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A cover that is retaining heat will have ice formed over the whole surface of the cover under the snow.
What do you want explained. Heat loss through the cover will keep ice from attaching to the cover under snow.
Let me explain the confusion Stabby.The development of ice from snow suggests melting and refreezing. Melting is associated with heat loss through the surface of the cover due to the transfer of heat. Heat loss is associated to lack of efficiency. Seemed like a simple question to me? I might add that you are the first person in the many years I've been doing this to suggest that ice on a cover is a sign of efficiency. That is the reason I was looking for an explanation as your post doesn't seem to make sense to more than just I obviously. Steve