by David Murphy
No! Grass, for example, will host ice a lot more quickly than level ground, because the blades are surrounded by cold air which cools the grass more thoroughly, quickly lowering its temperature. This same sort of mechanism is at work on bridges and overpasses, which often freeze before other road surfaces because they are being cooled from above AND below.
But a flat surface like a road or sidewalk may take longer to freeze, not only because just one side of the material is exposed to the cold air, but because the ice has to overcome the heat stored up by the stone or concrete during milder weather. Stone-based materials are dense with most of their molecules packed tightly together, making it easier for them to absorb, share and retain heat from the sun. Also, different paved surfaces will freeze at different rates, partly because some are more porous than others. Dark surfaces may freeze later than brighter colored surfaces, because dark surfaces reflect less sunlight and absorb more heat. Additionally, certain roads may not freeze as quickly as others because they've been treated with salt, either that day or previously. Exhaust from cars and friction from tires may heat road surfaces slightly, delaying any freezing.
