I don't know much about what causes fps to be affected, but for the most part, the viewable weather changes would be thicker cloud cover and different colored skies / clouds. I have seen what USRanger and Chewi have done with the new cloud editor to simulate sandstorms on a desert terrain. I am aware that once color of clouds is set it doesn't change, so as good as this all sounds and would be nice to see, I think that Special Events and Axis versus Allies Maps are the only terrains small enough to implement a map wide variance in cloud content, without favoring a single country and not the other (have had a fog bank over an enemy port in the MA and boy was it tough to attack).
Also, a very good place to do some experimenting is with WWI maps. A good place to hide from the 3 bad guys tearing you to pieces.
