Matt Miller’s Fog Blog
Saying that this past week has been a little bit foggy is like saying that we were a little cold in early January. Gross understatements. However, the two are very much related.
I've been asked many times in the last few days why we've been so foggy. Seems that no one has the foggiest idea (shameless pun) as to why we've been stuck with day after day of fog, some lasting all day long. The answer lies in what is currently lying on the ground...our snowpack.
After the Arctic chill in early January, the ground was extremely cold and snow covered. Although the air temperatures have returned to the 30s, not all of the ground temperatures have. That means you've got warmer air right on top of cooler ground. That warmer air keeps melting a little bit more of our snow and that adds more and more moisture to an already saturated air mass.
As that moisture is added to the air situated on top of the snowpack, the dewpoint is increase. Recall that the dewpoint is a very good measure of how much moisture there is in the air. As the air remains situated over a cold layer of snow on the ground, the air itself is cooled down a little bit.
As the air cools down, the temperature becomes closer and closer to the dewpoint. When the two numbers meet, you have widespread condensation since you've reached 100 percent relative humidity. That leads to fog, heavy frost (like earlier this week when we had freezing fog) and we are left sitting with our heads literally in a cloud.
There is some hope that we'll have fewer fog episodes later in the week as rain should help clear the air of some of the suspended particles that are easily forming into fog droplets each day and also help to melt away more of the snow and let the ground warm back up a touch.