Where the Snow Goes

Shedding Light on Sublime Sublimation

Feb 17, 2018
Sublimation

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Let’s talk about winter air. Ever noticed that if you have frost on your car, it will probably be totally gone by the time you reach your destination, even if the weather is subfreezing? The cause is sublimation! The snow is turning from a solid directly to a gas, bypassing the liquid watery stage.

Examples of sublimation include:

  • Sunny snowfields shrinking and eventually vanishing without melting into water.
  • Snowmen starting to shrink even though there’s still snow on the ground.
  • Snow disappearing from your roof even though it’s too cold to melt.
  • The snow on your picnic table in the backyard going down even though it’s not melting.
  • Ice cubes shrinking in the freezer. 

All due to ice turning directly into vapor without having to first melt.

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On a cold sunny day, you may have noticed that the huge piles of snow pushed onto the edges of parking lots by plows sometimes look as if they’re steaming. This is one sign that sublimation is underway.

And if your home’s walkway or driveway has gotten a thin coating of ice and snow and you’d rather not use salt, just leave it alone. If the air is dry, as it usually is this time of year, and especially if it’s sunny, all that white stuff will go away by itself. Even if it never melts.

This is because water molecules will happily go from their liquid or solid phase into their gaseous phase; the only thing that’s required is that they’re moving fast enough. Each molecule jiggles at a particular speed and in a huge mass of them, some are always moving fast enough to escape the water or ice and join their gaseous buddies in the atmosphere.

Dry air accelerates this phase change. So does sunlight, since water molecules readily absorb the sun’s infrared, which makes them jiggle faster. Solar infrared is also why the interior of your car heats up when parked in sunlight.

It’s fun when snow and ice sublime. And yes, that’s an actual verb.

Now find out why water is so amazing and unique!

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s blog on stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe

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