Have you heared about black ice? It’s a rare phenomenon and also called “congelation ice”. It occurs when water freezes without many air bubbles encapsulated inside. Then the ice is extremely clear so that the dark ground of the lake makes it look black.
So if there is a cold period with no wind for a while – hence no air bubbles in the water – and obviously no snow fall either then black ice developes, like in the picture at Lake Sils in Switzerland in early January.
Here a picture from the cable car up to Corvatsch.
A while ago I alrady posted about this topic from another lake in Switzerland.
structures & shapes 