Twister mystery

At first the high winds hitting Vail High School seemed like a bit of a laugh. They had caused a power cut, and the students giggled as they used their phones to light their classrooms. But then a teacher opened a door to see what was going on - and was immediately sucked out and thrown to the ground. Her pupils rushed to help her.

Luckily she was not badly hurt. But it showed the power of the tornado that hit Los Angeles last Wednesday, with winds reaching 110 miles per hour.

This was nothing, though, compared to the tornado MississippiA state in the south of the USA with a population of nearly 3 million. experienced on Friday night. In the small town of Rolling Fork it caused at least 26 deaths and left a scene of huge devastation. Houses were flattened, trees shredded and lorries piled on top of wrecked buildings.

The tornado was estimated to be a mile wide. Tracy Hardin, the owner of a dairy business, had a narrow escape. She and eight other people sheltered in a cooler - a huge metal box used for storing ice cream.

The US has the most tornadoes of any country - around 1,000 per year - and also the strongest.¹ Britain has an average of 33.

Tornadoes are created by thunderstorms which form when warm, humid air comes into contact with cold, dry air. The warm air rises through the cold air, resulting in a change of wind direction.

If the winds blow at very different speeds or in very different directions, a funnel forms. This draws in more warm air until it stretches from the ground up to the clouds.

Some people believe that climate change is making tornadoes worse, but it is very hard to say for sure.

Global warming means that the atmosphere can hold more moisture, making thunderstorms more likely. But it also means that wind shears - sudden changes in wind speed and direction - are less likely. 

Yes: The most destructive tornadoes are spawned by severe thunderstorms known as supercells. Studies show that we are likely to have more of these in future because of climate change.

No: There have always been tornadoes. The most violent one on record took place in 1925, when global warming was much less extreme: it travelled 219 miles and devastated three states of the US.

Or... It is impossible to say. Because most tornadoes are very short-lived, and the keeping of detailed records began comparatively recently, it is difficult to make comparisons between past and present.