Warning! Danger of death! How thunderstorms form and how to protect yourself Dangerous normality Our world is full of thunderstorms. On average, extent, is recognisable from far away as thunder- lightning strikes several thousand times a minute on storm cloud from its anvil-like shape. our planet. Every year more than 700,000 lightning The water droplets and ice particles formed within strikes are recorded in Germany. Lightning flashes the thunderstorm cloud collide as they are whirled up occur frequently within clouds. Less frequent are the and down by strong updraughts and downdraughts in dangerous cloud-to-ground lightning strikes when the cloud. The movement of the droplets gradually electric discharges flow between a cloud and the polarises the natural atmospheric electricity. Through ground. Here in Germany we experience an average various highly complex processes, a positive charge number of 20 to 40 thunderstorms at any one loca- builds up in the upper part of the cloud as opposed to tion. the negative charge forming at the base of the cloud. This causes an electric field to form in the Thunderstorms can occur anywhere and at any time atmosphere and gradually grow larger and stronger of the year. Thunderstorms generally occur more before temporarily unloading again by a sort of short frequently in the summer – when the air is warm and circuit, i.e. a powerful strike or flash. This discharge moist – than in the winter and more often in southern of electricity can take the form of cloud-to-cloud or Germany and higher regions than in northern cloud-to-ground lightning. Germany or lowland areas. An especially large number of thunderstorms appear to occur on the A streamer known as a stepped leader, which is Swabian Alb and in the Stuttgart area as well as in usually negatively charged, propagates in short the Rhön Mountains, Lusatia and Berchtesgadener stages, often strongly branched, from the cloud base Land and along the main chain of the Alps. downwards to the ground. One or several upward Thunderstorms are often accompanied by streamers leap from the ground towards the stepped gale- or hurricane-force gusts. Torrential heavy leader. A positively charged current then returns in rainfall can result in flooding, hail in crop losses. several pulses back through the fully opened channel up to the cloud. The entire process takes just a few How a thunderstorm develops milliseconds so that all we see is a single, powerfully A thunderstorm forms when air masses with very flickering flash. different temperatures meet or when temperature differences between different layers of air in the at- No lightning without thunder mosphere are particularly large. Unstable warm and Lightning involves brief electric currents of up to moist air then moves upwards, the water vapour con- 500,000 amperes. The diameter of the lightning chan- tained in it condenses and a powerful cumulonimbus nel itself is no more than a finger wide, which can cloud finally forms, which, due to the huge vertical sometimes be clearly seen on trees which have been struck by lightning. The air in a lightning channel superheats within a fraction of a second to around 30,000 °C, causing the air to expand rapidly outward. Thunder is produced by the acoustic shock wave that expands outward spherically. The lengthy rumbling, audible from some considerable distance away, is produced by the shock wave being broken and reflected several times between the ground and air layers at different temperatures. Lightning is always accompanied by thunder. However, thunder is not heard if lightning hits the ground more than 18 km away or if it only strikes across areas of cloud, in what is known as sheet lightning.

Special phenomena Sometimes, shortly before a thunderstorm, a strong electric field in the atmosphere can cause ionization and the glowing of air molecules, which can be seen on the tips of church steeples or ships’ masts. This phenomenon is known as St. Elmo’s fire. Occasionally, pearl-necklace lightning can occur. This lightning does not light up along its entire length, but varies in luminosity like pearls on a string. Another special type is ball lightning, but there is very little photogra- phic evidence for this phenomenon, which is reported to have a diameter of around 15 to 40 centimetres and to persist for just a few seconds near ground level. Ball lightning moves in entirely unpredictable ways. How it forms remains a mystery.

Protecting yourself in a thunderstorm

Look for shelter in an enclosed Take cover in a car. Avoid trees, towers, pylons. If you are outside, keep …and adopt a squatting building. your feet close together … position in a hollow.

30 m

It is extremely difficult to forecast the precise location of You should seek shelter immediately if you hear thunder summer heat thunderstorms. Thunderstorms of this kind often within less than nine seconds of lightning. occur within a space of a few minutes in a narrowly confined area. It is not unusual for one part of a town to remain dry The safest place in a thunderstorm is in a fixed shelter. It is also while a few streets away rain water is being pumped out of completely safe in the Faraday cage created by the frame of a cellars and basements. Many people are hit, either directly or car. If you find yourself in an exposed position outside, avoid indirectly, by lightning in Germany every year. Some people isolated trees, high towers and pylons, metal fences and any die after being struck by lightning. You can calculate how streams, rivers or lakes and make sure you are not the highest many kilometres away a thunderstorm is by counting the point in the area around you. The best thing you can do in seconds between a flash of lightning and the subsequent sound such a situation is to squat in a hollow in the ground with your of thunder and then dividing by three. feet close together and your arms around your legs. Publishing details Text and editing: Gerhard Lux, DWD Layout: Gabi Hopp-Niche, DWD Pictures and graphics: Claudia Hinz, Rüdiger Manig, Reinhold Mettig (alle DWD), Hartmut Kraatz, Adobe Stock Printed by Federal Ministry of Transport and Digital Infrastructure (BMVI) Paper: Printed on paper from sustainably managed Visit us on forests or other controlled sources. our website: ▸

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