Does fog eat snow?
I can remember as a child hearing my father’s sanguine observation that having a fog condition was good during winter because in his opinion it was making the snow disappear. As he described it, “fog eats snow.” Seemed logical to me at the time because the snow disappeared as the fog lifted and we could see the ground again. We wrote earlier about another method by which snow disappears — sublimation. One reader commented that he had missed seeing any significant fog events this winter (thanks for that observation).
Digging through the records of official NWS daily weather observations we find that there were very few days where the visibility was restricted by fog at Albany in February and January. Does fog really “eat” snow and does that explain why there is still much snow around? Not quite. Once again, this is a case of good observations but incorrect conclusions as to cause and effect.
Fog happens when the amount of invisible water vapor in the air is greater than the air can support and forms tiny droplets of the liquid form. We refer to that in terms of relative humidity or the dew point temperature nearly matching the air temperature. The rule of physics is that warm air can hold much more water vapor than cold air so that when air cools the vapor becomes tiny droplets. Clouds form in exactly that fashion so a good analogy would be that we have clouds on the ground when we see fog. During the winter with lots of snow cover a mass of warm air with its moisture which arrives in our area is suddenly cooled — sort of like opening the freezer door on a humid summer day. The result is fog but the action is that the warm air is actually melting the snow and changing it into its liquid form one tidy droplet at a time. Again, it melts, there is not a stream of water running across the ground, but the water is being moved into the atmosphere. Other factors are in play as well but are a bit more technical so we will leave them in the textbooks for now.
We saw that happen for a short while during the recent storm on March 12 where the dewpoint approached the temperature while it was still above freezing and a light rain was falling. Freezing fog is another matter which we addressed earlier.
Why not much fog this winter? We can look to the jet stream once more for the answer to that and remember that for us the air came mostly from the North which is typically very dry with very low dewpoints. In general, warm air with lots of moisture arrives from the South but our jet stream did not provide that transport route for us. Of course, the jet changes over time so it we have to wait for that to happen — perhaps August is a good bet!