Heat Days and Tropical Nights in Switzerland: The Data

In June 2026, Switzerland lived through the hottest ten-day stretch since measurements began more than 125 years ago. This page collects the key MeteoSwiss numbers on heat days, tropical nights and record summers, each with its source, and gets updated as new bulletins appear.

What counts as a heat day and a tropical night?

MeteoSwiss defines a heat day (Hitzetag) as a day with a maximum of 30 °C or more (MeteoSwiss). A tropical night (Tropennacht) is a night where the temperature never falls below 20 °C (MeteoSwiss). Both counts are climbing, and the nights are climbing faster.

Until the start of this century, tropical nights were rare outside Ticino. Today, low-lying regions across Switzerland record individual tropical nights almost every year, according to the same MeteoSwiss definition page.

How much has Switzerland warmed?

Switzerland has warmed 3.0 °C since pre-industrial times (1871 to 1900), more than double the global 1.4 °C (MeteoSwiss, Klimabulletin 2025). Summers alone are 3.4 °C warmer (Klimabulletin Sommer 2025).

The extremes moved even more, per MeteoSwiss's "Extremere Hitze" analysis of the Zurich, Geneva, Basel and Bern series:

• The hottest day of the year rose 3.4 °C between 1901 and 2024; the hottest week rose 3.7 °C.
• The warmest night minimum rose 4.5 °C, and the five highest night minimums on record all occurred in the last ten years.
• 2022, 2023 and 2024 are the three warmest years on record; 2025 ranks fourth (Klimabulletin 2025).
• Lucerne counted at most 10 heat days per year until the early 1980s; today that is about the average, and since 1981 no year has passed without one.

Which summers hold the heat-day records?

Summer 2003 is still the warmest on record, 3.0 °C above the 1991 to 2020 norm, and Geneva's 50 heat days that year remain the station record. Summer 2022 came second and broke southern records: Stabio counted 63 heat days, beating 2003's mark of 57 (Klimabulletin Sommer 2022).

Station	Heat days	Year	Note
Stabio (TI)	63	2022	previous record 57 (2003)
Geneva	50	2003	41 in 2022 (rank 2)
Sion	50	2003	49 in 2022
Lugano	47	2003	38 in 2022

More record marks worth knowing:

• The highest temperature north of the Alps: 39.7 °C in Geneva on 7 July 2015. The national record was set at Grono (GR) on 11 August 2003; MeteoSwiss publications list it as 41.5 °C in the Klimabulletin Sommer 2022 and as a revised 40.5 °C in the Klimareport 2023.
• Summer 2023 brought the most intense late-season heatwave ever measured: Geneva hit 39.3 °C on 24 August, the highest August value north of the Alps, during 15 consecutive heat days; Zürich-Kloten set a July record of 36.5 °C (Klimabulletin Sommer 2023).
• June 2025 was the second-warmest June on record, 3.8 °C above the norm, with 5 to 14 heat days in June alone depending on the station (Klimabulletin Sommer 2025).

What made June 2026 historic?

Between 19 and 28 June 2026, four stations recorded the hottest ten-day period since their measurements began 125 to 160 years ago: Basel/Binningen (36.7 °C mean daily maximum), Neuchâtel (35.7 °C), Zürich/Fluntern (34.5 °C) and Davos (26.3 °C). MeteoSwiss called it a heatwave "of historic proportions" (MeteoSwiss blog).

The supporting numbers from the same analysis and the event assessment:

• Geneva (35.6 °C), Bern (34.5 °C) and Lucerne (34.4 °C) ranked second, beaten only by August 2003.
• The lowlands saw 12 to 13 consecutive heat days from 17 June; for Bern that was the longest streak in about 160 years of measurement, beating a record from 1947.
• New June station records fell across the country: Bern 34.5 °C, Pully 35.5 °C, Payerne 36.1 °C, Aigle 34.5 °C, Buchs/Aarau 36.1 °C.
• All previous record ten-day heatwaves (2003, 1947) happened in late July or August. One this size in June had never been measured.
• Despite all that, June 2026 as a whole was "only" the third-warmest June since 1864, behind 2003 and 2025.

Why are tropical nights the bigger problem?

Because people can escape a hot afternoon but not a hot bedroom. Night temperatures in Swiss cities run up to 5 to 7 °C above the surrounding countryside, so cities collect far more tropical nights than their daytime numbers suggest (MeteoSwiss).

In the 1991 to 2020 reference period, inner-city Zurich averaged about one official heatwave per year lasting three to four days, central Geneva slightly more, while the rural reference stations logged fewer than one per year. In a 1.5-degree world, expected within years, the inner cities move to two or more heatwaves per year, with the longest lasting seven to eight days (MeteoSwiss blog, 2026). What warm nights do to people is its own research field; we summarise it in Sleep and Heat.

What do the CH2025 climate scenarios project?

The CH2025 scenarios, published by MeteoSwiss with ETH Zurich, quantify what comes next (MeteoSwiss):

• The hottest day of the year rises another 1.5 °C in a 1.5-degree world and 4.4 °C in a 3-degree world. For Basel/Binningen that means 38.8 °C instead of 34.4 °C.
• The hottest night of the year rises 1.1 °C, or 3.8 °C in the 3-degree case.
• Extreme heat days that used to occur once in 50 years become about 2.6 times more frequent in a 1.5-degree world and about 17 times more frequent at 3 degrees.
• Even in today's climate, a physically possible extreme event could beat current records by 5 to 7 °C.

What does this mean for cooling in Switzerland?

Cooling demand is already measurably rising: at Geneva/Cointrin, the number of cooling days and cooling degree days has increased markedly since the 1960s (MeteoSwiss). The market followed the thermometer: Swiss AC unit sales jumped from 12'253 in 2018 to 22'178 in 2020 in the last detailed industry statistics before the FEA stopped publishing them (FEA Marktstatistik).

For contrast: Switzerland installs 30'000 to 43'000 heat pumps every year (FWS), far more than the last counted AC sales. The country is well equipped for cold and thinly equipped for heat, while the heat keeps coming.

The result is visible every heatwave: sold-out shelves. If the tropical-night numbers on this page describe your bedroom, start with our sizing guide (How many BTU do you need?) and browse the portable split air conditioners; everything is currently sold out, and the free waitlist gives you 48-hour early access to the spring 2027 batch.