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You've probably heard of acid rain: rain that's more acidic than normal because of pollution in the atmosphere. But, if rain can become more acidic, shouldn't it also be able to become more basic?

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Pretty much everyone’s heard of acid rain: rain that’s more acidic than normal because it mixed with pollutants in the atmosphere. Acid rain can corrode buildings, weaken nutrients in soil, and harm plants and animals, so it’s not very nice stuff.

But acids are only one side of the pH spectrum. So what about basic rain? Why don’t we hear more about that?

Well, because even if it can happen, it’s probably not worth losing sleep over. While scientists do describe acidity in a few different ways depending on what they’re doing, one of the most common is using pH. And one way of understanding pH is to focus on the concentration of hydrogen ions in a substance.

As pH drops, the substance is more acidic, and essentially there’s more hydrogen ions available for bonding. And as pH goes up, the substance is more basic. It either grabs hydrogen off of other molecules, or forms hydroxide ions that can grab free hydrogen ions.

Pure water’s pH is exactly 7, which is perfectly neutral. And natural rain’s pH is about 5.6; it’s slightly acidic from mixing with carbon dioxide in the atmosphere. But rain can get more acidic if water in the atmosphere mixes with gases like sulfur dioxide or nitrogen oxides, which mostly come from pollution.

And once rain’s pH falls below 5, it’s officially known as acid rain. Acid rain can be surprisingly acidic: Some samples have been comparable to vinegar, or even stomach acid, with a pH of around 1.5. Because of how the pH scale works, that’s about 10,000 times more acidic than normal rain!

And in principle, things could go the other way, too. If atmospheric water mixed with the right stuff, it could become basic instead of acidic. But rain’s natural acidity means that it would need a lot of compound to turn basic, and there isn’t much stuff in high enough concentrations in the atmosphere to affect rain so strongly.

Plus, we just release way more pH-lowering chemicals than pH-raising ones, making the challenge even harder. Not that this is something we should strive for. That doesn’t mean that basic rain is impossible.

Rain near deserts and other dusty places with calcium-rich soil tend to have more neutral rain. Calcium compounds are a pH-raiser when they mix with atmospheric water. And there are even a few places where enough calcium or ammonia get into the atmosphere by natural sources or pollution and turn rain slightly basic.

But it’s not widespread. Because so many more pollutants turn rain acidic, scientists have actually been searching for ways to use the science of basic rain to their advantage. When acids and bases react, they neutralize each other, making a substance that’s much less harmful and reactive.

And some researchers have considered releasing calcium carbonate or another base-maker into the atmosphere, or leaving them on the ground in places affected by acid rain, to neutralize the extra acid. So even if basic rain isn’t a thing most of the time in most of the world, the science behind it might help us battle our environmental issues. Thanks to our Patreon patrons for asking this weird question!

If you want to learn about more strange precipitation, check out our video about animals falling from the sky. And if you want to keep discovering all kinds of science with us, you can always go to and subscribe. ♪ OUTTRO.