Question......

.......Why does the water which is being held back by the Tignes dam not freeze over when it's really really cold ( lower than minus 20 degrees), but freeze over when its nice and warm and mild?
Any ideas please let me know!
This was the Lac de Chevril (body of water held back by the dam) today with ice forming on it, as you can see! It hasn't had ice on it all season and it puzzles us quite a lot, (well, puzzles Richard and I, sure no-one else really gives it too much thought!).
It was a beautiful spring like day today with blue sky galore and hardly a cloud to be seen. Everyone had a fantastic day out on the slopes and many people have come back with sun burnt faces. Such a contrast to the last 48 hours with all the snow and wind!
The same glorious conditions are forecast for tomorrow so lets hope it's correct!!