I'm generally successful at keeping my pool's pH in the 7.2-7.4 range
without too much hassle. However, every time it rains, it takes what
I consider to be a dramatic drop. We got about .1" of rain yesterday
and, as expected, the pH dropped to 6.8. The pool is a 24x48 round
above ground, 13,000 gallons or so. If my math is correct, .1" is
approximately .2% of the total volume. With that much of an effect
on the pH of the pool water, I would expect to see my concrete patio
etched clean! I'm able to bring it back up to 7.2-7.4 with about a
pound of soda ash, so it's not a major problem. I'm just confused at
how so little rain can have so big an effect.

What am I missing?