How black ice is formed
A lake always freezes first at its surface. This is due to a special characteristic of water, which reaches its highest density at four degrees Celsius, and not, as is the case with other liquids, as soon as it reaches its freezing point.
Before water can begin to freeze, the entire volume of the lake has to first cool down to four degrees, after which the layer of water closest to the surface cools down further. Because it has a lower density, this layer will no longer sink.
While the water at the bottom of the lake remains at four degrees Celsius, the temperature at the surface quickly reaches freezing point. If the surface temperature sinks below zero, the water begins to freeze.
If no precipitation falls while the lake is freezing over, black ice forms. The less disturbed the lake surface is (i.e. the less wind and water currents), the smoother, more uniform and more transparent the ice.