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Heat warnings

MeteoSwiss has been issuing heat warnings since 2005, and continues to develop and refine its warning system. Since the summer of 2021, public warnings of impending hot spells issued by MeteoSwiss have been based on mean daily temperature, which allows the night-time temperatures to be taken into account alongside the daytime temperatures.


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Hot spells put the human body under extreme stress, and can be a health hazard. MeteoSwiss has been issuing heat warnings since 2005, and continues to develop and refine its warning system. Up until 2020, heat warnings were based on the heat index (HI). Since then, the daily mean temperature (abbreviated as Tmean) has been used as the threshold value. Tmean is defined as the mean of all temperature measurements from midnight to midnight taken on the day in question. Temperature readings are taken every 10 minutes at the weather stations in the MeteoSwiss automatic monitoring network, resulting in 144 measurements per day. Tmean is therefore a very robust parameter that can describe heat stress over the entire day.

Scientific basis

The threshold values and criteria for the heat warning concept have been established on the basis of new scientific findings from the Swiss Tropical and Public Health Institute (Swiss TPH). Epidemiological analyses show that there is a strong correlation between Tmean and the impacts of heat stress, especially when it comes to mortality. Even a single day with high temperatures can be dangerous for the human body. For this reason, an additional warning level (Level 2) was introduced in order to warn the public of short, intense hot spells.

It is not only the hottest temperatures in the daytime that have a negative effect on the human body, but also high temperatures at night. If the nights are not sufficiently cool, the body struggles to recover and is therefore less able to cope with heat stress, as demonstrated by a project of the NCCS (National Centre for Climate Services). Tmean is a very appropriate parameter for use in heat warnings, because it is an average of values over 24 hours and therefore includes night-time temperatures.

According to the Swiss TPH, humidity in the Swiss climate affects well-being but not mortality. Humidity is indirectly accounted for by Tmean, in that when humidity is high there is a lesser degree of cooling at night. A higher minimum temperature will therefore result in a higher mean temperature over 24 hours. Tmean thus incorporates the new epidemiological findings for Switzerland.

Heat warning concept

The MeteoSwiss heat warning concept consists of three danger levels. The warning criteria are the same throughout Switzerland, with no localised thresholds for different regions. The warnings are issued on the basis of the warning regions used by the federal natural hazards agencies.

The warnings are defined as follows:

  • A Level 2 warning (yellow) is issued for short, intense periods as soon as the mean daily temperature (Tmean) reaches or exceeds the threshold of 25°C for one to two days.
  • A Level 3 warning (orange) is issued as soon as the Tmean is equal to or greater than 25°C for at least three consecutive days.
  • A Level 4 warning (red) is issued when Tmean is equal to or greater than 27°C for at least three days.

How hot is it when the daily mean temperature is 27°C?

Two examples from the summer of 2018 illustrate how hot the temperatures need to be to achieve a mean daily temperature of at least 27°C.

  • Lugano: A Tmean of 27°C was reached on 5th August 2018. The minimum temperature was 21.3°C, and the maximum was 32°C. The coolest hour (average of six measurements) had a temperature of 22.8°C, and the warmest, 31.8°C. The temperature was over 30°C for seven hours.
  • Basel: A Tmean of 27.1°C was reached on 31st July 2018. The minimum temperature was 18.8°C, and the maximum was 34.5°C. The coolest hour had a temperature of 19.5 °C, and the warmest, 34.2 °C. The temperature was over 30°C for ten hours.

Heat wave

MeteoSwiss does not refer to a heat wave until the warning criteria have reached Level 3 – in other words, when at least three consecutive days are forecast with a mean daily temperature of 25°C or higher. If the criteria are only met for a maximum duration of two days, this is described as a short hot spell.

The public are warned via the MeteoSwiss App and the hazard map on the MeteoSwiss website, as well as via the Natural Hazards Portal. In addition, Level 3 and Level 4 heat warnings are conveyed directly to the cantonal authorities.

Behaviour recommendations

In view of the stress to the human body associated with extreme heat, it is important to be warned in good time of an impending hot spell, and to be informed of the appropriate behaviour recommendations.