Winter Thermodynamics: Foggy Glasses

We're having a brutal cold snap at the moment, and while today's early-morning dog walk was considerably warmer than yesterday's, it was still 0F/-18C out, which is way colder than I like. When I came back in the house after the walk, my glasses instantly fogged up. But I had to take some stuff outside for the recycling, and within a few seconds of stepping back outside, they cleared right back up.

Now, I knew that would happen (which is why I hadn't bothered to wipe them off inside), but in one of those physicists-are-a-little-weird moments, for the first time, it occurred to me to wonder why it is that that happens. Because it was single-digit Fahrenheit temperatures outside, and in that case you might naively expect the water droplets fogging my glasses to just freeze into a thin layer of frost, like the crap that forms on the car and makes getting ready to take the kids off to day care so annoying. After all, when I take out one of the beer mugs I keep in the freezer, that forms a layer of frost all over it very quickly.

This is one of those posts where I'm pretty sure I know the answer, but thermal physics has never been my strong suit, despite taking three classes in it (a combined thermo/stat-mech class as a senior at Williams, and both thermo and stat-mech in grad school, from the Chemistry department), so I'm going to talk it out here.

I think it's a sort of thread-the-needle kind of thing. My glasses, even after walking Emmy in the pre-dawn chill, are not in equilibrium with the outdoors, because they're in contact with my face. That's why, when I come inside, I get fog on the lenses, but not frost. If I left them sitting outside for an hour then brought them in, they'd probably frost up like my beer mug, but as cold as they are, they're not quite cold enough to freeze the moisture in the inside air.

This temperature difference also means that, when I go right back outside, they're a bit warmer than the ambient environment. And even though they're cold enough to condense water from the air inside the house, the air outside the house is so unpleasantly dry that my glasses are warm enough to drive the condensed water droplets back off the lenses in short order. Inside the house, the dew point is above the temperature of my glasses after a dog walk, but outside, it's lower.

This is also why they mostly stay clear on the actual dog walk, despite the fact that I sometimes get moist air on them when I exhale into the windbreaker I wear covering my lower face. There is a level of wretchedly cold, though, where that isn't the case-- I've had extremely cold dog walks where my glasses fogged up from my breath and then wouldn't un-fog, leaving me essentially blind. Yesterday morning's walk was pretty close to that temperature-- it took awhile, but they did eventually clear. wWich is a good thing, because Emmy doesn't remotely have the attention span to be useful as a guide dog...

If you would like an extra-credit homework assignment (and who doesn't like extra-credit homework? Other than faculty, I mean...), see if you can estimate the temperature of my glasses based on this-- it was 0F and 63% humidity outside according to Google, and 68F inside. Make sure to show all your work, and send it to Rhett for grading. Serves him right, living in Louisiana where it's all warm and festive...

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