Heatwave pushes ‘zero-degree’ line to record height in Switzerland

MeteoSwiss launches a weather balloon in May 2019 from its centre in Payerne, canton Vaud. © Keystone / Jean-christophe Bott

The heatwave in Europe has pushed the zero-degree limit - the altitude at which the temperature dips into the minus - to a record height of 5,298 metres in Switzerland.

This new record was recorded on the sweltering night of August 20-21, the Federal Office for Meteorology and Climatology (MeteoSwiss) said on Monday on the platform X, previously known as Twitter.

The previous record was 5,184 metres, set on July 25, 2022.

MeteoSwiss experts measure the zero-degree limit twice a day using weather balloons launched from their regional centre in PayerneExternal link, canton Vaud. In summer, the zero degree is typically between 3,000 metres (in June) and almost 4,000 metres (in July).

MeteoSwiss saysExternal link that the zero-degree isotherm near the ground, calculated in this case by measuring stations, has risen by 200-700 metres, depending on the season, since the beginning of surveys in 1864. Since the 1970s, this rise has accelerated, especially in spring and summer, says MeteoSwiss.

+ Switzerland swelters as temperature hits 36°C in Geneva

The zero-degree isotherm separates the layers of air which have a temperature above 0°C at low altitudes from those with a temperature below the freezing point at higher altitudes, says MeteoSwiss.

Owing to its very important influence on the development of vegetation, the limit of snowfall and the water cycle, the zero-degree isotherm is an integral part of weather forecasts.

Heatwave

Switzerland has been experiencing sweltering hot weather over the past week. MeteoSwiss recorded temperatures above 35°C on Sunday afternoon. The mercury even exceeded 36°C at the end of the afternoon in Sion (VS) (36.2°C) and Geneva (36.1°C). In Payerne (VD), Mathod (VD), Evionnaz (VS) and Mühleberg (BE), 35°C or more were recorded.

++Switzerland braces for heatwave

The temperature did not fall below the 20°C mark throughout the Lake Geneva basin from Sunday to Monday. The night was also tropical in Neuchâtel (21.9°C), Lugano (23.4°C), Locarno (24°C) and Zurich (20.4°C) in particular.

From Saturday until Wednesday, the whole of the country has been classified as level 3 heatwave danger below 800 metres altitude (on a scale of 1-5). In canton Ticino – and since Sunday also around Lake Geneva – the danger level is 4, which means a high risk of circulatory disorders and physical discomfort.

The heatwave episode is expected to continue until Thursday, with peaks at 37°C. From Thursday, showers, even violent thunderstorms, are likely in lowland regions.

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