The heatwave that just ended began on 9 August 2026 and lasted until 16 August, a stretch of eight consecutive days during which the daily mean temperature never fell below 25°C. The average daily mean temperature over the event was 26.4°C, with the thermometer peaking at 38.1°C and dipping to a low of 15.8°C. One tropical night occurred during this period, meaning that on at least one night the temperature stayed above 20°C, denying the body the cooler overnight recovery it normally relies on. This was not the first heatwave of the year: an earlier event ran from 18 June to 30 June and was considerably more severe, lasting 13 days with an average temperature of 27.9°C and five tropical nights. The August heatwave was therefore shorter and somewhat cooler by comparison, though still a significant strain for those living through it. MeteoSuisse defines a heatwave as at least three consecutive days with a daily mean temperature of 25°C or more, and this event comfortably met that threshold.

Heatwaves are not simply a matter of discomfort; they are a measurable public health threat. Prolonged exposure to high temperatures strains the cardiovascular and respiratory systems, and the risks are greatest for older adults, people with chronic illness, and pregnant women. Switzerland’s most severe heat-related mortality event was the 2003 heatwave, which caused an estimated 975 excess deaths, a 6.9% rise over expected mortality. The 2019 heatwave led to 521 excess deaths nationally, with people aged 85 and older bearing the largest share: 448 deaths in that age group, a 6.8% increase. Since 2003, excess mortality relative to peak summer temperatures has declined, 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 because the population has not yet physiologically adapted to the heat, which makes the June event of this year particularly concerning even before considering its length and intensity.

Since 1877, Switzerland has recorded 222 heatwaves, with an average length of just two days and a total of 448 heatwave days across the entire historical record. The longest heatwave on record is the one that occurred earlier this summer, from 18 June to 30 June 2026, lasting 13 days with an average temperature of 27.9°C. The August heatwave just ended, at eight days and 26.4°C, does not rank among the longest or hottest on record, but it is still well above the historical average length of two days. Figure 1 shows every recorded heatwave positioned by end date and average temperature, with the size of each point reflecting how many days the heatwave lasted. The figure makes clear that heatwaves have become more frequent and earlier, with June events appearing regularly only after around 2000.

Figure 1 also shows that since 2000, both the frequency and duration of heatwaves have increased visibly, and heatwaves beginning in June have become markedly more common. Before 2000, June heatwaves were rare; in recent years they occur regularly. Figure 2 shows how recorded heatwaves are distributed across the calendar months. Historically, July and August have produced the most heatwaves, which aligns with the peak of summer heat. A heatwave striking in June, as happened this year, carries a different risk profile than one arriving in late July or August, because the population has had less time to acclimatize to high temperatures. Figure 3 shows the number of heatwaves within five-year intervals. Except for the interval 1945–1949, there were only eight heatwaves per five-year period until 2000. After that threshold, the frequency increases sharply, reinforcing the pattern of a warming climate producing more frequent and earlier extreme heat events.