Snow science question

[funkendrenchman]

I'm teaching a high school class about phase changes tomorrow and I wanted to throw something in about sublimation.

Why does sublimation occur in snowpack? I know it does not occur with water outside of the snowpack situation? Does it have to do with the pressure? (I know you can make liquids such as caffeine sublimate by reducing air pressure.)

I know the temp gradient means that sublimation creates hoar, but I don't think the temp gradient in fact causes the sublimation. Or does it?

[INDY GS]

my kitty's breath smells like cat food.


not sure if it will help but check this link out.

http://www.enter.net/~jbartlo/articles/070997.htm[/URL]

[Snow Dog]

Sublimation is the phase change from solid to gas. So the solid (ice) will change to gas (water vapour) until the vapour reaches equilibrium. At that point the number of vapour molecules condensing to ice equals the ice molecules evaporating. It has nothing to do with air pressure -- only the partial pressure of the water vapour matters. When you hoover away the air you are also reducing the partial pressure of the water vapour and promoting faster sublimation. With a high relative humidity sublimation is slowed and low humidity sublimation is increased. It happens in the freezer all the time.

[funkendrenchman]

Got it. Thanks.

[Pinner]

Originally posted by funkendrenchman
I'm teaching a high school class tomorrow

V cool.

[Viva]

[starts waving hands] What we're talking about here is the relation between the pressure and temperature at which two phases of a substance are in equilibrium (I think that sublimation can go both directions). What you need in your snowpack is for the enthalpy of sublimation to be greater than the enthalpy of vaporization at that given temperature. [/finishes waving hands]

There's a mathematical expression that, I think describes this relation. I called it the Cassius-Clay equation


Hang on a sec... ( does a little web browsing) Okay, here we go:

ln(Pvap) = -DHvap/RT + b

I'm not sure this equation can be used to determine the sublimation of snow. It's for the equilibrium between a liquid and gas. Oh well, I tried, but I don't remember enough physical chemistry.

[Pinner]

So? How'd it go, prof?