|MadSci Network: Earth Sciences|
Here's the annoyingly short answer: "Yes and no." Here's the full answer: Frost will form at different speeds on different surfaces, but not because of the composition of the surface. It will form at different speeds due to the temperature of the surface, which itself is related to its composition. Whew! In other words, it will form faster on colder surfaces. Darker surfaces, such as blacktop and dark green grass, will absorb more sunlight than lighter surfaces like concrete or yellow grass, and hence will take longer to cool down so frost can form. A window in an unheated room will frost over before a window in a heated room will. The best illustration is seen in the road sign "Bridge freezes before road surface." The bridge has the same concrete surface as the road does (except maybe in Pennsylvania, but that's because our highway department has its head so far up its.....well, never mind), so why would it freeze first? The answer is simple. The road surface has warm, solid ground under it. That ground takes a long time to cool down enough to allow ice to form on the road. The bridge, however, is just a thin strip of concrete suspended in mid-air, with cold air blowing around under it. It will cool off very quickly. This is a good reason to ease off the gas pedal and keep the wheel very steady when driving over a bridge in the wintertime!
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