There is hardly anything more unpleasant in the summer holidays than to be surprised by heavy rain and lightning during a summit descent. How can you minimize this risk?
The first step is of course to inform yourself by means of a weather report. Will there be «April weather» throughout the day with rain showers, sunshine and dense clouds alternating in short succession? Will there still be some rain in the morning and will it then be sunny? Or will the sun shine until the afternoon and some showers will follow towards the evening?
The weather forecast text usually reveals this difference. A comparison between several weather forecast providers can be very revealing for your preparation. The more the texts of different meteorologists agree, the more certain the prediction.
If the hike is planned and the group is already on the road, a regular glance into the sky and onto current satellite images help to assess the weather.
Thunderstorm days are generally quite well predicted. Where and when exactly a thunderstorm arises, however, cannot be exactly calculated even with the best current models.
On the road, the development of clouds can help you predict the weather. As long as only small cumulus clouds decorate the sky, you can comfortably enjoy your hike. A kind of forewarning for the later part of the day are cumulus clouds which have small cloud towers formed above them. These are early signs of an unstable middle atmosphere weather layer.
The cumulus clouds often grow throughout the course of the day. As soon as the approaching clouds are higher than they are wide and their underside becomes very dark, the risk of a change in weather in the Alps is high. The first thunderstorms are possible within a few hours. It is time to start the descent or go to a hut. As soon as the thundercloud starts to fray at the top, it is fully developed – an «anvil» has formed.
A look at the precipitation radar will tell you whether there are already showers or thunderstorms with rain in the region. This allows you to estimate whether the forecasts of the previous day were correct, or whether the precipitation areas are developing much earlier.
The danger of thunderstorms in the mountains is mainly due to sudden poor visibility on challenging trails as well as the risk of lightning. However, it is very unlikely that a person will be directly struck by lightning. Instead, the current generally flows from the ground or rocks through the body.
If you get stuck in a thunderstorm in spite of all precautions, you should keep away from likely impact points: don’t stay close to trees, rock walls or metal objects (caution with transmission masts and hiking poles) and of course, get out of a mountain lake. Water conducts electricity very well.
On the other hand, you should minimize the so-called «step voltage» by placing your feet as close together as possible and squatting. If lightning strikes into the ground close by, a stronger current will flow through the body when standing with one’s legs wide apart.