Tornado Disaster: Clash of air masses in Tornado Alley

TORNADOES FORM in the unstable atmosphere of a thunderstorm, when winds of opposite direction meet and start a column of air spinning.

Think of rolling a cigarette. Fingers push one way round the cylinder of paper and tobacco, thumbs push the other - and it revolves. Tornado formation is this process, scaled up millions of times.

Optimum conditions exist in a belt of the United States Midwest, particularly Oklahoma, Kansas, Missouri and northern Texas - "Tornado Alley" - where cold dry air from the Rocky Mountains meets warm moist air from the Gulf of Mexico.

Where these two giant air masses meet, the warm air rapidly rises and its moisture condenses to form severe thunderstorms. In about one thunderstorm in a thousand, the air begins to spin.

What starts it is very strong "wind shear" - a sudden change of wind speed with either direction or height. Winds from the north-west suddenly slamming into winds from the south-east (as in the US) can create a vertical vortex, an upright spinning column of air which can then reach down to the ground and become a twister, turning its sinister black colour as it sucks up earth and debris.

But a horizontal vortex, a level spinning column of air, can also be formed, when winds of a higher speed above collide with winds of a lower speed below. And this, too, can turn into a twister, when the updraft of the thunderstorm gives it a tilt and turns it upright.

The power that is generated by these vortices is phenomenal. The intense updraft of a twister is capable of lifting buildings and cars into the air and dropping them some distance away; even railway carriages can be lifted.

The size and duration of tornadoes are very variable. Some will last for a few minutes, be between 20 to 100 metres in diameter and track across the land for up to three miles. But the worst can be a kilometre wide and travel more than fifty miles. They typically form in the afternoon or evening of spring and early summer, the peak times for thunderstorms.

Their severity is now measured on a scale devised by Ted Fujita, formerly Professor of Meteorology at the University of Chicago, which categorises them by damage and wind speed. Normally, an F-5 is the most serious, with wind speeds of up to 319 mph. But some reports last night said the winds in Oklahoma may have been even stronger. The Fujita Pearson index categorises tornadoes on the following scale.

F-0: Gale tornado; Wind speeds 40-72 mph; chimney damage, tree branches broken.

F-1: Moderate tornado; 73-112 mph; mobile homes pushed off foundations or overturned.

F-2: Significant tornado; 113-157 mph; considerable damage, mobile homes demolished, trees uprooted.

F-3: Severe tornado; 158-205 mph; roofs and walls torn down, trains overturned, cars thrown around.

F-4: Devastating tornado; 207-260 mph; brick walls levelled.

F-5: Incredible tornado; 261-318 mph; homes lifted off foundations and carried considerable distances, cars thrown as far as 100 metres.

F-6: Inconceivable tornado; 319-379 mph. Until yesterday, winds of this strength were considered to be theoretical.