Why is dew or frost thicker on some surfaces than others? Dew or frost will first form on substances that are either (1) the coolest or (2) the most moist. Objects can be cooler for two reasons: (1) the object is well exposed to the surrounding air (2) The object is efficient at radiating heat away.

Two surfaces that are good at collecting dew or frost are vegetation and metal. Vegetation has moisture evapotranspiring from its surface. This causes the dewpoint to be higher over vegetated surfaces and thus dew or frost will form on them first. Metal is very efficient at emitting radiation. Since a car is well exposed to the cooling of the air and the metal effectively radiates energy, metal surfaces are a prime spot for dew or frost to form. A surface dew or frost does NOT form on well is concrete. One reason is because the concrete is not well exposed to the air like grass blades or metal objects. Just as importantly, the concrete retains some of its heat gained during the day. As nighttime cooling occurs, the soil in many cases is warmer than the surrounding air. The warmer surface prevents dew or frost from forming on concrete first. The concrete also does not evapotranspire like vegetation. Therefore, both the combination of having less moisture and retaining warmth from the earth's surface causes dew and frost to have a difficult time forming on concrete. Next time there is a dew or frost, observe which objects have a thick coating, which objects have a light coating, and which objects have no coating of dew or frost, then think of the physical processes which caused the dew or frost to be thick on some surfaces but not on others.