Chlorine as a chemical is affected by the kind of sunlight often seen on bright summer days. That's because the ultraviolet rays present in the sun's rays quickly begin to degrade chlorine through a chemical reaction taking place in the water. This degradation or loss of chlorine can be halted or greatly diminished by using another chemical stabilizing agent. The most common agent in use for decades has been cyanuric acid, or CA.
Though too much sunlight isn't good for pool chlorine, light is actually needed for the chemical to do its job. When sunlight or the rays from overhead lights interact with the chlorine, free radicals are produced as the chlorine dissipates. It's really these free radicals that bind with all the organic matter in a pool, including bacteria, and then kill it off. Once degradation occurs, it's necessary to rechlorinate to recommended levels.
Heat itself has little effect on swimming pool chlorine levels unless it raises water temperatures excessively. Of course, a pool that hot would be uncomfortable to swim in and superheated water could even cause itching or other problems for swimmers. People often confuse the intensity of the sunlight or overhead lighting with the heat that's created from such lighting, and they mistakenly believe it's heat that's acting negatively on chlorine levels in pool water.
Evenly distributing chlorine in a pool will make it less susceptible to rapid dissipation from sunlight. You should also ensure that the pool water's pH levels are within an optimal range of 7 to 8. If the pH is below 7, chlorine will rapidly disappear. If it's above 8, it won't work as efficiently. Ensure that chlorine is maintained at 1 to 2 parts per million.