Question: What causes fog? If the cloud is heavy, why doesn't it just rain?

Chris Armstrong answered on 23 Jun 2015:

So fog is just a thin stratus cloud at a low level, kinda. But instead of the stratus cloud sinking its formed at low levels.

It is generally from a local source of water, hence its associated with creepy lakes, swamps, and marshes, rather than clouds which are more often than not formed from ocean water.

It happens when the air just cooler than the temperature at which the water vapour can condense back into liquid (varies per situation but around 2-5degrees).

Weirdly it does kinda rain as well, the fog eventually deposits the water its “carrying” onto the ground, its just incredibly soft rain fall.