What happens to the water in a poolon a hot, sunny day ?​

I'm assuming you meant pool instead of poolon, i'll take it as a grammatical error. Anyhow, the water in a pool on sunny days tend to have less chlorination then on regular days. This is because chlorine forms hypochlorite ions in the water, and these are broken down by the ultraviolet light from the sun. Rainy or shiny, the weather is always affecting the chemistry in your pool.

Now, if you want to get more complicated into it, you can add a stabilizer into the pool. The common one safely used in pools is cyanuric acid. This almost completley prevents all de-chloronification.

I hope this information was of any use to you.