With rain in the forecast for Newcastle this week, now is a good time to talk about why pools go green after a downpour - and what you can do before it gets ugly.

If your pool has ever gone from sparkling blue to swamp green seemingly overnight after heavy rain, you are not imagining things and you did not do anything wrong. Rain is genuinely one of the most disruptive things that can happen to a pool, and it does its damage quietly, while you are inside watching the weather radar and feeling smug about not having to water the garden. By the time the sun comes back out, the conditions inside that pool have shifted enough to give algae exactly what it needs to throw a party.

The short version is this: rain dilutes your chlorine, throws your pH out of balance, and dumps a fresh load of organic material into the water. Each of those things on its own is manageable. All three happening at once, over a sustained period of wet weather, is where pools start to lose the plot. Newcastle gets its share of decent rain events, especially heading into the warmer months, so this is not a theoretical problem. It is a very real, very green one that we see regularly.

Let's break down what is actually going on. When rain falls into your pool, it is not just adding volume. Rainwater is naturally slightly acidic, which means it pushes your pH downward. A pool with low pH is uncomfortable to swim in and, more importantly, it makes chlorine less effective even if the chlorine level itself looks fine on a basic test. That is the part that catches a lot of pool owners out. They check the chlorine, it seems okay, and they cannot work out why the water is still looking off. The answer is usually pH, and rain is often the culprit.

At the same time, rain rinses everything off your roof, your garden, surrounding surfaces and the pool deck itself. Leaves, dirt, bird droppings, fertiliser, sunscreen residue from the last time anyone swam - all of it ends up in the water. This is what pool technicians call organic load, and it is essentially free food for algae. Chlorine has to work hard to break all of that down, which burns through your sanitiser faster than normal. So now you have less effective chlorine, you have more stuff for algae to feed on, and you have conditions that actively favour algae growth. That is why the green can appear so quickly - sometimes within 24 to 48 hours of a significant rain event.

There is also the matter of water volume. If you get a heavy enough downpour, your pool may overflow or come close to it. That dilutes every chemical in the water proportionally. Even if your levels were perfect before the rain, they may be well outside the ideal range afterward. This is especially relevant for pools that do not have an auto water leveller or overflow drain, where the water has nowhere to go and the dilution effect is concentrated.

The good news is that none of this is hard to fix if you catch it early. The key word there is early. A slightly imbalanced pool after rain is a straightforward fix. A pool that has been left for a week after a rain event, in warm weather, with sun hitting it every day, is a different conversation altogether. Algae establishes itself fast under the right conditions, and once it does, getting on top of it takes more time, more product, and more patience than most people want to deal with mid-summer.

So what should Newcastle pool owners actually do after rain? Start by giving the pool a visual check. If the water is still clear, that is a good sign, but do not stop there. Skim out any debris that has blown in, check that your skimmer baskets and pump basket are clear, and run your filter a bit longer than usual to help the system catch up. Beyond that, a proper water test is really the only way to know where your levels sit and what needs adjusting. A dip strip can give you a rough read, but it will not tell you the full story.

Bringing a water sample in for a full analysis is genuinely the most useful thing you can do after a wet spell. It takes a few minutes and it gives you a clear picture of exactly what is going on, so you are treating what the water actually needs rather than guessing. If the pool has already started to turn, or if it has been a while between services and a run of wet weather has arrived on top of that, booking a service call is the smarter move. Getting a qualified eye on it early is far cheaper and less disruptive than dealing with a full green pool later.

With wet weather forecast across Newcastle this week, now is genuinely the right time to pay attention to your pool. Do not wait until you can see the problem from the back door.

Bring a water sample into Poolside Newcastle for a free water test, or get in touch to book a service with our qualified pool technician. We are here to help you sort it out before the green takes hold.

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