I think the pH of rain has a lot to do with what's in the air and mixes with rain. Many places have acid rain. Here we have usually "neutral" pH (relative to the pH of pool). Sometimes it may be a little alkaline, though, mixing with the chalk and clay dust.
Thinkly, are you near, or surrounded by major metropolitan area(s). How about the prevailing winds. Where do they usually come from. Any factories within a few hundred miles? We get a lot of dust that blows in from far west Texas, hundreds of miles away. As I'm south of Dallas/Fort Worth, with winds and storms mostly from south, and southwest, in summer, we don't get the pollution from the Metroplex mixed in our rain; just lots of dust from sparsely populated areas.
People who live in urban areas usually have dust mixed with pollution and it is darker in color than what we have from our mineral dust/powder.
Last year, one of our member's neighbor's house burned down. Her pool was a greasy oily mess due to the fire close by.
gg=alice