How thunderstorms form

Warm, moist air is lighter than the air around it and rises. As it does, it cools and its water vapour condenses into cloud droplets. Condensation releases heat – the same heat that was needed earlier for evaporation. This heat keeps the rising air warmer than its surroundings and drives it further upwards. If the atmosphere is “unstable”, meaning cold enough higher up by comparison, the rising continues on and on.

This is how a heaped cloud grows into a thundercloud, the cumulonimbus. It reaches far into heights where it is icy cold. At the top, where it cannot rise any further, it often spreads out sideways and forms the typical top that looks like an anvil. Inside the cloud, strong updraughts and downdraughts sit side by side.

In this turmoil, small ice crystals and soft ice pellets, called graupel, collide. This separates electrical charges: lighter particles with one charge are carried upwards, heavier ones with the other charge stay lower down. A huge voltage builds up. When it is large enough, it is discharged as lightning – within the cloud, between clouds or to the ground.

A lightning bolt heats the air in its channel to tens of thousands of degrees in a tiny instant. The air expands abruptly and creates a pressure wave that we hear as thunder. Close to the strike it sounds like a sharp crack; further away it is a long rumble, because sound from different parts of the lightning channel takes different times to reach you.

Light reaches you practically instantly, sound takes about three seconds per kilometre. If you count the seconds between flash and thunder and divide by three, you get roughly the distance in kilometres. That is a nice estimate – but not a measure of safety. Lightning can strike well ahead of the rain area and even behind a departing storm. If you can hear thunder, you are within reach.

Heat thunderstorms form on hot, humid summer days. The sun heats the ground through the day, the air rises ever more strongly, and the thunderstorm risk is usually greatest in the afternoon or early evening. In the mountains, sunlit slopes strengthen the rising air. A warning sign is heaped clouds that already grow upwards quickly with sharp edges in the morning.

Frontal thunderstorms form when a cold front quickly lifts warm, moist air. They can occur at any time of day and sometimes form entire lines of storms. A thunderstorm typically goes through three stages: growth with a billowing cloud, maturity with precipitation, gusts, lightning and possibly hail, and finally dissipation.

Shortly before a thunderstorm, cool gusts can often be felt. This is cold air from the cloud's downdraught spreading out along the ground. A sudden drop in temperature and freshening, gusty wind mean the storm is close.

There is no truly safe place outdoors in a thunderstorm. Safe places are substantial, enclosed buildings and hard-topped metal vehicles with the windows closed. Tents, open-sided shelters and isolated trees offer no protection – an isolated tree is in fact a particularly dangerous spot. Do not wait until it rains; move as soon as you hear thunder or see a storm approaching, and stay inside until about 30 minutes after the last thunder.

Thunderstorms can trigger flash floods, even in valleys where no rain is falling, because the water comes from further up. Avoid stream beds, gorges and underpasses, and never walk or drive through flooded roads. If someone has been struck by lightning, call the local emergency number (112 in the EU) immediately; the person carries no electrical charge and can be touched safely. Detailed rules of behaviour are in the entry on lightning safety outdoors.

On summer days, watch the sky deliberately: when do the first heaped clouds grow, how fast do they tower up, and what does it look like an hour later? Compare your observation with the forecast and the weather service's rain radar. This gives you a feel for how quickly a harmless cloud can develop into a thunderstorm.

Plan summer trips so that exposed places such as summits and ridges are behind you as early in the day as possible, and decide in advance where you would find shelter in an emergency. Anyone who starts early and keeps to a fixed turnaround time has to make difficult decisions under pressure less often when storms come.