Considering some of the answers given in a previous question, I would like to ask what climatic phenomenon would be necessary to generate a mega-fog, thick enough to at least reduce the intensity of sunlight on a hypothetical planet.
In the previous thread it was indicated that perhaps one convenient way to achieve this end would be the continuous generation of volcanic ash, but it would be unoriginal in relation to sagas such as Mistborn, by Brandon Sanderson, or Tolkien's Mordor itself.
What I'm looking for is a mega-fog. A fog originated naturally, but exaggerated. What causes would this mega-fog have? What effects would it have on the ecosystem?