MeteoSwiss has been issuing heat warnings since 2005 and is continuously optimising and refining its warning system. Until summer 2020, heat warnings were based on the heat index (HI), which was originally developed in the United States and is given in Fahrenheit. From summer 2021, MeteoSwiss will warn the public of an impending hot spell based on the daily mean temperature (Tmean). Tmean includes night-time as well as daytime temperatures and is defined as the mean of all measured values from midnight to midnight.
The scientific basis
The threshold values and criteria for the heat warning system were determined based on new scientific findings by the Swiss Tropical and Public Health Institute (Swiss TPH). Epidemiological analyses show that Tmean correlates well with the effects of heat stress, especially mortality. Even one day of high temperatures can be dangerous for the human body. For this reason, an additional warning level 2 will be introduced from 2021 to warn of short, intense periods of hot weather.
As well as high daily maximums, high night-time temperatures in particular have a negative impact on the human organism. If the nights are not cool enough, the body has little chance to recover and is less able to cope with heat stress (german). Tmean is a suitable measure since it is averaged over 24 hours and so takes night temperatures into account.
According to Swiss TPH, humidity in the Swiss climate has an impact on well-being, but not on mortality. Tmean also takes indirect account of humidity since when humidity levels are higher, the air cools down less at night. Consequently, a higher minimum temperature results in a higher daily mean temperature. Tmean thus factors in the latest epidemiological findings for Switzerland.
Heat warning system
The MeteoSwiss heat warning system is based on three danger levels (Figure 1). The warning criteria are the same across Switzerland, i.e. different regional thresholds are not applied. The warnings are issued according to the warning regions used by the federal natural hazard agencies.
Figure 1 shows the threshold values for the various warning levels. The warnings are defined as follows:
- A level 2 warning (yellow) is issued for short, intense periods during which the daily mean temperature (Tmean) reaches or exceeds 25 °C for one to two days.
- A level 3 warning (orange) applies when Tmean is greater than or equal to 25 °C for at least three consecutive days.
- A level 4 warning (red) is issued if Tmean is 27 °C or above on at least three days.
MeteoSwiss only uses the term 'heatwave' if the warning criterion for level 3 is fulfilled, i.e. if the daily mean temperature is 25 °C or higher for at least three days in succession. If the criterion is met for only one or two days, this is referred to as a 'short hot period'.
The public is alerted via the MeteoSwiss app and website and on the Natural Hazards Portal. Significant (level 3) and severe (level 4) dangers are also relayed directly to the cantonal authorities.
Recommendations on what to do in hot weather
Periods of hot weather can place extreme stress on the human body and endanger health. This makes it all the more important to have early warning of an impending hot spell and to find out what action you should take.