# Water Sublimation Question

“The loss of snow from a snowfield during a cold spell is often caused by sunshine acting directly on the outer layers of the snow.”

Can someone explain in layman’s terms how this works? It’s sunny and 9 degrees F outside and the snow that fell a few days ago is slowly disappearing.

Where is it going exactly?

It’s subliming- changing from a solid to a gas without first becoming liquid. Ice cubes in freezers do the same thing. Sublimation is a large part of “freezer burn”, too.

Ever wonder what freeze-drying is? They take the stuff they want to freeze-dry and freeze it. Then it’s put in a cold chamber where the air pressure is dramatically reduced, speeding the sublimation.

The snow on the ground or the ice in the freeze-dried food is going into the surrounding atmosphere.

Into the air, the same way liquid water goes into the air when it evaporates.

Ice is made up of H20 molecules, just like liquid water. And just like liquid water, those individual H20 molecules can sometimes get a kick with enough energy (from a ray of sunshine or just from a random collision from a gas molecule) to set them free. The H20 molecule in ice requires a bigger kick to be set free than the H20 molecule in liquid water. That is why liquid water usually evaporates faster than ice sublimates.

It works a lot better when the air is very dry, too. We get a lot more sublimation around here (semi-arid) than in my hometown on the Great Lakes.

And it works better when it’s dry because the the idea runs both ways… in other words deposition, sublimation’s counterpart, is a competing factor.

So it’s 47% humidity outside. Is that considered dry? The dew point is -2 degrees F. Does that play into it somehow?

47%? I’d call that moderate-to-dry. The dew point of -2F tells you at what temperature 100% humidity is for you (ie at what temperature the rate of deposition is greater than the rate of evaporation). If it is below freezing, the deposition will result in frost. If it is above freezing, the deposition will result in dew. In your case, it’s telling you that (assuming it’s below freezing so the ice doesn’t melt) some ice outside will sublimate. If it get’s colder than -2F then the opposite will occur – the ice will grow frost.

To clarify, the relative humidity is a ratio of how much water vapor the air is currently holding, compared to how much it could hold at the current temperature. If you change the temperature, then you change the amount of water the air can hold. If you lower the temperature without changing the actual amount of water vapor, then eventually you reach a temperature where the air can’t hold that amount of water vapor any more, and any excess is dumped out as dew or frost.