The heatwave that just ended began on 10 July 2026 and lasted five days, concluding on 14 July. During this event, the average daily mean temperature reached 26.9 degrees Celsius, with a peak temperature of 35.5 degrees and a low of 15.9 degrees. The heatwave included one tropical night, a night when the temperature did not fall below 20 degrees Celsius, which is particularly taxing on the body because it denies the recovery period that cooler nighttime hours normally provide. This was not the first heatwave of the year; a longer heatwave had already occurred from 18 to 30 June, lasting 13 days with an average temperature of 27.9 degrees and five tropical nights. For more on temperature definitions, see MeteoSuisse.

Heatwaves are a measurable public health threat, not merely a matter of discomfort. Prolonged heat exposure strains the cardiovascular and respiratory systems, posing 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 percent rise over expected mortality. The 2019 heatwave caused 521 excess deaths nationally, with people aged 85 and older bearing the largest share at 448 deaths, a 6.8 percent 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 218 heatwaves. The longest on record lasted 13 days, with an average temperature of 27.9 degrees, occurring earlier this same year from 18 to 30 June. The heatwave that just ended, lasting five days with an average temperature of 26.9 degrees, was a comparatively short and mild event by historical standards. Figure 1 shows every recorded heatwave positioned by end date and average temperature, with the size of each point reflecting duration, allowing the reader to see this event among all others since 1865. The figures show more frequent and earlier June heatwaves starting around 2000.

Figure 1 shows all recorded heatwaves, with marker size representing event duration and color indicating the month of occurrence. Since 2000, both the frequency and duration of heatwaves have increased visibly. Heatwaves beginning in June have also become markedly more frequent — before 2000, June heatwaves were rare, whereas 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, but a heatwave striking in an unusual or early month carries a different risk profile, as noted earlier regarding acclimatization. Figure 3 shows the number of heatwaves within a five-year interval. Except for the interval 1945 to 1949, there were only eight heatwaves per five years until 2000. After this threshold, the frequency increases sharply.