Although pool evaporation is always an issue, it’s a bit of a counter intuitive dynamic. When the weather is at its hottest and most humid there is very little evaporation. When the weather is cool and dry, particularly after a hot spell, the evaporation rate skyrockets. Cool night air is the culprit. When we have a body of warm water meeting to cool dry air of a late August night we lose heat and water quickly. I’m sure that many of you have seen your water “steaming” in the chill of a September morning. It’s almost like clock work, on the first set of chilly nights as the season begins to wind down, the calls begin. Everyone is convinced that their pool is leaking. In extreme circumstances, (very cool night, very warm water), a pool can lose 2″ in 12 hours. If you find that your pool is suddenly losing more water than you may be accustomed to, before you panic, look to the night air.