And I have the empirical evidence. This is the first year in the four I've been living in the south, and owning my pool, that we had a significant early summer drought. Virtually no rain week after week after, well you get the idea. Interestingly, I noted that I didn't have the usual summer algae, either. Algae that I long ago decided to ignore because no amount of shocking would get rid of it. I should mention that the few little pockets of the green stuff never made the pool green -- I always have sparkling clear water.

So no algae this summer, then poof, we got our first drenching rain, and within 24 hours I had the little pockets of algae again. Then we had another few weeks without rain, and no algae. Then again got a good drenching, and again the little swirls of green appeared.

Interestingly, I also saw that my pH didn't have the usual rise (using bleach) that it historically does with my pool. Instead of having to add a cup of acid every week, I was going several weeks in between doses. When the rain came, though, the pH immediately rose.

Anyone care to opine on the rainwater-algae-pH connections?