The amount of 'normal' fog set in the mission editor is overwritten with the new 'enhanced' fog as soon as the weather module is placed by Zeus and the fog setting is altered.
Unfortunately you can only set the density of the enhanced fog as Zeus and not the necessary settings: density, decay or base altitude - or even if you want the new height dependent fog or the old regular fog.
That way after altering the fog setting in a mission for the first time you can no longer have normal fog - which is unfortunate.