Often before sunrise on a clear, calm, cold morning, ice (frost) can be seen on the tops of parked cars, even when the air temperature is above freezing. this condition happens because the tops of the cars are cooling by:

The tops of the cars are cooling by radiation. Throughout the day the cars are taking in heat from the sun, in relation to the temperature. When the sun goes down at night the temperature drops. Atmospheric moisture condenses quicker than it can be evaporated, which results in the frost, or frozen dew.