Why would an open carport stop frost?

31 October 2010

Question

I recently began renting a car port from my landlord. Though it is
little more than a roof, fully exposed to the elements, I found that
there was no frost on my car windows this morning. Yet not eight feet
away, a car parked outside the car port had a thick layer of frost on
the windshields. None of the cars parked in the car ports had frost,
and all of the ones parked outside the car ports had frost Why is
this?

Luke Collins from Concord, NH.

Answer

Dave - To understand this, you've got to understand first of all, what is frost and how is it happening? Basically what happens is, at night especially, there's a clear sky at night, anything which is warm radiates heat. You're radiating heat all the time, that's the thing which the heat cameras you sometimes see on these films on the TV are seeing when people glow really brightly. That's because they're warmer. They're radiating more infrared heat. So basically, the warmer something is, the more heat it loses. If you're inside the house, you're losing heat all the time, but so are the walls. You're heated up by the heat coming off the walls and it's heated by you, so you don't feel that cold, because actually quite a lot of energy is beamed into you from the walls of the house.

But if you go outside on a cold clear night, you're basically just seeing the sky and the temperature of deep space is about 2.9 degrees above absolute zero - about minus 270 degrees centigrade. That's really, really cold. So there's no heat coming down onto you and so you're just radiating heat and you lose heat very quickly. The same thing happens to the ground. The ground loses heat very quickly. It cools down and then once it gets below the frost point of the air, water starts condensing out of the air onto it and starts forming frost. Eventually all the water is condensed out of the air and it becomes very dry. There's nothing more left to condense out.

Whereas under a carport, the top of the roof will cool down quite a lot, probably down to slightly below zero, but that's still a lot, lot warmer than the sky. So the roof is still radiating some heat down onto the ground, inside the carport, so that keeps it slightly warmer than everything else. Because of this, all of the moisture tends to condense everywhere else rather than inside the carport. So, even if, by the end of the night, it did get down to below zero, all the water vapour is already condensed everywhere else. So, there's no frost inside the carport.

Chris - And I will add that perhaps the water which is going to condense is going to come down out of the sky and if there is a physical barrier there, then it's likely to settle on that roof, more than on the car underneath the roof, and therefore, there isn't anything to freeze on the glass even though the glass is very, very cold anyway. You're going to end up with a clear windscreen rather than a frosty one.

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