Making dark fog in certain locations that comes and goes?

Is there a way to make certain parts of the map dark and yet other parts normal?
I know about making the whole map that way but I've yet to find out how to make it so only certain places are dark while other places have no fog. Also, I want to make it variable so it can come and go as well as control how dark or light the fog is.

Any help is appreciated, thanks in advance.

In the region layer, create a new region where you want the fog.
Then in the edit-region-dialog you can already assign a weather effect to the region.
If you need to enable/disable the effect, then use the Create Weather effect in region trigger (i think it's in the Environment - category).
Then assigne the Last Created Weather Effect to a variable.

Thanks Mike but is there a way to make the fog lighter or darker in real time?