Funny you have observed this!!! During some of the thunderstorms we had this summer I was noticing that my ph and alkalinity would fall off the chart. We had a 5+" rain overnight the other night and I woke up to water levels above the skimmers. My first thought was that this would be an ordeal to get things back in order. I dumped about 3" of water from the pool with the pump and however much went out the overflows during the night, added some chlorine to get the cl back up and that was it. Ph was at 7.5 and alkalinity was at 70 while cya stayed in at below 30. This surprised me because I figured that on a 20x40 Ig pool I probably pumped out several thousands of gallons to get out 3" of water. Maybe it was not as much as I figured and the overnight rainfall has a different effect than a thunderstorm during the daylight hours.