The heatwave that began on June 18 and ended on June 30 was a 13-day event, making it the longest on record in Switzerland. During this period, the average daily mean temperature was 27.9 degrees Celsius, with a peak of 39.0 degrees and a low of 16.9 degrees. It included five tropical nights, when the temperature did not fall below 20 degrees Celsius, denying the body the nighttime recovery it needs. This was the first heatwave of the year, and it stands as a discrete, named event that the reader has just lived through.

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 heatwave just described is the longest on record, with a mean temperature of 27.9 degrees Celsius. By historical standards, it was an exceptionally long and intense event. 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, 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 than one arriving at the historical peak of summer heat. Figure 3 shows the number of heatwaves within a 5-year interval. Except for the interval 1945-1949, there were only 8 heatwaves per 5 years until 2000. After this threshold, the frequency increases sharply.