Periods of hot weather place extreme stress on the human body and can endanger health. Among other things, they can trigger cardiovascular and respiratory conditions and impair mental and physical performance. A hot spell can also have adverse effects on nature and infrastructure. For example, bodies of water often heat up considerably, causing fish to die, while high temperatures can lead to buckling of road surfaces and deformation of railway tracks.
Heat risk in Switzerland
In Switzerland, an excess mortality rate of 7% was recorded in the hot summer of 2003, with 975 deaths attributed to the heat. Across Europe, excess deaths totalled around 70,000. Since 2015, the Federal Office for Civil Protection (FOCP) has classified heatwaves as one of the greatest threats from natural hazards that Switzerland faces. The Federal Office of Public Health (FOPH) has published a list of steps that people can take to protect themselves from heat (available in German/French/Italian). For measures to be taken before, during and after a heatwave, see this page.
Heat in towns and cities
In certain weather conditions and at certain times of the year and day, towns and cities experience consistently higher temperatures than the surrounding countryside. The temperature differences are particularly pronounced at night. During the day, the differences between urban and rural areas are usually smaller and the air temperature in towns and cities is often only slightly higher.
This urban heat island (UHI) effect arises from the fact that the local climate in towns and cities differs from the rural climate due to dense development, lack of vegetation, anthropogenic emissions, less ventilation and waste heat. Towns and cities warm up more during the day and cool down more slowly at night than the surrounding countryside because buildings are good at storing heat. The UHI effect is greatest during periods of high pressure, when there is little wind or cloud and high levels of sunlight. This coincides particularly with spells of hot weather.
Higher night-time temperatures in towns and cities pose a particular health risk for urban dwellers. If the nights are not cool enough, the body has little chance to recover and is less able to cope with heat stress during the day.
Urban planning and architectural measures, close contact with cantonal authorities, and timely and effective warnings of hot periods are all important for preventing or mitigating adverse impacts. MeteoSwiss highlights the greater heat stress in towns and cities in its heatwave warnings.
Increasingly intense hot spells
In recent decades, periods of hot weather have become more frequent and intense than in the past due to the general warming of the climate. For example, temperatures at the Basel station during the most intense three-day hot periods have increased by 2–2.5 °C in recent decades and the longest continuous periods with a Tmean over 25 °C now last for several days, whereas they once hardly occurred at all, or only lasted for a brief time (Figure 2 and 3).
The 30 °C temperature threshold will continue to play a major role in climatological evaluations. The familiar term 'hot day' is based on this threshold, and many weather services will continue to use this term in their forecasts as it is well known to the public.
According to the CH2018 Climate Scenarios, Switzerland can expect to see a further increase and intensification of hot periods by the end of the 21st century as a result of continued global warming. The strength of these trends will depend on future global greenhouse gas emissions. A scenario without global climate change mitigation measures will mean a significant increase in hot days at low altitudes throughout Switzerland (Figure 4). The Geneva region, Valais and southern Switzerland are expected to see the most additional hot days.
Continued global warming will also entail an increase in heat stress and its negative effects on people and society. Studies show, for example, that heat stress could become a problem in Central and Northern Europe, especially for those working in the blazing sun. In Switzerland, it has been calculated that labour productivity could drop by up to 25% in summer.