Snow is a good insulator. Ice not so good.
Soil is also a better insulator than water.
If it is cold and the ground is covered with
snow, the warmth of the subsoil will not
reach the air as much as the warmth of
the water under the ice.
If the surrounding air is 20 degrees below
freezing, the air over the lake may be 7
degrees below freezing and thus 13
degrees warmer, causing thermal lift
to occur.
Jenny