There is air in the snow .
My hives looked like that 3 weeks ago , now there is an open space between the snow and the hives , I think the bees vent out air and the wind causes an open space between the hive and the snow,
If this doesnt happen then you know that the hive did not survive the winter. :(
In Sweden we have ventilation through the bottom , or over the floor of the hive not through the roof.
mvh edward :-P