Some co-workers and I are trying to hash out this question:
Why is it that cars parked in the open are frosted over on cold mornings, while cars parked under a rudimentary carport (i.e. a roof and not much more–no insulation, not attached to another structure) are not?
We have four theories:
It somehow warmer under the carport, and so the cars do not cool sufficiently to cause condensation on the windshields.
Since the carport is shielded overhead, it prevents the heating of the pavement underneath during the day, and so less heat is radiated around the car overnight, meaning less warm air, less moisture, and less condensation.
The air mass under the carport, while presumably not warmer than that in the open, is more stable. The air around the car condenses and then settles and cools, shielding the car from other warmer, moister, air masses that move more freely outside the shelter.
There is an element of precipitation to dew/frost, and simply having a cover overhead keeps that off of the car.
Does anyone know the real answer to this? Muchas gracias.
I don’t know the answer, but I suspect that 1) and 3) are important, but not necessarily for the reasons you suggest. 1) without the carport, heat can radiate away into space, whereas with the carport, it gets reradiated back to the car. 3) the carport reduces the speed of air blowing across the car, which causes evaporation of condensed water, which cools the remaining water until it freezes.
Put my vote for number 4, notice that vertical or nearly vertical windows get little or no frost, or that only one side of the car gets frost, presumably because the wind is coming from that direction.
Heat being blocked from radiating upward’s may also have an effect.
Radiation. The same reason you can protect plants from frost with a sheet(if it’s not too cold). The car is radiating heat, but not much is returning from space. Put a reflective surface between the car and the night sky and the radiation bounces back. If there is a significant cloud layer the clouds do the job.
Is Montana far enough north?
It is the radiation, mostly. You can feel the effect yourself if you go out one night when it’s clear (things cool off fast) and another when it’s cloudy (it stays warm). The same thing works if the reflector is a building or stand of trees instead of clouds.
Take it from somebody who grew up in Saskatchewan winters!