You say rain doesn't affect chemistry levels much...

[JohnN]

And even in the most extreme cases, it doesn't do nearly what I thought it would. People often ask if rain will affect their pool's chemical levels, and the answer is always "no." Well, that is not always true. The past 2 days, I got 8.43" (according to my manual rain gauge) or 9.62" (according to my automated gauge) of rain. As you might be able to guess, my levels are not what they were before the rain. I honestly thought everything would be worse, but I'm still going to have to work to fix everything.
_____Before___Date____After____Date
FC___7.0_____7/13/14__2.0_____7/16
CC___0.0_____________0.0
pH___7.8_____________7.6
TA___70______7/5/14__70
CH___20_____________20
CYA__40______6/27/14_30
Temp_Low/Mid 90s_____86
Gal.__~12200_________~12700

Obviously, in just about every case, rain WILL NOT affect your levels. I just posted this, because I thought you might like an example of what extreme rainfall can do. Keep in mind, this was 2-3 months worth of rain in 2 days.

 

[jblizzle]

Good post.

9" of rain in say a 48" deep pool could potentially replace 19% of the water. So at most the CYA, CH, salt, borates could be reduced by 19%.
pH and TA affects depend on the pH and TA of the rain water and may have little effect (as you saw).
And FC is always being consumed and replaced, so does not really matter.

 

[Gilrock]

Well I'll add that when I used to pay a pool guy my pool always looked bad within a day after it rained so I always thought it had a big effect. Now that I'm keeping the levels in check on a daily basis rain doesn't seem to affect it much. I think I have seen it raise the pH more than normal.

 

[Waldog]

Maybe this is why I've been having trouble for weeks battling algae. My rain gauge said 5" which is maxed out just this week. I have dumped the rain gauge at least three times in the past four weeks.

 

[JohnN]

I'll share these images that are representative of how much rain fell, just to give you an idea of how much water we are talking.
Using numbers from the National Weather Service and my rain gauge, I figured that almost 1.5 MILLION gallons fell on my 6.5 acres Mon-Tues, or enough to fill my 24 ft (approx 12,200 gal) pool about 122 times.
20140716_130337_Lanes Mill Rd by JNapoli01, on Flickr
20140716_130430_Lanes Mill Rd by JNapoli01, on Flickr

 

[Smykowski]

Good point, but how many standard deviations is that away from the mean?