If there is a heavy rain,you could say it's fog..you won't see a
thing,even with wipers at high speed..
Zero,fog is usually caused by temp inversion.Usually,but not always.
there are also:
radiation fog ,advection fog,evaporation fog,precipitation fog,upslope fog, valley fog (caused by inversion) ,ice fog,freezing fog,garua fog .
To learn more about those,check this:
http://en.wikipedia.org/wiki/Fog