I love fog, and we had serious fog last night as you can see from these (blurry) pictures.  Driving in it was a bit dicey though, mostly because people packed in really dense and everyone was afraid to spread out.  That seems more dangerous, as opposed to more safe!  But anyway, fog.  Fog is cool...

 

What causes fog?  When I was a little boy my Dad used to tell me "Fog is just a cloud on the ground."  He was basically correct, although the processes that form fog are typically not the same as the ones that form clouds.

Clouds form when humid air rises into the atmosphere and cools.  When humid air cools, water droplets condense in the air which makes it look foggy.  From a distance we see this blob of cooled humid air as a cloud.

Like a cloud, fog is humid air that has cooled, but obviously not because it rose into the air.  At night, the ground tends to radiate away infrared energy picked up during the day, this makes the ground cooler.  If a warm humid air mass is present over the cooling ground, the air mass will start to cool, and fog will form.  This is referred to as "radiation fog" since infrared radiation plays a part in it's formation.

You can also get fog during and after rain because the falling rain moistens the air near the ground, as well as cooling it and the ground.  This sort of fog has as much to do with saturating the air as it does with cooling it.

Last night we had a different variety of fog.  A humid air mass is cooled by moving in over a cooler surface, in this case the surface was snow and ice.  This kind of fog is called "advection fog" and can accelerate snowmelt.  This kind fog is also formed on the coastlines when warm moist air moves over colder coastal water and landmasses.

Here's a wiki article on fog if you want to learn more.

Humorous side note... when I typed "Types of Fog" into google, the first page that came up was this.

Scary sidenote, fog near factories can sometimes be deadly.   Also, fog containing zombies is apparently dangerous too.