The snow has melted from the roof of this house, due to energy coming from within. Energy has made its way from the heated portion of the house to the roof, warming it from the inside. Notice that the roof over the front porch is still snow covered -- the porch is cold and has no energy to provide to melt the snow.

In the foreground is an unheated garage. Its snow cover has not yet begun to melt because there is no source of heat to cause melting. The roof on the house is different, however. The snow is completely gone from the lower part -- this part of the roof is directly over the ceiling of the living space. The upper roof still has some of its snow cover -- there is an unheated attic, but the attic is apparently somewhat warmer than the unheated garage.

Here are two very similar houses, side by side. Most of the snow on the roof of the house in the background has melted, while the one in the foreground has managed to keep its snow. The difference surely is how well the houses are insulated. It takes a lot of energy to melt snow, and this energy is leaking out even when there is no snow cover to reveal it. Think about the gas bills ...

The snow pattern on this roof reveals where the support beams are. Apparently they are made of some material that is a good insulator. This keeps heat from leaking out that part of the roof, and so it remains snow-covered

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