One of the challenges of having a pool is keeping debris and rainwater out of it. That said, if your pool is low on water, you could  be tempted to keep the cover off during a rainstorm to let it fill up with water. It may seem like a great idea, but it can actually be harmful to your pool for the following reasons.

Algae

Algae will always find a way into a pool no matter what sort of prevention steps that you take, but rainwater will cause algae to grow more rapidly. Much like plants, algae will thrive on consuming nitrogen, and rainwater is a way that additional nitrogen can be introduced to your pool water. This is due to the nitrogen in the rainwater that is caused by lightning. The end result will be walls of the pool that are coated in algae, which could easily be prevented by covering your pool and filling it up with water from a municipal source.

Contaminants

Rainwater not only takes in additional nitrogen with it when it enters your pool, but it can bring all sorts of pollutants with it as well. All of the dust and chemicals that are floating in the air get brought down to the earth with the rainwater, which gets into your pool as well. It will make the water look very cloudy as opposed to looking clear.

Water Dilution

Adding additional water means that the ratio of salt or chlorine to water will be thrown off very suddenly. Having your water diluted of these substances that are designed to disinfect water isn't ideal. Any fungi or bacteria in the pool water will have a greater chance of growing, which increases the risk of you getting sick when you swim in it.

What To Do If Rainwater Gets Into The Pool

You should always make sure to cover your pool prior to a rainstorm, but accidents do happen. The best thing you should do if rainwater does get inside it is to immediately check your pool's salt or chlorine levels and increase them if necessary. Leaving the levels as is will only cause the algae to grow, which will make it difficult to clean your pool later.

If you do need help with professional pool cleaning, or with swimming pool liner replacement because it has been destroyed by algae, contact a contractor in your area that can handle it for you. It is the best way to ensure that your pool is free of those pollutants that can get you sick.