Why does the rain affect my pool's pH so drastically?

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?
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Mike Hartigan wrote:

Not what you are missing, but maybe what you are getting - Acid Rain the most acidic rain falling in the US has a pH of about 4.3.
--
Joseph Meehan

Dia duit
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observed change in the pool calculates to.... four point three.
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Regards,
Doug Miller (alphageek at milmac dot com)
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Rain - contains lots of things and in turn that changes the pool's ph. You won't see etching etc. from those strengths unless a very long period of time is studied, but obviously it's what's in the rain that causes it. Collect some rain and check its ph; you'll see what I'm talking about. Don't overthink it; just use common sense and logic.
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On Tue, 5 Jul 2005 20:02:03 -0500, Mike Hartigan

As the other poster suggested, check the pH of your rain water. Even pristine rain water will run in the mid sixes. That comes from rainwater scrubbing CO2 from the air, making carbonic acid. If that rain is getting heavier oxides in the air the pH will be lower. Bear in mind pH is a log scale so a 5 is 10 times stronger than a 6.
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Boost your alkalinity levels. It acts as a buffer against large ph swings. Most strip tests kits have it listed.
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Mike Hartigan writes:

Maybe your pH tests or procedures are no good. (What is your basis for trusting them? False results are easier to get than true, and it takes knowledge and diligence to keep them in line. If you've never calibrated your tests, then they can't be trusted.) Adding about 1 part in 1000 of rainwater does not seem like it could have such an effect.
The first step is to confirm the readings.
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Whats your total alkalinity number? Alkalinity is your buffer (kinda like buffered aspirin). It is there to prevent wild pH swings. If its within nominal than I would just say you have some very serious acid rain and alot things around you should be dissolving by now.
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