Not being a rocket scientist, I do not fully understand all that is known about these. We have both of them in my area of Southern California. From what I read, acid rain and acid fog are caused by factory and automobile emissions as well as other sources. We should have a definite lower pH and TA here after a rain, but I have never noticed any drop in these in my service route pools. Others servicing areas near here tell me the same. In most instances, after a noticeable amount of rain, pH and TA are actually higher. I attribute this to the rain bringing in alkaline materials from the atmosphere; but I do not have a Ph. D in this phase of the pool business. Acid fog would be expected to have the same results higher pH and TA- if any change at all. At least we all agree that rain brings in enough contaminants with it to knock out or reduce the chlorine substantially; in most instances we superchlorinate after a noticeable rain. Teat pH and TA after each rain; bring these up or drop them accordingly before you superchlorinate.