Hot and cold on Lake Geneva
One fine summer's day on the beach at Vidy, you must have wondered why the water is suddenly so cold... One day the water is 25 degrees and the next it's barely 18... And yet the outside temperature has remained stable... But what's happened? First of all, you need to know that in summer, the lake is stratified. In other words, the surface water warms up while the deep water remains cold. This heat spreads very slowly to the bottom. The temperature drops sharply between 3 and 10 m below the surface (thermocline). During very cold winters, the water cools at the surface. It then becomes denser and sinks to the bottom, stirring the lake. Find out more about the stratification of Lake Geneva next week! Let's get back to our temperature change. The most plausible explanation is that the Bise has risen. This northerly wind pushes the (warm) surface water towards Geneva. The warm surface water is then replaced by the colder water at the bottom, which is much colder. The longer and stronger the wind episode, the greater the drop in temperature. The opposite phenomenon occurs when the wind blows from the west.
