What is Fog?

We've had several foggy days this fall in our neighborhood, which led my 7 year to ask me, while waiting for the bus one foggy morning, what is fog?  Is it a cloud?  Is it something else?  What is it?

We've all seen fog come and go, but what is it?  How does it occur?  Fog is nothing more than a collection of water droplets and/or ice crystals.  It's basically the same thing as a misty day, except the visibility is much worse in fog than it is when it is misting.  This makes it the same as a cloud, but on the ground.  That begs the next question.  How does a cloud form on the ground?  

Fog forms when water vapor near the ground cools and condenses (changes phase) from a gas to a liquid, hence the tiny water droplets.  How this happens can differ depending on the type of fog.  A great source for this is:

How Does Fog Form?

In most cases, the fog will "burn" off as the sun rises.  The temperature of the water droplets increase and change phase back into a gas.