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December 19, 2011 12:33 AM   Subscribe

The air is usually very dry in the winter. Why are my car windows always frosted over without exception?

If my car windows are very dry when I park my car at night, and it doesn't rain or snow or anything, and the air is so dry that my skin starts to flake, why is there a millimeter of ice on my car windows? Where is this moisture coming from? Obviously it comes from the air, but how does this work? During the summer when it's humid, my car windows aren't usually wet in the morning.
posted by WhitenoisE to Science & Nature (6 answers total)
The windows can be colder than the rest of the car interior. The air inside may be warm enough to hold some moisture (including moisture from your breath), but lose that moisture when it hits those cold windows. If you breathe some moisture into the car's interior and then park it for the night, that moisture will collect as ice on the windows when the car windows cool.

Also, if there's a guy hiding in the back, he may breathe heavily.
posted by pracowity at 1:02 AM on December 19, 2011 [4 favorites]

Best answer: Cold air can't hold nearly as much moisture as warm air.

What happens when air gets cold and can't hold all the moisture that's in it, is that the moisture falls out as dew, and the air ends up very dry.

In the summer, there's plenty of room for all the moisture in the air that you're likely to get in your climate, so the moisture stays in the air overnight and you probably don't get much dew.

You may notice in the autumn when the temperature overnight is cooler than it is in summer, your car windows probably start to get wet in the morning, as does the grass.

In the winter the car window and the grass get wet and then freeze, leaving you with white grass, possibly icy roads, and a scraping job in the morning. So you scrape all the ice off and leave it to evaporate a little bit in the morning sunshine, making the air that little tiny bit more humid. That moisture can't stay in the air when it gets cold again overnight, so it will end up back on your car again.
posted by emilyw at 1:36 AM on December 19, 2011

Best answer: We experience winter air as 'dry' because we heat it. If our bodies and houses weren't warm, the air would feel just as moist as it does in the summertime.

Cold outdoor winter air isn't actually dry in terms of relative humidity, which is what matters when you're dealing with frost. There's less water in the air in absolute terms, but there's still water all over the place -- mostly on and in the ground. The sun rises, the air warms and picks up some of that moisture. If the nighttime temperatures drop past the dew point then that moisture leaves the air and is deposited as frost. If your car is always covered with frost in the morning then the temperature is dropping enough every night to cross the dew point.

Is the frost forming on the inside or outside surfaces of the windows? Moisture can build up inside the car from snow tracked in on your boots, a windshield leak, a failing heater core or even from your breathing, and is worse if you leave your ventilation set to recirculate instead of admitting lots of fresh air.
posted by jon1270 at 2:57 AM on December 19, 2011

Response by poster: Good answers, thanks.
posted by WhitenoisE at 3:31 AM on December 19, 2011

Where is this moisture coming from?

Out of the air. That's why the air is dry; it's dumped its moisture all over everything else.
posted by flabdablet at 4:23 AM on December 19, 2011

When hot meets cold you get condensation. You'll get general frosting when temperature drops to or below freezing but you'll get additional condensation and frosting on your car because after you park because its interior is likely warmer than the outside to start out with.

This, by the way, is why roofs need to be vented and why certain walls are built with plastic sheeting in them (thermal breaks).
posted by Hairy Lobster at 1:23 PM on December 23, 2011

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