Hi! 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. Me: 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!!