mysterious-or not

In the spring and the fall, with temperatures around freezing, there are some very interesting thermodynamic phenomena that take place. In some places there's snow, and in some places there isn't. There is snow where it's been piled up by the plows. There's snow along fence lines, and in depressions in fields where snow can collect. There's snow on north facing slopes; that's easy to understand. There's more snow on east facing slopes than west facing. Even that's not really hard to figure out. It is colder in the mornings when east facing slopes get more direct sunlight, and warmer when the west facing slopes get direct sunlight.
On the sidewalk at work, however, about 4/5ths of the walk can be snow free while the remaining fifth harbors ice or snow or both. The snowy section is about 5 feet lower in elevation, and on a slightly different angle. Could that make a difference? I have seen at least a couple of places on roads where the straight sections are snow free, and curved sections are snowy. Sometimes the bridge next to our house will be without snow while the areas on either side will contain snow.
It is fun, at this time of year, to be able to figure out the physics behind the observable phenomena. But it is actually more satisfying to observe patterns that are totally unexplainable.