June 2021: a nice increase in water temperatures in Lake Geneva
The water temperature at the surface of the lake has increased significantly from last month.
It was 17°C on June1 and 21.5°C on June 14. On June1, the surface waters were particularly low in transparency due to algal growth. The Secchi transparency actually rose to a depth of 2.7 m. By June 14, the surface waters had returned to their normal transparency, with Secchi transparency falling back to about 6 m depth.
At this time of year, the temperature decreases rapidly with depth. On June 14, for example, the temperature at 9 m depth was 10 °C lower than at the surface. This situation is stable because water at 21.5 °C is lighter than water at 11.5 °C. But let's remember what happened in 2017 at the same period: a windy episode had lowered the water temperature at the surface of the lake between Geneva and Rolle by 15 °C1. This phenomenon is called coastal upwelling.
June 2021 also saw a new heat record. The lake bottom water temperature exceeded 6°C. The previous record was set in 2003. Note that since 2012, the lake bottom water temperature has been increasing on average by 0.11 °C/year.
- 1 In Lake Geneva, an abysmal cold freezes the waters of Geneva - Le Temps
- 2 Evolution of the temperature at the bottom of Lake Geneva since 1957