Why Does Frost Form on Cars?

To understand why frost forms on cars, one must understand why dew forms on your car windows. It is the dew or condensation that freezes into frost. When the sun sets, the temperature of the glass cools more rapidly than the air around it. The air around the glass then begins to cool and condense faster, thus producing condensation that gathers on the glass. This is because the water molecules in the warm air were spread farther apart, but now are brought closer together due to the temperature of the air and its contraction.

Frost will form on the car windshield and roof as the temperature drops below freezing. In some cases, frost forms even before the temperature drops below 32 degrees Fahrenheit. This is due to the cooling properties of evaporation and the inability of the glass and metal to retain heat. As the temperature cools, the car loses heat through transduction. This is enhanced by the perfectly flat surface and that makes frost form even in mildly cool weather.

A typical frost cycle starts out with a sunny, mild day where temperatures reach about 45 degrees in the afternoon. The air is somewhat moist, retaining moisture from evaporation throughout the day. As night falls, the air is no longer heated by the sun. The air temperature drops and becomes even cooler around the surface of a car, due to the coolness of the metal and glass. Water condenses around these areas and gathers. As night goes on, the temperature drops toward the freezing mark and the water beads freeze as they evaporate in the drier, colder air.