What levels does rain affect? If I remember correctly, rain will lower pH levels.
Rain might either raise or lower the PH, though raise is quite a bit more common.
Yes, rain aerates the water, which tends to cause the PH to rise. Rain is also just slightly acidic. Unless your TA level is fairly high or there is a lot of rain, there won't be too much change in the PH.
If you get enough rain to make the pool overflow somewhat, you are replacing pool water with water that will dilute all values (TA, CH, CYA, FC).
Over a normal wet winter here I will typically see CYA decline rather a lot, like 15 or 20 points. TA can be kept up with just a small amount of tap water since my tap water has TA of like 340-ish.
Also, rain will typically add organics to the pool with a bit of dirt and leaves from my badly sloped flower beds nearby and the leaves and stuff that blows in. So that uses up FC also.
23,000 gallon in ground pool with rock waterfall and spillover spa, Aqualink control system, Polaris 380 cleaner, Purex Triton Clean&Clear Plus cartridge filter. Located in The Woodlands, Texas.
Pool owner since Nov 2008, Trouble Free since April 2009. Happy to help when I can.