text: so metal is a good conductor...why can we see frost on cars in the am when the temp is above freezing?   any surface with air below it becomes colder quicker than others in the absence of solar radiation. note how water freezes on a bridge before it does on other parts of a road, or how snow accumulates first on a deck before it does on the ground when the temperature is right around freezing. the ground is a good conductor of heat (warmth from below), air is a poor one. same applies to frost on the blades of grass but not the bare ground in many spots.  add to that the fact that especially during a calm, clear night, surfaces such as the top of a car or deck radiate heat to space and become cooler than the air above them. the air cools in response, but at sunrise the temperature of that surface may be 5-10° lower than the air at 2 meters above ground (which is reported).