text: 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.\n\nAdd 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).