All that would make sense to me because thunderstorms could come from an area around here that had a paper plant or industrial boiler but any weather that came from the north would come from a national forest. Thus one weather event might produce a very different ph than another. Then again what do I know!! My handle is "chemistrydropout". But I'm learning. I have found that if I test the water and keep the chemicals on hand it doesn't really matter that much. It is scary though when you check the water and it is off the ph scale, very little or no chlorine, and alkalinity is out of whack. Panicky might be a better word. This happened during one heavy rain event this summer. At least this week I knew to check things out the morning after instead of waiting till the normal afternoon test routine.