Fog is a finicky weather phenomenon. It can come and go quickly, or stick around for days if the conditions are right. So what creates it? Fog is essentially a cloud at the surface. It is defined by creating visibility about 1 km, whereas misting has visibility of 1-2 km. There are many different types of fog including Radiational, Advection, Upslope etc. Fog formation requires the dew point temperature to be less than 4 degrees Farenheit, or 2.5 degrees celsius away from the air temperature.

There must be dry air aloft, or at the upper levels of the atmosphere, and a moist layer passing over a cooler surface, with relative humidity close to or at 100%. Very light winds, typically 2-5 mph mixes moisture at the surface, often times left over from heavy rain is conducive for fog. Advection fog, which is what we have been experiencing lately, is often when a warm front moves over an area.  When the air is 100% saturated, it can no longer hold any water vapor, and moisture condenses into water droplets which creat fog.

If you'd like to read about the other many different types of fog, Wikipedia.org provides easy to understand summaries of each!