I live in DC where it does get cold, and we often get decent snowfall (but not in recent years).
My house was uninsulated all last winter. We had a couple of inches one storm. I noticed my roof (on the shady side of the street) melted most of the snow within a day. I could also walk around up in the attic with a t-shirt and jeans to do some electrical work and not be too uncomfortable. This year, I insulated the entire attic floor. We had a snowfall a few weeks ago and I watched the snow hang up there for a couple of days.
Many of the houses have been rehabed and gutted in my neighborhood. Insulation was added to these houses and they are the ones that retain snow on the roof. The older houses, that haven't been remodeled, melt snow faster than the streets! These are also the same people with much higher heating bills than the rest of us.
Of course, average temps from year to year muck around with the comparisons, but I'm holding onto my theory that the clean roofs are the houses with little to no insulation up in the attic.