Over 90% of a pool’s heat loss is from the surface, there’s very little heat loss from the sides and bottom. Heat loss is due to a combination of evaporation, convection and radiation.
Considerable heat loss occurs at the surface of the pool through evaporation. This is a continuous process, even during times when the sky is cloudy. As water changes from a liquid to a vapour, there’s a transfer of energy involving heat. The heat is drawn form the water, making it cooler.
Convection occurs when the air moves over the surface of the water and removes heat. When it’s windy, for example, and the air temperature is cooler than the water temperature, more heat is lost. This is similar to blowing on a hot drink to make it cooler. Radiation occurs when the air temperature is lower than the water temperature. The heat in the pool radiates into the atmosphere, lowering the temperature of the pool water.