Originally Posted by ps0303
If you put a glass of water in direct sun and another one in the shade with the outside temp being 85, the water in direct sun light will heat up.
Actually both will heat up as long as the one in the shade has a lower water temperature than the air and/or environment. The one in the sunlight just heats much faster and to a higher temperature.
There are four basic methods of heat transfer for a pool (both heat gain and heat loss):
Radiation - Sunlight hits the pool and raises the water temperature. A pool also radiates heat back out to the environment for heat loss even during the day.
Evaporation/condensation - Water temperature higher than the dew point will lose heat via evaporation while water temperature lower than the dew point will gain heat with condensation.
Convection - The air above the water can either add heat or remove heat via convection depending on the temperature difference of the water and air.
Conduction - This will occur through the shell of the pool and can again be a net positive or negative depending on the water temperature and ground temperature. For a pool, this is usually a heat loss component.
All four components of heat transfer are always present in a pool. However, evaporation can be significantly reduced with a solar cover and helps with convection and radiation as well.
But as I pointed out earlier, you can reduce the heat loss slightly with circulation during the day when the water can develop a temperature gradient. The heat loss due to evaporation is dependent on water surface temperature so when the pump is not running during the day, the surface water can become warmer than if the pump where running. Therefore the evaporation component of heat loss can be higher without the pump running than with the pump running.