Tornado: shelter in an interior room on the lowest floor

Tornadoes usually form in very powerful thunderstorms. In such clouds, warm, moist air rises quickly while the wind at different heights blows at different strengths and in different directions. This can set the rising air spinning. Under certain conditions, a narrow, fast-rotating vortex forms that reaches the ground.

A tornado's wind can be much stronger than in an ordinary storm, but it is often confined to a narrow track. That is why one house can be badly damaged while the neighbouring house is almost untouched. Most injuries are caused not by the wind itself but by flying debris and collapsing parts of buildings.

The whole protection strategy follows from this: every additional wall stops debris, and low, small interior rooms have the most solid walls all around. Head and neck are the most vulnerable and are therefore covered as well – with your arms, a blanket, a mattress or under a sturdy table.

Many weather services distinguish two levels. A watch means: the weather situation makes tornadoes possible – now is the time to check your shelter spot, put shoes and a torch ready and follow the news. A warning means: a tornado has been sighted or detected on weather radar – go to shelter immediately. What the levels are called and which channels are used for warnings differs from country to country.

Sometimes there is hardly any warning time. That is why it pays to know the typical signs: a very dark, often greenish sky, large hail, a low-hanging rotating cloud or a loud, continuous roar reminiscent of a freight train. Anyone who notices something like this does not wait for an official message but takes cover.

Warnings only reach you if you can receive them. A warning app on your phone, a battery-powered radio and knowing the local siren signals are therefore part of preparation.

It is best to choose your shelter spot before you need it – at home, at school and at work. Ideal is a designated safe room or a basement. If there is none, use the smallest windowless room on the lowest floor, preferably in the middle of the building: hallway, bathroom, storage room. Away from windows, doors and outside walls.

In tall buildings without a basement, go to an interior hallway or stairwell as low down as possible and do not use lifts. In large halls with wide roofs, such as gyms or shopping centres, move to smaller interior rooms such as toilets or corridors and follow staff instructions. Local shelter plans always take precedence.

Practise the route with everyone in the household, especially children. If everyone knows where to go, it saves valuable seconds in a real event.

Caravans, mobile homes and light structures do not offer adequate protection in a tornado. Anyone living or holidaying in one moves in good time to a sturdy building or designated shelter when warned. Sheltering under motorway bridges or overpasses is dangerous: the wind is funnelled and strengthened there, and debris flies through.

A persistent myth says you should open windows to equalise the pressure. That only wastes time and does not protect. Going to the window or filming is just as dangerous – that is exactly where glass splinters fly.

If you are caught in a car, drive to the nearest sturdy building if possible. If that is not possible, Ready.gov advises either staying in the car with your seat belt on, ducking your head below the window line and covering it, or, if reachable, lying in an area noticeably lower than the road and protecting your head. Both are last resorts, not safe places.

Stay in shelter until the warning has ended – sometimes a second tornado follows. Afterwards you need sturdy shoes and a light, because glass and nails can be lying everywhere. Watch for gas smells and hanging lines and do not enter damaged buildings.

Tornadoes also occur in Europe, usually weaker and less often than in parts of North America. The basic rules apply everywhere. It is also honest to say: in the very strongest tornadoes no interior room is completely safe. The right shelter spot still lowers the risk considerably, because it protects against the most common danger – debris.