So, this morning I noticed a phenomenon I’ve noticed before, and I’m finally curious enough to ask. I walked out to my car to go to work, and it was covered in frost. Was it whore frost? I don’t know. Anyway, the top of the car, and to a lesser extent the hood, trunk and windshield, were all frosty. Yet the overnight low was something like 42 degrees, which means it should never have been below freezing. How does that happen?
Your car will lose heat primarily by both conduction and radiation. When the sky is clear and the car is in the open, it can radiate heat away faster than it can absorb it through conduction from the surrounding air. Often the final equilibrium temperature can be quite a bit lower than the air temperature.
It was 42 at the official weather station which is probably on a hill or higher elevation than you’re at. It’s not uncommon for a 10 degree temperature swing to occur at night, even several hundred yards from one location to the next.
I’ve ridden the motorcycle at night through low lying areas where the temperature differences were/felt like 15-20 degrees cooler.
Are you sure it was frost? Did you scratch the material and did it stick to your finger nail?
Finally, screw off! We just got 20 inches of snow yesterday.
1/32 of an inch of frost. :rolleyes:
Yes, that is hoarfrost.
See Frost - Wikipedia about 1/4 down the page.
I thought it might be something along these lines, but lacked the requisite scientific acumen to articulate it.
Oh, I’m sure. I had to scrape it off my windshield with a CD case (real ice scrapers are not a common item here). Plus I grew up in Maryland and lived for ten years in Utah. I know frost. 
Hey, the trade off is that by St. Patrick’s Day, it will already be 92 and sweltering here.
An excellent and clear explanation.
Did it ask for money? If not, it was just plain old slut frost.
OK, next question: I can understand how my car is losing thermal energy through radiation as long as the air temperature is lower. For example, if the top of my car is 60 degrees and the air is 40, the metal is going to give up heat to the air. But what we’re saying here is that even when the surface of the car hits 40 (in this case), it continues to radiate heat, so that the car temperature drops below 32 and frost forms? Why would that be?
Thanks to those who are fighting my ignorance here, as well as those who are keeping it lively with zingers about the virtue, or lack thereof, of my car’s frost.
This is the distinction between conduction and radiation that flex727 was making. On a microscopic level, your car will lose heat in two different ways: through direct contact with the air molecules, and through the emission of infrared radiation (photons). If the night is clear, and there’s not very much water vapor in the atmosphere (obviously you need some, otherwise there wouldn’t be any frost at all), then the infrared photons that the car emits will zip off into outer space, taking heat energy with them.
If photon transfer was the only heat transfer mechanism available to your car, it would eventually cool down to about 2.7 K — the temperature of the photon radiation in outer space, also known as the cosmic microwave background. Of course, your car is safely on Earth’s surface, so once the car gets below the temperature of the atmosphere, the air surrounding the car will actually work to warm the car back up. But if the air is still and fairly dry, then it can’t transfer its heat to the car quickly or efficiently enough to compensate for the loss of heat to outer space. Net result: a car that’s below freezing, even though the atmosphere isn’t.
This phenomenon can be taken to extremes: it’s possible (or so I’ve heard) to make ice in the desert at night, simply by putting an shallow, insulating bowl of water and making sure that the water can’t “see” anything but the sky. This lecture on radiation transfer is pretty fascinating, if a little technical.
flex727 and MikeS, thanks a lot. You just explained that more concisely and cogently than any science teacher ever could.
Every object emits radiation, simply because the atoms are moving, and moving electrical charge generates electromagnetic waves. (Unless the object is at absolute zero temperature.) The hotter the object, the more radiation it emits, and at shorter wavelengths - your body emits mostly infrared, the filament in your light bulb emits visible light, and the million-degree corona around the Sun emits UV and X-rays.
Objects also receive and absorb radiation. The desk in front of you is emitting radiation in all directions, but also absorbing radiation from the walls, so they eventually reach the same temperature and there is no more net transfer of heat from one to the other. That is to say, the desk is radiating at the same rate it’s absorbing.
Now, consider the car parked outside. During the day, if the sun is shining, the car absorbs more radiation than it emits, and gets warmer than the surrounding air. Eventually it gets so hot that it loses heat to the air (convection/conduction) at the same rate that it’s absorbing the radiation from the sun, and the temperature stops going up, but it stays warmer than the surrounding air.
If it’s night, and the sky is cloudy, then the car still receives some infrared radiation from the clouds. The clouds will be somewhat colder than the air at ground level, but not much. The temperature of the car’s surface will be very close to that of the surrounding air.
Now, consider a clear night. The car still radiates heat, but there’s almost no radiation coming from above. Clear air does emit some infrared, but it’s a very small amount. So the car loses heat and this heat is not replenished. The car’s surface becomes colder than the surrounding air.
Another example of this sort of radiant cooling:
The front lens of some telescope designs will tend to fog up. Telescopes are, of course, aimed at the night sky which sucks enough heat out of the lens that its temperature drops below the local dew point; hence condensation. The rest of the 'scope is at about ambient air temperature, but heat can’t flow from the body to the lens fast enough to keep the latter from “chilling out”.
scr4, would the color of the car make any difference at night? During the day, clearly it does; darker colors will reflect fewer of the photons, meaning that the car’s surface absorbs more heat energy. But would the color make any difference in how much energy it gives off at night?
It’s common to hear of weather forecasters talk of “a touch of ground frost” when the predicted overnight low is around 2 to 4 deg C (36 - 39F). That means you may see hoarfrost on the grass. Cars, being better radiators than grass, tend to get iced up before the ground does.
If you’re talking about cars, the color of the car shouldn’t make much difference. Black paint and white paint are both pretty efficient at emitting (and absorbing) infrared radiation. In other words, they both look dark in the infrared.
But there are some surfaces that emit (and absorb) less IR than others, like a clean shiny metal surface. So if you wrap your car in aluminum foil, it should prevent frost from forming. The glass surfaces of a thermos bottle is coated with aluminum for this reason - the vacuum stops conduction, and the shiny surface minimizes heat loss by radiation. The “space blanket” is another application.
By the way, I said “emitting and absorbing” because that’s always the same number for a given wavelenth. You can’t have an object that emits IR but doesn’t absorb it - if you did, it’d cool down on its own, which violates the laws of thermodynamics. (You’d be able to use it to power a perpetual motion machine.)
Excellent! I’ll do this tonight!
So why does my windshield frost up, but the side windows remain clear?
Probably because the windshield is at an angle, and is exposed to the sky. It doesn’t “see” (receive IR radiation from) much else. The side windows are nearly vertical, and it mostly sees the ground, trees, building walls, etc which are “warmer” than the sky (i.e. emit more radiation).
Although if the car is in a very exposed location (like our driveway) the side windows are also exposed to the sky, and they do frost up.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAH!!!