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Why does ice grow in the freezer rather than evaporate?

  1. Oct 16, 2015 #1
    I hope I put this in the right place. I apologize if I didn't.

    We we're having a discussion here, as to why frost builds up in the freezer.

    The discussion has been something like this:

    Guy 1:

    The water in this room temperature bowl will weigh less and less over time. This is because as long as it's not below zero degrees Celsius, the water will evaporate.

    That's right. It actually does evaporate even below zero degrees aswell, known as sublimation. If you have a tray of ice cubes in your freezer, you'll see that it shrinks over the months/years. The hotter the water is, the faster it evaporates, but even at negative temperatures, it will evaporate.

    Guy 2:
    If the water is frozen and the humidity is high, then the ice will attract the water and freeze it. So the ice will grow in negative temperatures regardless of the humidity. Just check your freezer.

    Me (being on thin ice (ba-dum-ts), realizing my freezer does grow ice):
    That requires satured air (ie 100 % humidity). If the air isn't saturated, then the ice will evaporate, rather than grow.

    Guy 2:
    Hmm. That doesn't make sense. Can you explain to me why frost grows on my windshield and why you get frost on a beer glass you've had in the freezer, even though the relative humidity is below 50 %?

    ... so now I'm trying to figure out how this actually works. I found this meteorological explanation, but I don't really understand how the relation between the dew/frost point and relative humidity work, and I don't really understand the formation of frost when the air isn't saturated. I have also of course read the Wikipedia articles on dew point, frost point, relative humidity and deposition, without really understanding it all. Can you help me bring this discussion back on track? :D

    Thank you!!
  2. jcsd
  3. Oct 16, 2015 #2
    Seems that I have read that to find the dewpoint something like a mirror is cooled until a reflected beam of light
    decreases in intensity.
    That should explain the frost on a beer glass.
    It does appear that frost will appear on a windshield before the rest of the car. But you also have to consider that
    1 gram of water has to give up 80 calories of heat before it can turn to ice.
  4. Oct 16, 2015 #3
    In equilibrium thermodynamics, this is a phase separation between the two. I don't know your level of knowledge in thermodynamics, but to start with in common English, below certain temperature (freezing point) where there is a phase separation, the amounts of the two phases (ice and vapour) are determined by the total amount of the material (water molecules), i.e. more humid, more ice would form.

    Wind shield is not in thermal equilibrium which is way more complicated. But let us assume in the vicinity of the glass there the layer of air is quenched to an equilibrium system below the transition temperature. Apparently, immediately after quenching, there is no ice phase at all; then, vapour phase definitely would like to go into ice phase.
  5. Oct 16, 2015 #4
    Even if the relative humidity is 50% in the room temperature air, the surface of the glass is at a much lower temperature, and, if the air were at that temperature, its relative humidity would be greater than 100%. Don't forget that the relative humidity is defined as the partial pressure of the water vapor in the air divided by the equilibrium vapor pressure of water vapor at the prevailing temperature. The glass cools the air right next to it to the point where the relative humidity of that air is at or above 100%. This allows the water vapor to condense onto the glass surface.

  6. Oct 16, 2015 #5


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    Every time you open the freezer, you mix the air with the humid air in your house. Warmer air can hold more water, so when the house air gets cooled by your freezer, it becomes super-saturated with water and it frosts. If you somehow remove humidity from your freezer, then ice will sublime. But the humidity doesn't really have anywhere to go, unless you fill your freezer with water-absorbing materials. (That's one way to freeze dry things at home.)
  7. Oct 16, 2015 #6

    Andy Resnick

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    The vapor pressure of -20C ice is very low- only 1 mbar (100 Pa)- so the sublimation rate is very low. Frost builds up on the evaporator coil (and surfaces near it) because it is at a lower temperature than -20C, and so water vapor from the warm exterior air which enters the freezer by opening the door preferentially condenses on it.

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