[nudephil (OP)]

This question was sent in by Bob:

Why do the very tiny water droplets in fog remain suspended and not collide together to form larger/heavier droplets, falling as rain and clearing the fog?

Does anyone have the foggiest idea?

[Janus]

Fog needs a couple of conditions to exist.
The temp needs to be below the Dew point.
You need some slight air movement, but not too much.
This second one is significant here.   When the air is fairly stagnant, air resistance tends to keep the droplets apart. Then there is also the fact that the droplets aren't constant, but always in the state of forming or evaporating. In other words, a fog bank is formed by water droplets condensing, then evaporating, and new droplets condensing to replace them.... 
However, if the air becomes more turbulent, then the droplets will start bouncing against each other, forming larger droplets, and eventually fall out as rain.
So fog forms in that quasi-stable state where water vapor condenses, but the droplets don't interact enough to collect into larger droplets.