June 2021: a nice increase in water temperatures in Lake Geneva

The water temperature at the surface of the lake has risen significantly compared to last month.

It was 17°C on1 June and 21.5°C on 14 June. On1 June, the surface waters were particularly low in transparency due to algal growth. The Secchi transparency rose to a depth of 2.7 m. By 14 June, the surface waters had returned to their normal transparency, with Secchi transparency falling back to around 6 m depth.

At this time of year, the temperature decreases rapidly with depth. On 14 June, for example, the temperature at a depth of 9 m 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 time: a windy episode caused the water temperature at the surface of the lake between Geneva and Rolle to drop by 15°C1. This phenomenon is known as coastal upwelling.

June 2021 was also marked by a new heat record. The water temperature at the bottom of the lake exceeded 6°C. The previous record was set in 2003. It should be noted that since 2012, the temperature of the lake bottom has increased by an average of 0.11 °C/year.