tornado is measured by the Fujita-Pearson Tornado scale. The highest - F-5 - has winds of up to 318 mph, and is strong enough to lift houses off their foundations and carry them considerable distances.

Invisible vortexes

To satisfy the definition of a tornado, the funnel of spinning air must be in contact with the ground. If you see a funnel touching the ground it is definitely a tornado.

But the funnel only becomes visible once it contains debris such as dust or moisture. Often, a tornado may be invisible, spinning below a funnel forming cloud, but without picking up debris.

Experts still do not know exactly what makes a thunderstorm spawn a tornado. They are also difficult to predict because they are so small and short-lived.

Tornadoes normally stay on the ground for no longer than 20 minutes - but they can touch ground several times in different areas.

Tornadoes can travel distance over 100 miles (150km) and reach heights over 60,000 feet (18,000 metres) above ground.

Safety tips

• The best place to shelter during a tornado is in an interior room, on the lowest level of the building - preferably a storm cellar or a basement.

• Stay away from windows.

• If you are in a car in a populated area, do not try to drive away. Abandon your vehicle and seek shelter inside.

• If you are travelling through a rural area, drive away from a tornado at right angles to its path.

• If you are caught in the open, try to find a low, sheltered spot such as a ditch, and cover your head with your arms.