The period between 23 July and 23 August is often called the dog days. This term traditionally refers to the hottest phase of summer.
The dog days are named after the constellation Canis Major ("greater dog" in Latin). A period of 30 to 31 days elapses from the emergence of the constellation until it is visible in its entirety, and these days are therefore referred to as the "dog days".
The star Muliphein represents the start of the constellation's head but is so faint that it can only be seen in full darkness. As the brightest star, Sirius is even visible at daybreak. When the star Aludra appears, the Canis Major constellation has fully risen. The time period quoted for the dog days (dies caniculares in Latin) – 23 July to 23 August – stems from the Roman Empire.
Before 1990, hot days were frequently recorded between 23 July and 23 August, e.g. in 1911, 1921, 1943, 1947 and 1983. Individual extremely hot days with daily maximum temperatures exceeding 35°C were also recorded on 18 August 1892 and 23 August 1944.
Summer temperatures in Switzerland are generally increasing due to climate change, and the total number of very hot days has increased significantly since 1991. Specifically, the exact same number of very hot days were recorded in the period between 1901 and 1990 – that is, over a period of 90 years – as have been recorded in the 35 years since 1991. The increase in summer temperatures has intensified since the 1960s, leading to more frequent and more intense hot spells. The dog days are nevertheless recorded as a hot spell – including, for example, between 2 and 13 August 2003.
Hot days can already occur in June:
Daily maximum temperatures of over 35°C are also possible after 23 August (e.g. 24 August 2023). Days like these quickly become more scarce in the lowlands, and it is rare for the temperature to reach 30°C in the Central Plateau from 10 September onwards.
The median date for the latest hot day is around 16 August in Zurich, around 22 August in Basel and around 24 August in Geneva.
The very latest hot days were recorded on the following dates (partially incomplete data series):