Regarding swimming pools, why does the pH of the water go up at night when the pool cools and down when warms?

Question

Regarding swimming pools, why does the pH of the water go up at night when the pool cools and down when warms?

Answer 1

From strictly a chemistry perspective, as the water temperature of pure water rises then more hydrogen ions and hydroxide ions will be formed and the pH will go down.  In pure water the variation is small (7.0 at 20C to 6.63 at 50C).

Chlorine reacts with water to form HCl (acid).  Sunlight accelerates this reaction and increases the HCl level, which lowers the pH.