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Jan. 14, 2011

Science Dad on Snow and Sublimation

by Vince Harriman

Click to enlarge images

Rowan's first snow

I don't know about you, but after last year's crazy weather, I'm already pretty tired of snow. It has only really snowed here once, and not that much, but when I look out my window, the world is cold and covered in white. And I heard on the news that every state except Florida had snow this week. Even Hawai'i had snow on top of a volcano!

So as we drove to school this week on a very cold morning, Beckett asked the perfect question for a snowy day -- where does the snow go when it 'melts' but doesn't turn to water first?

When Beckett was just a little boy around Rowan's age (Rowan is 2 1/2, Beckett just turned six), he would cry when we drained his bath. Not because bath was over, but because the water was all going down the drain. We tried everything to console him -- saying goodbye to the water, draining the bath after he got out, etc.

The Water Cycle, image courtesy USGS

Finally, I began to explain to him the water cycle -- where the water goes, and the pathway it takes to become rain or snow. This finally satisfied him. Beckett has been a scientist as long as I can remember!

 
After our discussion about the water cycle, we started to say 'see you later' to the water when we drained the bath. So Beckett was already pretty familiar with the water cycle in general -- water evaporates from ground water, oceans, lakes, and even small puddles, or it transpirates from plants; then condenses into clouds before falling back to earth as precipitation such as snow, rain, hail, or sleet. But, this week, Beckett noticed that in really cold weather the snow 'disappears' without turning to water first. So what is happening? We called his Aunt Melanie, who lives in Anchorage, Alaska, to ask her, but she didn't know.

Here is what happens:

Beckett in last year's snow

Water, like most matter in the universe, mostly occurs in three states: solid, liquid, and gas. Some substances, under special circumstances, can transform from a solid state (in this case snow) directly into gaseous state. Scientists call this process sublimation.

So even though it is below freezing outside, the heat energy from the sun gives the water molecules enough energy to break free from their bonds, which knocks the water molecules free into the atmosphere. The reverse of this process (going directly from a gaseous to a solid state -- for example, frost) is called deposition.

Sublimation happens to all kinds of different substances, we usually just don't notice it. Most home air fresheners sublimate from a solid, waxy state directly to the air. Dry ice is visible sublimation. I am old enough that I can remember my Grandmother hanging clothes out to dry in freezing weather. First, the clothes would freeze solid and stiff. Then they would slowly lose the water in them to sublimation. I was always amazed that they would 'dry' in such cold conditions.

If you want to 'see' sublimation, there are lots of great experiments you can try here ,here, and here. These all require strict parental supervision -- dry ice can be very dangerous and any chemicals should always be handled by adults, even cleaning products and air fresheners.

I don't know about you, but my weather forecast says snow again as early as tomorrow!

_____
Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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