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It was very cold outside, this morning, when I took the car that slept in the snow, with a simple cloth on the windshield. I entered the vehicle, drove a kilometer or so. The air inside was so cold I could see my breath (or maybe I forgot to brush my teeth). Warm air was blowing on the windshield from the inside and suddenly…

The glass in front of my became opaque, starting from the bottom (the place where the hot air was blowing) and freezing up, up, up until the whole screen was filled in about 5 seconds. It reminded my a little bit of how some baterias spread.

When I tried to remove the mist, I realised it was ice in the car. My windshield instant froze.

I know the theory behind supercooling: a very cold and still liquid can freeze when moved. I'm not sure what happened here, but there were some pretty big turns in the road before it froze. It really looked like the hot air froze the place and not the movement.

Any idea about what happened ?

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I don't feel confident enough to make it an answer, but my guess would be a combination of three factors. 1) Your breath was adding humidity to the interior environment; 2) the warmth from the heater allowed the air to hold the moisture; 3) an initial freezing spot provided nucleation sites for crystal growth. –  AdamRedwine Jan 31 '12 at 13:17
    
@Adam: That's what I would guess too (though I'm doubtful about 3). –  Mike Dunlavey Jan 31 '12 at 18:55
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2 Answers

up vote 1 down vote accepted

Maybe that was just window frost (http://www.its.caltech.edu/~atomic/snowcrystals/frost/frost.htm - "forms when a pane of glass is exposed to below-freezing temperatures on the outside and moist air on the inside")?

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You removed the insulation that the snow provided on the windscreen. With the snow on the windshield the temperature of the glass windscreen was the same everywhere, say - 10C (for example). Outdoor temp was say - 20C. Then you start driving, the exterior of the glass now dives to -20, the inside wall to say -15C while the inside air is now up to say -6C, so moisture starts to accumulate on the cold windscreen, which of course will be in a frozen state, giving you frost.

Had you driven with the snow still on the windscreen (not recommended), then the glass would be insulated from the cold outside, and would not fog. I always clear the windscreen last, after the rest of the car, and hopefully after the engine has warmed things up a bit.

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