Heat always moves from higher to lower temperature. The rate at which it moves depends on the composition of the two substances, but the equilibrium temperature does not. A bit of metal or glass in contact with air will always gain heat from the air when it is cooler than the air, and lose heat to the air when it is warmer than the air. If the metal is losing heat by radiating infrared to space, that cooling is communicated to the nearby air via conduction; i.e. the air immediately above the metal is cooled. For frost to form, the air must cool to the freezing point of water, otherwise the water molecules in the gas will have too much kinetic energy to stick to a growing crystal. Likewise, if the air immediately surrounding an ice crystal is above freezing, that crystal will melt.
Again, this shows the problem with the usage of the term ambient temperature. If you measured the temperature of the air above the surface of the frosty car, you would find that it drops to freezing during the night. As Napier pointed out, the temperature you call “ambient” depends on where and how you read it. I took the term “ambient air temperature” to mean the temperature of the air immediately surrounding the exposed object. If you read a thermometer somewhere else, you might easily conclude that ice crystals sometimes form when the temperature is above freezing.
The ground, or car or whatever is losing heat by radiation to space. As soon as the temperature of an object falls below that of it’s surroundings, heat begins flowing into the object by conduction. If the object is exposed to air then the ambient temperature of the surrounding air decreases. A radiating object will get colder and colder until the rate of heat conduction from the surroundings to the object exactly balances the rate of heat loss via infrared radiation. Since conduction through solids, liquids, and gases is usually much faster than radiative cooling, the temperature difference between a radiating object and its surroundings will be small.
Boy, I’ll bet that’s about as clear as mud . Sorry, I’m probably looking at the question from a smaller perspective than the OP intended. I See that ImInvisible just made the point himself; if you don’t prevent conduction by insulating, the temperature differences you’ll see from radiative effects will be minor.
This is correct in the context of this thread. In the “backyard experiment” ImInvisible is remembering, they probably suggested measuring different parts of the sky (clear sky, a cloud, the Moon, a tree, …) and comparing temperatures. For that, you would want the thermometer at the focus, to maximize directionality.
Thinking about this some more, to get the clearest proof about the frost on the car, tape one thermometer to the car windshield, and hang another nearby to measure the ambient temperature (in the sense of the air temperature away from anything, not right near the windshield).
For nitpicker’s only: Is the air temperature even rigorously definable? Temperature is strictly only defined at equilibrium. It’s not hard to imagine a black thermometer and a metallic thermometer giving different temperatures, even at night.
I heard this is the principle by which the ancient Egyptians made ice in the desert. Supposedly if you dig a hole several feet deep, and maybe 1 foot across, and possibly line it with aluminum foil or other reflective material, it will get below freezing at the bottom. The hole must have a clear shot to the sky. The way this was explained to me was
the same explanation ImInvisible posted from Scientific American- basically the hole is seeing the coldness of deep space. Although the ground can act as a heat sink around the sides of the hole, it can still get below freezing in the center, depending on the geometry of the hole, etc. This is a good experimental setup because the ground is usually at a relatively constant temperature several feet down.
This as also given as an explanation as to why the inside of a car can be colder than the surrounding air temperature at night.
It seems there might be some confusion as to what causes radiation. Neither infrared nor any other type of radiation depends on a temperature differential. A material at a given temperature will always radiate the same amount period.
All radiative cooling means is that the object is emitting more radiation than it is absorbing.
At the risk of making a total fool of myself with but one post, I’ll say I’m surprised dew point hasn’t been mentioned explicitly:
I’d also add that the empirical evidence that hoar forms at ambient temperatures above 0 degrees Celsius neglects the fact that heat transfer between solid (the car), liquid (the dew), and bag of mostly water (one’s hand) is a lot faster than that between the air and one’s hand. All things being equal, your hand would eventually get as cold in the air as it does touching the car. The thermal conductivities of Silicon and iron are a lot higher than those of nitrogen and oxygen, and that speeds the process up, but the system will still reach the same equilibrium. The upshot is that the ambient temperature has to be at or below freezing for hoar to form on one’s car, presuming it’s on earth and at or near sea level. That the ice feels colder than the air doesn’t mean it is, it just means it’s sucking heat from your hand faster.
I hope I’m not stating the now-obvious. This seems to be a good place and I’d like to wait a while before giving people a good reason to shun me.
Thanks The Tooth and welcome to the SDMB. I would like to add that hoarfrost formed by sublimation, as you state, is also the mechanism by which snow is formed. Water vapor sublimates to ice crystals, which fall as snow. If there is no sublimation in the clouds, there can be no snow.
