June 2021: a fine rise in Lake Geneva water temperatures
The water temperature at the lake's surface has risen sharply compared with last month.
It reached 17°C on June1, then 21.5°C on June 14. On June1, the surface waters were particularly non-transparent due to algal growth. In fact, Secchi's transparency rose to a depth of 2.7 m. By June 14, surface waters had returned to their normal transparency, with Secchi transparency falling back to around 6 m depth.
At this time of year, temperatures drop rapidly with depth. On June 14, for example, the temperature at a depth of 9 m was 10°C lower than at the surface. This situation is stable, as water at 21.5°C is lighter than water at 11.5°C. But let's not forget what happened at the same time in 2017: a windy episode lowered the water temperature at the surface of the lake between Geneva and Rolle by 15°C1. This phenomenon is known as coastal upwelling.
June 2021 also saw a new heat record. The water temperature at the bottom of the lake exceeded 6°C. The previous record was set in 2003. Note that since 2012, lake-bottom water temperatures have risen by an average of 0.11°C/year.
- 1 Abysmal cold freezes Geneva's waters - Le Temps
- 2 Temperature trends at the bottom of Lake Geneva since 1957