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.
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.
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.
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:
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.
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.
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.