Rainwater is acidic and involves the chemicals in the pool’s water. Rainwater can also change the pH balance and alkalinity level. Acidic rain can also make water corrosive, which can affect plaster and ceramic over time.

Here are some other factors on how rain may affect your swimming pool:

• Rainwater causes pH levels to rise and TA levels to fall due to chemical dilution.
• Chlorine is less active, so it will not kill the microorganisms and keep the water clean.
• Rain can bring algae spores and other debris (like dust) that can submerge into the water which can make it murky and, if not treated quickly, the sun’s rays can accelerate algae growth.
• The appearance of algae, mold and mildew on the tiles is another situation. Rain can also disperse algae, which will give the water a green color. When rainwater falls into the pool, the water’s pH and chlorine alter and can cause the appearance of fungi and bacteria in the pool.
• Rainwater is usually a different temperature than the pool, contrasting the temperature between the rainwater and the water in the pool, which alters the pool’s normal temperature levels. 
• Overflow is another situation that is possible if rainfall is continuous. In a heavy rain the water level can rise considerably and, eventually, overflow.

Rainfall Pool Maintenance
Some recommendations on how to treat your pool before and after it rains are as follows:

• Install an automatic or manual pool cover. A pool cover helps protect the pool’s water from being contaminated by rainwater. 
• Drain the water until levels are normal. This only needs to be done if the pool water overflows from continuous heavy rain. 
• Rain brings debris, so remove it from the pool, including leaves and insects. Use an automatic vacuum to clean the pool’s sides and bottom, preventing dirt from retaining any organisms and fungi growth.
• Check skimmers for any debris and clean thoroughly.
• Turn on the filter system to avoid algae growth. This will help circulate the water and prevent the development and growth of algae spores. We recommend running the filter pump for at least 12 hours daily during the summer months.
• Test the pool’s water. It may need to be adjusted to normalize pH and chlorine levels.