Thunderstorms and lightning outdoors

In a thundercloud, ice crystals, soft hail and water droplets collide in strong updrafts. This separates electrical charges until the voltage becomes so great that it discharges in a lightning bolt. Lightning can jump within the cloud, between clouds or between cloud and ground. It prefers to strike high, isolated points, but it can also occur well ahead of or behind the actual storm – sometimes even before it rains.

The danger is not only the direct hit. After a strike, the current spreads through the ground and can injure people nearby. It can also jump from a struck tree to a person next to it, and it travels along metal such as fences or wires. That is why the spot under an isolated tree is one of the most dangerous of all.

A building with electrical wiring and water pipes, or a car with a metal body, conducts the current around the people inside and into the ground. That is why these places protect you – and why a tent, a pavilion or an open-sided shelter does not. In a car, by the way, it is the metal shell that protects you, not the rubber tyres; a convertible or a vehicle with a plastic body does not offer this protection.

Safety in thunderstorms is almost entirely a matter of timing. It starts with the forecast before every outdoor activity. In summer, thunderstorms often form in the afternoon; if you start early and leave exposed places such as summits, ridges or open ground in good time, you avoid the most dangerous situations. Think beforehand about where the nearest safe shelter is.

The US National Weather Service's motto is “When thunder roars, go indoors.” If you hear thunder, stop the activity immediately, even if it is not raining yet and the sun may even still be shining. Afterwards wait about 30 minutes after the last thunder before going back out. Many accidents happen because people stop too late or carry on too early.

Indoors there are a few more rules: keep away from windows, and during the storm avoid contact with plumbing – so no showering or washing up – and with devices connected by cable to the power or telephone network. In a car, close the windows and stay seated.

If no safe shelter is within reach, you can only reduce the risk, not remove it. Leave summits, ridges, open ground and the water immediately, and move away from isolated trees, metal fences, poles and power lines. Go to lower areas, but not to ditches or gullies with running water. Do not lie flat on the ground, because that way you touch a large area of ground through which current can spread.

Groups should spread out with space between people. That way, if lightning strikes nearby, the whole group is not injured at once, and someone remains who can help. Water sports must also stop immediately: anyone swimming, rowing or standing on a board is the highest point on an open surface.

A person struck by lightning carries no electrical charge and can be touched straight away. Call the local emergency number (112 in the EU) at once and give first aid as far as you are trained. A first-aid course is very good preparation, for this topic too.

Many old pieces of advice are stubborn. A German folk rhyme says you should avoid oaks but seek out beeches – that is wrong: lightning does not distinguish between tree species, and every isolated tree is dangerous. The idea that lightning never strikes the same place twice is also untrue – tall buildings and masts are hit again and again.

In the past, the so-called “lightning crouch” was often recommended: feet together, crouch down, head tucked in. The US National Weather Service does not present it as a safe measure. It certainly does not replace getting indoors in time. The real protection lies in planning, not in a particular body position.

In high mountains, on via ferratas and on high-altitude tours, additional rules apply that alpine clubs and mountain rescue organisations describe in detail. Anyone going there should get specific training and plan early starts and fixed turnaround times.