What ‘dangerously hot’ means in Switzerland

Keeping cool and hydrated is essential during a heatwave, and not just for people, as this 2018 photo from the Zurich-area Lake Katzen illustrates. © Keystone / Melanie Duchene

High temperatures expected across the nation this week have caused MeteoSwiss to issue the highest heatwave warning since 2015 for some regions. How hot is considered a health hazard in Switzerland?

Remarkably hot weather is forecast from Tuesday to Thursday, including record-breaking temperatures pushing 40° Celsius (over 100° Fahrenheit) in some regions.

“In Switzerland, heatwaves usually occur in conjunction with high-pressure conditions and a south to south-westerly flow of air, so that warm, dry air is pushed toward the Alpine region,” notes the federal Natural Hazards PortalExternal link, which defines a heatwave as “a period of extreme heat stress, which can endanger human health”.

MeteoSwissExternal link, the Federal Office of Meteorology and Climatology, bases its heat warnings on a heat index that takes air temperature and humidity into account. The warnings are issued via media release and teletext, with regular updates online.

“Danger level 3” is when the heat index hits 90+ on three consecutive days. So during the hottest part of the day, a Swiss heatwave can mean anywhere from 29°C with 75% relative humidity to 34° with 30% humidity. This is what most of the nation started experiencing on Monday afternoon. [You can find the heat index tables hereExternal link.]

“Danger level 4” – an extreme heatwave warning – is when the heat index is 93+ for at least five days. This translates into anywhere from 29°C with 80% humidity to 35°C with 30% humidity. From midday on Tuesday, this is the forecast for the Basel region in northern Switzerland, in parts of canton Valais in the southwest and in canton Ticino in the south.

Both levels bring a risk of circulatory complaints and physical discomfort; level 3 means “significant danger” while level 4 means “severe” danger. According to MeteoSwiss, the most recent level 4 was issued in August 2017 in Ticino and all over Switzerland in 2015 – the hottest year ever recorded in Switzerland.

Unlike with avalanche, flood and other warnings issued by the hazard portal, these don’t exist for heatwaves. “Heat warnings are issued only for categories 3 and 4, since a heatwave more extreme than this is very unlikely to occur in these latitudes,” notes the portal.

This varies considerably depending on location and weather, but 30-degree days are becoming more common. For example, there’s been a steady increase in the number of hot days in canton Lucerne since 2000.

The all-time Swiss heat record of 41.5° Celsius was set on August 11, 2003 in Grono, canton Graubünden. However, it was recorded using an old measurement technique of weather huts, which generally recorded temperatures a few degrees higher than modern instruments.