Even on overcast days, your pool’s chlorine levels can diminish due to ultraviolet (UV) radiation. While clouds may block visible light and heat, they are surprisingly ineffective at blocking harmful UV rays. According to the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA), over 90% of UV rays can penetrate light cloud cover. This means your pool’s chlorine is still being consumed, even when the sun isn’t shining brightly.