Recognizing an Approaching Thunderstorm in the Alps
Sometimes the regular weather forecast isn't accurate enough
In the Alps in particular, thunderstorms can develop within a matter of hours. Anyone caught off guard by lightning, thunder, and heavy rain while hiking can quickly find themselves in a dangerous situation. This makes it all the more important to recognize the signs of an approaching thunderstorm early on.
The weather forecast is as much a part of trip planning as hiking boots and a backpack. Still, you shouldn’t rely solely on weather apps. Especially in the mountains, thunderstorms often form over small areas and develop much faster than in flat regions.
Checking current weather radar images can therefore be helpful. They show not only forecasts but also the actual weather conditions and the direction of movement of rain and thunderstorm cells. If you check the conditions regularly before a hike and while on the trail, you can often avoid unpleasant surprises.
Of course, the staff at your hiking lodge will also be happy to help you with weather advice if you’re unsure. Locals can often correctly interpret signs in the sky that go unnoticed by visitors. They can also tell you which hiking trails have shelters along the way where you can take shelter if necessary.
If you watch the sky closely, you can spot signs of an approaching thunderstorm hours in advance. So-called Castellanus clouds are particularly telling. They form small towers that rise into the sky like the battlements of a castle wall.
These clouds usually form in the morning and are considered a sign of an unstable atmosphere. They do not necessarily mean that a thunderstorm is imminent. However, they do indicate that the conditions for thunderstorm formation can increase significantly as the day progresses. Anyone who spots such clouds should carefully reconsider longer or particularly exposed hikes and keep an eye on further weather developments.
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The closer a thunderstorm gets, the more striking the cloud formations become. Typical features include massive cloud towers that grow rapidly upward. These eventually develop into classic thunderstorm clouds.
A clear warning sign is the so-called anvil shape. In this formation, the cloud spreads out laterally at high altitude, resembling an anvil. In the mountains, however, this shape is not always fully visible, as peaks and mountain ranges obstruct the view.
Often, only the upper part of the cloud is visible—a fibrous, frayed veil of ice crystals. Anyone who spots such an “ice shield” in the sky should take notice. Thunderstorms are often already present in the immediate vicinity, and you should turn around immediately and look for suitable shelter.
If you can already see lightning and hear thunder, you can roughly estimate the distance of a thunderstorm. To do this, count the seconds between the flash of lightning and the subsequent clap of thunder, then divide that number by three—sound travels about one kilometer in three seconds.
Example: If 30 seconds pass between the flash of lightning and the thunder, the thunderstorm is about 10 kilometers away.
As soon as the thunderstorm is less than 10 km away, individual lightning strikes can hit you even outside the actual thunderstorm cell. The situation should therefore be taken seriously, and you should immediately implement those safety measures that you are able to take.
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