Tap water PH

I use tap water to water with. I let it sit out for 24 hours before using or adding nutes to it.

It used to come out very close 7 and would drop a tad in the 24 hours to what seemed to be a perfect number - 6.5 - 6.8.

2 months later (being now) the outside temp has risen from freezing to 50-70 depending. I have also noticed my waters PH dropping to around 5.5 and then finally rising when it sits out.

Does anyone know why this might happen?

when u leave it out the chlorine dissolves so the ph becomes more neutral

Makes sense but why is there that much more, or less, chlorine now that it is hotter out? Or does it just heat up and cause a different effect?