Snow Eating Fog-It's a good thing.
February 2005 Weather Feature
Frank Watson, Meteorologist

Who would have thought snow would aid in keeping our winter skies brighter and visibilities up? That's right snow actually aids in keeping the formation of fog away.

The best way to explain this phenomemom is to take a look at vapor pressure and water molecules.

The term, vapor pressure, by itself, usually alludes to the pressure of vapor over water.

However, there is such a quantity as water vapor pressure over ice which acts differently.

Suppose in a closed metal box, with water to one-inch depth, we had air that was saturated-that is, as molecules entered the air from the water, an equal number of water molecules from the air entered the water. In another words we have an equal exchange of molecules for water to air and visa versa.

Now suppose that the water froze.

We would now see (if we had some kind of microscope) water molecules leaving the air and sticking to the ice - the balance shifts to greater number of molecules staying on the ice surface than being released equally to the air. So we're now taking water molecules out of the air.

This would continue for a while, but then the movement of the molecules would become equal back and forth across the ice surface. As such time, the air would be said to be saturated with respect to the ice surface. We could then speak of vapor pressure over ice.

Clearly, we can see that vapor pressure over water represents more water in the air than does vapor pressure over ice, if both are at saturation at the same temperature.

We witness an effect of this in the late fall and in the winter. Snow may fall on the ground, followed by a few days of cold weather. Then, a southerly wind begins to blow, a warm moist air moves into the area. The snow then chills this air, and the air becomes saturated with respect to a water surface. We may see fog form.

Snow, however, is made of water molecules in the solid state, so the snow begins to pull water molecules out of the air. The formation of fog is inhibited, and, indeed, fog may even go away close to the snow.

So, in meteorology, we say that air that is saturated with respect to a water surface is supersaturated with respect to an ice surface. This is fortunate in the northern winters, or else we would see a lot more fog in snowy climates. The snow surface, by pulling water molecules keeps our winter days brighter.

Frank Watson is a White Bear Lake, Minnesota meteorologist and can be found on the web at WeathermanWatson.com