The State of the Snow

This morning's walk and the very cold weather we've had in the past week brought up a very good question.

We got five inches of snow last week. And the temperature has not got above freezing since then. But the snow looks now like less than five inches, and it is ragged and crunchy where the sun shines on it. But if the temperature has continued well below freezing, then what happened to the snow?

It's a very good question. I really like it when N. asks questions like that. Technically, N. is doing science through his Kamana II studies, which is mainly the ecology of the Sandia Mountains, as well as related natural phenomena. But weather and climate are part of the local ecology.

In the picture to the left and the picture above, you can see that there is no longer a uniform covering of five inches of snow on the ground. Where did it go, indeed!

The answer is not magic, it is sublimation. On earth, matter exists primarily in three states: solid, liquid, and gas, listed here in order of increasing energy. A fourth, and very energetic state of matter, plasma, is not so common on earth, but is very common in the universe.

Normally, we think of snow--a solid state of water--as being removed by melting to become liquid water, which happens when the temperature gets above 32 F (0 C), which is the freezing point for water.

So what is happening in the picture on the right, where there is no water--and, in fact, the temperature was 8 degrees F, which is well below the freezing point? Shouldn't the snow just hang around as a solid until the temperature gets high enough for a phase transition from solid to liquid?

In two words, not always.

Sometimes, when the vapor pressure at the surface of the solid is lower than the triple point for that substance, the whole liquid state is skipped. The state transits directly from a solid to a gas. This kind of phase change is called "sublimation."

Here, in our desert mountains, we lose a lot of snow to sublimation because the air is not capable of holding very much moisture due to altitude. Nor does it retain heat well, because of how dry it is.

This means that on a sunny, very cold day when there is snow on the ground, the sun hits the surface of the snow and as it reflects back, it warms the air above it. This lowers the vapor pressure at the surface of the snow, so that sublimation occurs. Sometimes, when the light is right, you can actually see the waves of water vapor coming off the snow. As sublimation occurs, the snow becomes pitted and crunchy, not from melting and refreezing, but from sublimation.

In the picture, you may notice that the dirt now visible due to loss of snow from sublimation is frozen, and quite dry.

This continued very cold and clear weather due to a high pressure parked over the Four Corners region means that we will not get a lot of mud from melting of this snow cover.

And the 'shoe yekke' in me likes that. This means reduced vacuuming and mopping and washing of rugs.

On the other hand, this very common way for the snowcover to disappear also means that we do not retain as much water in the soil, perpetuating the dryness of our desert mountains.

Science:It's everywhere!

Even on our morning walk.