Yes, it will cool overnight. How much is the question? Check the temp in the morning and let us know how much it dropped. If say it got down to 95 in the morning, how much warmer during the day tomorrow would it get when starting off at 95 degrees. Then tomorrow night to Wednesday morning, it should lower another 4 degrees or so. I would think, but I really don't know. It would be great for us overall to find out in the Houston heat how much water temperature can drop overnight with evaporative cooling when starting off at 99 degrees.