Fog is essentially a stratus cloud that forms in contact with the ground. Clouds will typically form when relative humidities near 100%. In other words, when the temperatures drop to the dew point. However, often only dew forms when that is the case.

While there is not a definitely “fog will always form when…” type of answer, there are certainly some things that can make fog formation more likely.

Wet ground conditions is one. As the temperature falls to the dew point, it is more difficult for condensation to occur when the ground is already wet. So, often the result is condensation on suspended particles in the air. (Since every suspended droplet needs to have something as its nucleus).

On that note, the more nuclei that you have in the air, the more likely there is to be fog. So the “dirtier” the air, the more likely fog and clouds are. Air that is too clean would eliminate our potential for clouds and precipitation. Air that is too dirty is just unsafe to breathe. Lucky for us, we have a nice balance nearly all of the time.

I just wanted to give you a few thoughts to have the next time you drive through a patch of fog. Don’t think about it too hard, you don’t want to rear end the car in front of you!