Originally Posted by Ghost
Dusting off weather and chemistry from grades 9 and 10...
Fog is caused by warm air sitting on top of colder ground (or snow). What happens is the cold ground (or snow) cools off the air to the point that the water vapour in the air condenses into water droplets.
A gram of water vapour has more energy than a gram of liquid water, which has more energy than a gram of ice. You will notice that when you put an ice pack on your injured shoulder; as the ice melts it sucks thermal energy from you shoulder making it colder. If you have ever beened burned by a jet of steam, you will know that steam gives off a lot of energy when it condenses.
When ice or snow melts and becomes a liquid it takes heat (absorbs thermal energy), when the liquid evaporates and turns into a gas it takes heat (absorbs energy). When vapor condenses it gives off heat (releases thermal energy), when liquid freezes it gives off energy. Ice turning into water at 0 C takes [Spock voice] approximately 333. 55 [/Spock voice] Joules; water turning to water vapour takes about 2257 joules per gram. It's the same amount of heat going the other way.
What happens to the thermal energy of the water vapour when it turns to tiny water droplets in the fog? for every gram of water vapour turning into fog, 2257 joules of thermal energy goes into the air, energy that could melt over 7 grams of snow. It heats up the surrounding air, and the surrounding air gives up its energy to melt the snow.
So, yes fog makes snow melt, but it doesn't eat it. The water won't want to evaporate into the already saturated air; it will erode the snow pack from underneath.
Warm dry air might evaporate snow faster than fog can melt it though.
Ghost is correct. The thing is that the energy that the vaper gives off to condense into fog is a one time thing. It happens at the moment that the fog condenses and is gone. When the fog changes back into vapor, the enegry is reabsorbed. I think that the effect of this energy exchange would impart less energy into the snow pack than direct solar radiation would. In my opinion this implies that the net effect of fog is to insulate the snow. In the atmosphere, the moisture and the temperature remain in equilibrium relative to each other and I don't think the ground absorbs a significant amount of this energy. In fact if there was enough moisture colliding with cold air, the individual fog droplets would increase in size until they got heavy enough to fall out the atmoshere and it would then be raining. Rain does more damage to snow than fog.
I'm basing this on my college chemistry, I took a fair amount. We did fractional distillation experiments where a mixture of different liquids is heated and the time/temperature graph is plotted as the different components of the mixture are vaporized, condensed, and collected. The temperature flat-lines during the time that each fraction is being collected. Even with surplus energy being pumped into the system, the temperature of the mixture can't increase past the individual vapor points until each liquid fraction has been fully converted to vapor. Each collected component can be identified by looking up the vapor point in a reference manual.
In the case of fog this works in reverse. The air (oxygen, nitrogen, and other things) can't warm up while the vapor is condensing. If it does, the fog will burn off. When we see fog, the temperature is at or below dew-point or we wouldn't see fog.
The other thing that is needed in the atmosphere is a condensation nuclei. This is usually a piece of dust or something like that. When there is a surplus of moisture relative to the temperature and not enough condensation nuclei to accommodate this moisture, the atmosphere becomes supersaturated with super-cooled water vapor and in the winter we get rime growing into the wind. The supercooled water vapor flash freezes onto any cold object it encounters. The rime ice grows into the wind.
Also warm dry air doesn't evaporate snow, it sublimates it. Evaporation is the change of state from a liquid to a gas. Sublimation is the direct change of state from a solid to a gas. Dry Ice sublimates from solid CO2 to CO2 vapor at room temperature. That's why its called "dry" ice.