Switzerland has just emerged from a brief but intense heatwave that gripped the country from July 29 to July 31, 2026. Lasting three days, the event met the official MeteoSwiss definition of a heatwave: at least three consecutive days during which the daily mean temperature does not fall below 25°C (MeteoSuisse). The average daily mean temperature over this period was 27.7°C, with the thermometer peaking at a scorching 39.7°C and dipping no lower than 17.9°C. Crucially, the heatwave included one tropical night, when the temperature remained above 20°C, denying the body the overnight cooling it relies on for recovery and amplifying physiological stress. This was the second heatwave of the year, following an earlier and far more severe event in June that lasted 13 days and registered an average temperature of 27.9°C.

Heatwaves are not merely a matter of discomfort — they are a measurable public health threat. Prolonged heat exposure strains the cardiovascular and respiratory systems and poses particular danger to 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 caused 521 excess deaths nationally, with people aged 85 and older bearing the largest share (448 deaths, 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 220 heatwaves. The longest on record occurred just weeks before the recent event, lasting 13 days from June 18 to June 30, 2026, with an average temperature of 27.9°C and five tropical nights. By comparison, the July heatwave was a relatively short and mild event by historical standards, ranking neither among the longest nor the hottest. Figure 1 plots every recorded heatwave by end date and average temperature, with marker size reflecting duration, allowing the reader to see this event among all others since 1865. The visualization reveals a clear pattern: heatwaves have become more frequent and are increasingly occurring earlier in the year, particularly in June, a shift that began around the year 2000.

Figure 1 also uses color to indicate the month of occurrence, highlighting how June heatwaves, once rare, have become a regular feature of Swiss summers. Figure 2 shows the distribution of heatwaves across calendar months, confirming that July and August have historically produced the most events. A heatwave arriving in an unusual or early month carries a different risk profile than one at the historical peak of summer, precisely because of the acclimatization challenge described earlier. Figure 3 reinforces the trend by displaying heatwave counts within five-year intervals. Except for the 1945–1949 period, there were only eight heatwaves per five-year window until 2000, after which the frequency increased sharply.