We got a couple inches of snow here in Atlanta over the holidays, and I love to get outside and take pictures after the snow has been on the rooftops for a day or so. In the photo below, you can see what happened after that snow had been on the roof for only 12 hours.
Near the ridge, the snow has all melted. Further down, you can see the outline of where the rafters are. At the eave, the snow is as thick as it had been all over the roof when the snow first came down.
The reason is that in this building, one of the buildings in the condo complex where I live, there's a lot of heat loss to the attic. The building envelope - the air barrier and insulation - is supposed to keep the heat in the house. If, however, there are air leaks through the ceiling or the insulation is inadequate or both, the home loses heat to the attic, and that heat melts the snow on the roof.
By contrast, the building below, also in our community, shows no evidence of melting snow.
One serious problem that can occur in areas where you have significant accumulations of snow on the roof is ice dams. When that snow on the roof melts, it runs down to the eave as liquid water. At the eave, it's not being heated from below anymore, so it refreezes, as shown below. I've heard of one huge ice dam that fell off a house in New Jersey and crushed a car below.
Who needs an infrared camera when you've got snow!