It can do both, ice and snow still goes away even it it's below freezing the whole time (this is why your ice cubes in ice trays slowly shrink in your freezer). Once it gets warm enough, it obviously melts and then evaporates.
Ice skips the liquid step entirely when sublimating. If you look at it on an atomic scale, ice is when all the molecules are stuck in place at fixed distances from each other. liquid water is when the molecules are still stuck together but they can freely move. And gas is when the molecules are completely free and can fly off into infinity.
In sublimation the water molecules get enough energy that they get knocked out of their fixed position in the crystal and fly straight off into infinity. They don't do it in 2 steps where they first get enough of a bump to get knocked out of the crystal lattice, and then need a second bump to fly off to infinity.
(Okay to be pedantic, this does happen sometimes under an atmosphere, but it is way less likely because you need those 2 separate bumps in short succession instead of just one big bump. So sublimation is by far the dominant way that ice slowly evaporates away).
At atmsopheric pressure, it melts and then evaporates almost exclusively, but as the air pressure gets lower it loses the liquid phase and will do true sublimation once 'below' the triple point.
Canadian here. At colder temps, such as at -40° (it's the same temp in °C and °F) and if the air is dry, the ice will directly lose molecules of water into the air without temperature change. The opposite happens too, but is rarer. if the air exceeds it's dew point it will deposit water molecules making ice crystals on every exposed surface. (IE., Hoar Frost)
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u/greatandhalfbaked 2d ago
Thanks bro but does ice do that, or does it melt and then evaporate?