Dangerous weather: The storm warning service for the lakes in the Alpine region

Particularly in hot weather such as we are currently experiencing, the lakes in the Alpine region are popular destinations for water sports. Dozens of bodies of water in southern Germany, Switzerland and Austria attract kitesurfers, windsurfers, sailors and motorboat enthusiasts in the summer. However, conditions can change rapidly due to specific circumstances and can become quite dangerous in the event of extreme weather.

That is why the storm warning service exists: it uses a visual signalling system – the storm warning lights – to alert people to impending danger. It is not only visitors who need to be familiar with how they work. During the summer season, two particular weather conditions in particular trigger the activation of the storm warning service: thunderstorms and the Föhn wind.

Short warning times for heat thunderstorms

Thunderstorms are not unusual in summer. They occur at the leading edge of a cold front as it passes through, or as localised heat-induced thunderstorms. A frontal thunderstorm often brings heavy, torrential rain or hailstorms. These are usually accompanied by gusty winds that can reach gale force.

Heat thunderstorms, on the other hand, usually form as individual cells, recognisable by their typical tower-shaped cumulonimbus cloud. In the Alpine region, these thunderstorm cells can form and rise to great heights at particularly high speed in summer, meaning that the storm warning service often has only a short lead time before the first gusts occur.

Föhn storm: Danger despite blue skies

The Föhn can also lead to warning situations and, under certain conditions, develop into a storm. The Föhn occurs when a low-pressure system north of the Alps meets a high-pressure system south of the Alps. The air flowing northwards loses its moisture on the southern flanks of the mountains, descends again on the leeward side of the ridges, warms up and, against a mostly blue sky, provides clear visibility over long distances.

A Föhn storm can strike suddenly and turn calm conditions on their head within minutes as the first gusts sweep across the water. They can reach gale force. In addition, a short, steep wind-driven swell develops within a short space of time. On larger bodies of water with sufficient fetch, a wave several metres high can even build up. Due to its orientation, the eastern part of Lake Constance is particularly vulnerable to this weather phenomenon.

Further detailed information on the weather for water sports enthusiasts in the Alpine region can be found in Nicola Möckli’s new book: “Weather on the lake”.

The Storm Warning Service on Lake Constance

As an international body of water situated in the border triangle of Germany, Switzerland and Austria, Lake Constance also has a standardised warning system for this reason: Several dozen orange warning lights have been installed along the shores of the Obersee, Untersee and Überlinger See, so that, in theory, at least one signal is visible from any position on the water.

The entire lake is divided into three warning zones: West (west of the Konstanz–Meersburg line), Central (east of the Konstanz–Meersburg line and west of the Arbon–Langenargen line) and East (east of the Arbon–Langenargen line). From 1 April to 31 October, the system is active from 6 am to 10 pm, and in some cases for longer during major events.

Orange flashes: standard warning signal

The storm warning service operates on a two-tier system: strong wind warnings and storm warnings.

• Strong wind warning (40 flashes per minute): There is a risk of gusts of wind with speeds of between 25 and 33 knots (or 46 to 61 kilometres per hour, or 6 to 8 on the Beaufort scale, i.e. strong to gale-force winds).
• Storm warning (90 flashes of lightning per minute): There is a risk of gale-force winds with speeds of 33 knots or 61 kilometres per hour (force 8 on the Beaufort scale).

As soon as the signals are activated, all skippers must take the necessary measures to ensure continued safety, such as preparing their boat accordingly or, if necessary, heading for the nearest port as quickly as possible.

The storm warning service on the Bavarian lakes

In Bavaria, the same rules generally apply to the storm warning service as on Lake Constance: The system has two levels based on the same guidelines, and the visual signals – consisting of orange flashes (40 per minute for a strong wind warning, 90 per minute for a storm warning) – are also the same.

The lakes equipped in this way include, amongst others, the Ammersee, the Starnberger See, the Tegernsee, the Chiemsee and the Schliersee. A complete overview is available online on the website of the Bavarian Ministry of the Interior.

Storm warning in Switzerland and Austria

Switzerland, too, has implemented the system described above for its numerous lakes. Here, the Federal Office of Meteorology (MeteoSwiss) In cooperation with the cantonal police forces, it is responsible for the storm warning service. In addition to Lake Constance, the waters covered include Lake Geneva, the Jura Lakes, Lake Zurich and Lake Lucerne.

In Austria, orange storm warning lights are also used, although the system is not quite as standardised as in Germany or Switzerland. The principle, however, is the same: if the lights are switched on, the wind conditions may become dangerous. The system for Lake Neusiedl in Burgenland, for example, is a two-stage system and is therefore identical to that in neighbouring countries. In the province of Salzburg, on the other hand – which includes Lake Wolfgang, for example – or at Lake Traunsee in Upper Austria, there is only a single warning level.