Switzerland has just emerged from a five-day heatwave that gripped the country from August 2 to August 6, 2026. During this event, the daily mean temperature averaged 26.9°C, peaking at a scorching 37.8°C and dipping to a low of 17.0°C. The heatwave included two tropical nights, where temperatures never fell below 20°C, denying the body its essential overnight recovery period and amplifying physiological strain. This was not the first heatwave of the year; an earlier, more severe event had already occurred from June 18 to June 30, lasting 13 days with an average temperature of 27.9°C and five tropical nights. The official definition of a heatwave, as used by MeteoSuisse, requires at least three consecutive days with a daily mean temperature not falling below 25°C, a threshold distinct from a mere hot day where the maximum reaches 30°C.

Heatwaves represent a measurable public health threat, not simply a spell of uncomfortable weather. Prolonged exposure to high temperatures strains the cardiovascular and respiratory systems, with older adults, people with chronic illness, and pregnant women facing the greatest danger. Switzerland’s most lethal heat event remains the 2003 heatwave, which caused an estimated 975 excess deaths, a 6.9% rise over expected mortality. The 2019 heatwave resulted in 521 excess deaths nationally, with people aged 85 and older accounting for 448 of those fatalities, a 6.8% increase in that age group. Excess mortality relative to peak summer temperatures has declined since 2003, a trend public health authorities attribute to improved heat warning systems and cantonal heat action plans. Heatwaves that strike earlier in the season tend to be more dangerous, since the population has not yet physiologically adapted to the heat.

Since 1877, Switzerland has recorded 221 heatwaves, with an average length of just 2.0 days across all events. The longest heatwave on record is the one that occurred earlier this summer, lasting 13 days from June 18 to June 30, 2026, with a mean temperature of 27.9°C. By comparison, the August heatwave was a relatively short and mild event, neither ranking among the longest nor the hottest in historical terms. Figure 1 plots every recorded heatwave by its end date and average temperature, with the size of each point reflecting event duration, allowing the reader to see this recent event among all others since 1865. The visualization reveals that heatwaves have become more frequent and are increasingly occurring earlier in the year, with June events appearing regularly only after the year 2000.

Figure 1 also uses marker color to indicate the month of occurrence, showing that since 2000 both the frequency and duration of heatwaves have visibly increased. June heatwaves, once rare, now occur regularly. Figure 2 shows how recorded heatwaves are distributed across the calendar months, with July and August historically producing the most events. A heatwave arriving in June, before the population has acclimatized to summer temperatures, carries a different and often greater risk profile than one arriving during the historical peak of summer heat. Figure 3 underscores the accelerating trend: except for the interval 1945 to 1949, there were only eight heatwaves per five-year period until 2000, after which the frequency increased sharply.