Last weekend was quite interesting as far as weather is concerned. We had near record warmth during the days and dense fog developed Friday afternoon and lingered through mid-morning on Saturday. We’re used to seeing fog in the mornings, but what causes it to form in the middle of the day?
In Friday’s case, rain had a lot to do with it. Fog is water droplets suspended in the air, trapped very close to the earth’s surface, and limiting visibility – basically a cloud trapped near the ground. Without Friday’s rain, we wouldn’t have had enough moisture to cause such dense fog. Warm air helped as well, but not in the way you might be thinking.
I’m sure you’ve heard the phrase “warm air rises.” It’s true that a parcel of air (think of it as a little balloon) will rise as long as it is warmer than its surroundings. When that parcel of air hits a layer of air that is warmer than itself, it stops moving upward. Normally, the air in the troposphere, which is the lowest layer of our atmosphere where most weather happens, gets cooler with increased altitude. When the temperature actually increases with height within this layer, we call the pocket of warmer air an inversion.
Not all inversions cause fog, but if an inversion close to the surface traps warm, moist air like what we had after the rain on Friday, we get fog. The more humid the air, the denser the fog will be. There are other types of fog that can form, but most of what we see here in the Triangle, is created by inversion.