I was watching BBC Planet Earth and on the deserts show, it is said that the snow in gobi desert evaporates, rather than melt. Can someone explain?

I believe the term is sublimate.

http://en.wikipedia.org/wiki/Sublimation_(phase_transition)

I believe the term is sublimate.

http://en.wikipedia.org/wiki/Sublimation_(phase_transition)

How does it happen? [On a chemical/ molecular level]

As far as I know, it doesn't. Ice only sublimates significantly at low pressures. (Less than 1% of atmospheric, or about 5mm of mercury.) So I don't see how it could go directly from solid to gas under the conditions found in an earthly desert.

I can only guess that some degree of artistic licence was taken, and the strict physical meanings of "melt" and "evaporate" weren't adhered to.

I take it to mean that the layer of melt-water on the snow evaporates into the dry air so quickly you'd never see a puddle of it.

Here is a diagram which I think shows this;

http://upload.wikimedia.org/wikipedia/commons/thumb/0/08/Phase_diagram_of_water.svg/700px-Phase_diagram_of_water.svg.png

I can't see any way to get from solid to gas without either going through the liquid phase or being below a fraction of normal pressure.

I was watching BBC Planet Earth and on the deserts show, it is said that the snow in gobi desert evaporates, rather than melt. Can someone explain?

We associate, desert is hot , but the Gobi in the norther part might be cold ,
so if there is a constant high wind and the temperature is below zero the it sublimes .

I learned the term many years ago from watching the local weather forecast. The meteorologist said that the light dusting of snow we had was going to skip the liquid stage and immediately evaporate into the atmosphere. I'll have to defer to someone who knows what they're talking about as to the actual molecular process.

As far as I know, it doesn't. Ice only sublimates significantly at low pressures.

Ice sublimates in freezers even when the temperatures are never above freezing.

It is very common for ice and snow to sublimate. The only thing that is needed is snow, temperatures below 0C and low humitdity. The water molecules simply evaporate from the snow without going through the water phase.

Ice sublimates in freezers even when the temperatures are never above freezing.

Oh. So it is a significant effect. The phase diagram is just an approximation then.

I didn't know that.

The temperature swings massively, with daytime highs in summer of 50 C and nightimes in winter at -40 C, so maybe that could explain it - but no, its doesnt melt at all, animals have to eat snow rather than drink water.

Ice sublimates in freezers even when the temperatures are never above freezing.

Sure, Ice have a vapor pressure, So there is necessary an air current to remove the vapor and the vapor builds on again and the process goes on and call it sublimation .

Ice sublimates in freezers even when the temperatures are never above freezing.

Correct.

Think of it like this (very simplified): Ice molecules don't adhere to each other like--say--Iron atoms. Over time, Ice molecules simply "cast off" away from the ice. Exterior molecules float away with the wind. It very closely mimics erosion. Put a block of ice in your freezer near the vent (where cold air blows in). Watch how the part nearest the air circulation "erodes" away.

~String

We have a word, here in the southwest US, for rain that evaporates before it hits the ground . . . "vergas" . . . interesting phenomenon.

Think of it like this (very simplified): Ice molecules don't adhere to each other like--say--Iron atoms. Over time, Ice molecules simply "cast off" away from the ice. Exterior molecules float away with the wind. It very closely mimics erosion. Put a block of ice in your freezer near the vent (where cold air blows in). Watch how the part nearest the air circulation "erodes" away.

~String

Yes, that makes sense.

Of course, just because the statistical average kinetic energy of the molecules is too low to be a gas at atmospheric pressure, at the small scale individual molecules can still have enough energy to break free. And then as long as the partial pressure of water in the atmosphere is lower than the vapor pressure of the ice, you can have a net exodus from the ice to the atmosphere, so it will eventually diffuse away.

"Sublimation" of ice in the Gobi is also likely related to very low atmospheric humidity . . . cold air 'holds' less moisture than hot air . . . . may also have to do with winds . . . IMO, the ice-in-the-freezer analogy is a good model for the Gobi . . . .

Ice often evaporates directly from the solid to the gaseous state. A frozen lake may have all the ice on it disappear without the temperatures ever rising above freezing if there is a strong wind blowing across the surface of the ice.

If you've ever played with dry ice, you will understand sublimation - it just burns off as a gas without first melting.

A certain amount of ice will always sublimate under normal conditions but on the Gobi you have factors that contribute - dry air, wind and no shade. Also, some areas are at elevations where there is a slight reduction in atmospheric pressure, and the lower the pressure, the faster the sublimation. Snow will sublimate faster than ice since it is porous to the air so there is more surface available to evaporate, and it will react to sunlight more quickly than ice because the air between individual snowflakes impedes thermal conductivity.

Ice often evaporates directly from the solid to the gaseous state. A frozen lake may have all the ice on it disappear without the temperatures ever rising above freezing if there is a strong wind blowing across the surface of the ice.

Amazing. I've never seen that. It doesn't get that cold here, or dry.

Michael, I saw it often in New Jersey, which is neither cold nor dry. Most people just never notice it, and assume that the ice melted from below, turning to water.