You’re missing the important point that there is more than one temperature involved here. There are the temperatures of the air and of the sky. The car is thermally connected to the air by heat conductivity, and thermally connected to the (much colder) sky by radiation. Since the car is connected to two heat reservoirs at different temperatures, the car will have a temperature between the temperatures of the heat reservoirs. That temperature will depend on how well connected the car is to each of them, and their temperatures.
Think of it this way. Everyone knows that on a sunny day, a car in the sun can be several tens of degrees hotter than the surrounding air temperature. This is because it is in radiative contact with the sun. The sun is very hot (6000 C), but covers only a tiny fraction (1/185,000th) of the total solid angle around the car. The temperature of the night sky is much closer to the ambient air temperature, but it can cover up to 1/2 the total solid angle around the car. It shouldn’t be surprising that on a clear night, a car exposed to the cold sky can be a few degrees colder than the surrounding air.
Or I’d back this up a little better. ZenBeam said:
I understand that there are going to be a couple of mechanisms for heat loss, but it seems to me that Squink is right, that the heat loss due to radiation is negligible in comparison to convection and conduction. Is this what you mean by the temperature of the air as well as the sky? Otherwise, I’m not sure what you mean. I don’t think counting the coldness of space as a heat sink is valid. Isn’t that what
means?
But regardless of the mechanism of heat transfer, I have a problem wrapping my head around the idea of an object becoming colder than the environment to which it’s losing heat. Heat sinks just don’t work that way. Equilibrium between a hot and cold object is always somewhere in between the two starting temperatures of the objects. Always. If you take it to the extreme, what you’re saying is a car can eventually cool down to nearly absolute zero sitting in air at 10 degrees. Free refrigeration for all! The problem is, the heat from the car isn’t radiating into space, it’s radiating into the air, warming it to some degree. The car won’t cool below the surrounding temperature by this mechanism, it’ll meet somewhere in the middle, as you said. You’re essentially saying that space is a heat sink for the car but not the air, I think. But the car is insulated from space by the air.
It had better be!
In the end, I can’t think of any way in which a warm object can drive the ambient temperature of the air surrounding it down below the dewpoint, which must happen for dew or frost to form. If there’s dew or frost on the car, it’s because the temperature of the air around the car dropped below the dewpoint. That’s only going to happen if the weather changes or the car was cold enough to change the air temperature in the first place.
I just brought up thermal conductivities to illustrate that the car/hand system is going to reach equilibrium sooner than the hand/air system, even if the same temperatures are involved. Cold metal feels colder to us than cold air, but that’s just because it cools us off faster. We can’t replace the heat as fast as the metal takes it, so we feel colder. So saying there’s ice on the windshield when the air feels warmer isn’t a guarantee the car is colder than the surroundings, whether that should be the case or not.
You can’t say the object is getting “colder than the environment” - the environment is a complex system and doesn’t have a single temperature. The temperature of an object settles to a point where heat input is balanced by heat output.
Say you have an object in the room, and the object’s surface does not emit or absorb infrared light (i.e. it’s very shiny). Conduction is the only source of heat loss/gain. Under these conditions, the temperature of the object will be the same as that of the surrounding air. Now if you paint the object black, it will absorb and emit infrared. The object emits infrared radiation, but so do the ceiling and walls, and the object absorbs that radiation. They ancel each other out and it will remain at the same temperature. (This assumes that the ceiling and walls are at the same temperature as the air.)
Now you take away the walls and ceiling. The object continues to emit infrared radiation, because the rate of radiation only depends on the temperature of the object. But now it has no infrared to absorb, because there is nothing around the object that is emitting infrared. So the temperature of the object starts to drop. But not forever - because as soon as the temperature starts to drop, heat begins to flow in from the air by conduction. At a certain temperature below the air temperature, the rate of heat input by conduction equals the rate of heat loss due to radiation, and everything is balanced again. In this equilibrium condition, the object’s temperature is lower than the air temperature - the temperature difference must be maintained, otherwise there will be no heat input to cancel the heat loss due to radiation.
Sorry, I was oversimplifying and assuming the closed box conditions. I should have remembered how desert nights work. Kind of embarassing, actually.
With regard to the OP, the air from which the water was drawn to form frost in the first place must have been cooled to below its dew point. The car has spent the night cooling the immediately surrounding air by pulling heat out of it to make up for heat lost to radiation. So on a clear night, the objects radiating the most should cool the air the fastest, and end up covered in dew or frost first should that air be cooled enough.
. Sorry, I’m probably looking at the question from a smaller perspective than the OP intended. I See that ImInvisible just made the point himself; if you don’t prevent conduction by insulating, the temperature differences you’ll see from radiative effects will be minor.