Just 20% of America’s twisters are rated at F2 and above. Even so, that’s still 200 tornadoes every year with winds of over 322kmph (200mph). The amount of F4 and above tornadoes clocks up just 20 a year, and thankfully F5 monster tornadoes generally strike just once a year. This is still enough to kill an average of 60 people a year in the US alone, and cause billions of dollars-worth of damage.

Even a relatively gentle twister can send a piece of wood crashing through a brick wall as if it was made of paper. The safest place to shelter from a tornado is below ground level in a basement. Amazingly, it’s estimated that less than half of the houses in Tornado Alley have cellars.

Experts recommend that people living in tornado-prone areas without basements should line the walls of a room in the house with 10mm steel sheeting. Ideally, this would be a windowless corridor in the interior of the home surrounded and protected by other rooms. The family should cover themselves with mattresses and cushions to prevent injury from flying glass and debris.

Safety experts from America’s Storm Prediction Centre recommend a list of tornado warning signs to look and listen for:

• Strong, persistent rotating storm clouds.
• Whirling dust or debris on the ground under a cloud base.
• Hail or heavy rain followed by either dead calm or a fast, intense wind shift.
• A loud, continuous roar or rumble, which doesn't fade like thunder.
• Small, bright flashes near to ground level beneath a thunderstorm which indicated that power lines are being snapped by very strong wind.

On average, people living in their path have just 18 minutes warning to take shelter for an approaching tornado. However, public awareness of tornado safety is extremely high in Tornado Alley, and consequently casualties are often lower than other parts of the USA.

On 3 May 1999 an incredible 66 tornadoes trashed Oklahoma leaving a 38 mile trail of devastation into Oklahoma City itself. The biggest of these twisters, an F5, damaged almost 8,000 buildings and caused over a billion dollars worth of damage. Despite this incredible destruction, fewer than 40 people were killed. People in these parts know to keep an eye on the sky.