Danger to health What is a heatwave anyway?

Heatwaves are occurring more and more frequently in Switzerland. But what it is and how best to deal with it is not so easy.

"Federal government declares danger level 3", "The heatwave is on its way", "Switzerland will be sweating profusely at the weekend" - weather services and media(including blue News) have announced the heatwave over the course of the week. This usually happens when widespread temperatures of over 30 degrees are expected for at least three days.

Heatwave? It's complicated

But if you look for a precise definition of the term heatwave, things get a little more complicated, as the following comparison illustrates:

While in Switzerland, summer temperatures of 30 degrees and above are classified as a heatwave, in India, for example, temperatures of around 30 degrees are normal. In the capital Delhi, for example, the average high in May is 40 degrees.

Because heat in a particular region is measured on the basis of the typical weather conditions there at a particular time of year, it is difficult to come up with a standardized global definition - there is no official one.

Even within Europe, the criteria for a heatwave vary from country to country.

New measurement method since 2021

The World Meteorological Organization once defined it like this: If the average maximum temperature is exceeded by 5 degrees for five consecutive days, it is called a heatwave.

The Swiss Federal Office of Meteorology and Climatology does things differently and focuses more on people in its definition of a heatwave: "A period of extreme heat stress that can endanger human health." It is important to note that heat is not always perceived in the same way.

Humidity has a significant influence on how a person perceives the heat.

The following applies: high temperatures are easier to tolerate with low humidity than with high humidity.

MeteoSwiss has been using a new method for warnings since summer 2021. Previously, it used the so-called heat wave index. This describes the perceived temperature and is calculated on the basis of temperature and relative humidity. If the index exceeds a value of 90 for at least three days, there is a significant risk to people's health.

However, MetesoSwiss has been using a different method since 2021. The focus is on the daily mean temperature, abbreviated as Tmean. Both daytime and night-time temperatures are taken into account for the calculation, over a period of 24 hours, from one midnight to the next.

The MeteoSwiss website explains: "The temperature is measured every ten minutes at the stations of the MeteoSwiss automatic measuring network, so there are 144 individual measurements per day. Tmean is therefore a very robust parameter and describes the heat load over the entire day."

This change was introduced on the basis of the latest scientific findings, according to which high night-time temperatures also have a negative impact on the human organism.

Heatwaves - the greatest threat to the Swiss population?

The Federal Office for Civil Protection (FOCP) even considers heatwaves to be one of the greatest threats to Switzerland and has therefore compiled a risk dossier. Because temperatures are also rising in Switzerland. Since 1864, it has become 1.8 degrees warmer on average.

The Federal Office of Public Health (FOPH) has also formulated recommendations on how best to protect yourself and others from the heat.

Note on transparency: An earlier version of this article still quoted the old MeteoSwiss warning measurement method. We have subsequently updated this.

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