The temperature of Lake Geneva (in French "le lac Léman") reaches it peak during a heatwave like the one we are experiencing at the moment. In the right spot it will be 24C at "swimming depth", if you tread water or dive down a metre or 2 then you'll find the temperature of the water dips considerably. In fact if you dive down below 40m the temperature hovers around 5C for the entire year!
I'm quite keen on following the temperature from afar, it means I can plan my lake swims to coincide with the best conditions. I've written about my last swim across lac leman on this blog in 2015.
The Lausanne University website has recently been updated and it shows some very interesting effects. I had no idea how much the temperature varies across the lake surface, especially towards the outflow in Geneva. There are even currents and eddies created by the shape of the lake and the flow from West to East.
The daily variations in temperature are most easily seen at the Buchillon measuring station.
The Meteolakes website also has the following cross-sections.
Lake Geneva facts
#Lake Geneva is one of the largest lakes in Western Europe, 3rd after some massive lakes in the Netherlands (IJsselmeer, Markermeer).
#Surface area: 580.03 km2
#Max depth 310m
#Time from a drop of water entering upstream to leaving downstream in this lake: 11 years!
#60% Swiss, 40% French
#14km at it's widest point and 73km at the longest.
#Apparently in the 70's is suffered from pollution. That problem is now solved. It is clean and safe to swim in throughout now. Which considering the fact it is surrounded by large cities is incredible. More info on the water quality down the Swiss end here. And a newspaper article on the fact that all the beaches are safe to swim from, 86% are rated "excellent or very good" for water quality.