Al is correct about pH of rain water being dependent on local area. I have measured the pH of rain water from the area where I work and home. I commute about 45 miles and have noted a pH at work around 5.0 and at home around 7.0. There is a coal power plant not far from the hospital and it causes a lower pH in rain water. I have also noted a variance in rain pH when the storms come from different directions at home. I have a rain gauge on my deck and plot rain fall and check pH each time I dump it. Next year I also plan to add ALK to the measurements to compare rain pH and ALK to pool pH and ALK.
Steve