For places that tend to have rainy and dry seasons, monsoons are typically associated with torrential downpours during warmer summer months.

These wet monsoons have a lesser known sibling, though … dry monsoons take their turn when winter comes. And together, they play a surprising role in radically reducing air pollution.

But with Earth’s climate changing, the current monsoon seasons are becoming far less reliable. In order to help save ourselves, scientists are scurrying to figure out how monsoons do a whole lot more than rain. [♪ INTRO] Monsoons happen all over the world, but they tend to pack a bigger punch in places where a large area of elevated land meets an ocean, such as the Indian subcontinent. They’re driven by winds, but if you’re looking for the source of those winds, you’ve gotta look to the Sun.

Not at the Sun. Do not look directly at the Sun. I feel like I have to tell people these things now.

They’re going to eat stuff... They’re going to look at things… The Sun heats different parts of Earth’s surface unevenly. As the summer sun beats down on both South Asia and the Indian Ocean, its light warms the humid air over the water more slowly than the arid air over the land.

This creates two pockets of air with different temperatures. And when there's a difference in air temperature, you find a difference in air density. Warmer air is more spread out, because it’s bumping into itself.

Cooler air is more condensed. Now, when something becomes less dense than the stuff around it, it starts to float up. So that warm land-based air rises, and the cool ocean air rushes in to take its place.

There you go. That’s where wind comes from. And in the case of summer monsoons, ocean air also brings a bunch of water vapor with it, and several months of rain.

When September rolls around, the temperature difference between the land and sea evens out, and the rain stops. But by December, the situation has flip-flopped. The continent turns chilly, but the ocean still maintains a little of that summer heat, and the winds shift seaward.

Now, these dry monsoons don't look like very exciting weather from the perspective of someone living in Mumbai or Bangkok. They’re just a long string of windy days. But they have a super important job worldwide.

They carry away pollutants. When humans create a bunch of air pollution… say, by burning crops, running industrial factories, or driving gas-powered cars… it often forms an atmospheric haze that just hangs out over whatever land it came from. Not so nice to breathe day in and day out.

But nature has a way of cleaning it up. When the winter monsoon winds show up, they carry pollutants like carbon monoxide and lead in the air out to the ocean. Once it’s there, the dry air can pick up some moisture.

And since a single raindrop can attract dozens or even hundreds of aerosolized particles as it plummets through the air, these pollutants can sink to the bottom of the ocean whenever it rains. Back in 2018, researchers reported that dry monsoons are responsible for removing almost 80% of the sulfur emissions in South Asia. Sulfur is dangerous because it's prone to react with other compounds in the atmosphere, resulting in pollutants like sulfur dioxide and even acid rain.

But dry monsoons can’t get all the praise. With their torrential rainstorms, wet monsoons can get in on the pollution-busting action, too. Using the same process that dry monsoons do once the air reaches the ocean, wet monsoons gather pollutants in the air and send them into the ground.

So it’s great news for humans and ecosystems that both wet and dry monsoons can clear out some of humanity’s mess. But unfortunately, they aren't perfect air scrubbers. The same researchers also found that some pollutants extracted by winter winds aren’t snatched by raindrops and carried to the ocean’s bottom.

Instead, they’re pushed up above the monsoon clouds into the stratosphere… the part of our atmosphere where the ozone layer lives. The ozone layer is great because it blocks most of the Sun’s harmful ultraviolet light from reaching living things down here. We love that ozone layer!

But when pollutants get up there, they can react with the ozone molecules and break them down…especially over Earth’s poles. If all this sounds familiar, it should. We've worried about the thinning of the ozone layer before.

But thanks to an international treaty passed in 1987, banning the use of some ozone-depleting chemicals, we managed to reverse the trend. It would be a shame to see all that hard work be undone, so we can’t just keep letting pollutants leak into the atmosphere assuming monsoons will do all the hard work for us! Scientists are still learning about this ozone piece of the monsoon puzzle, although they might not have much time.

Due to warming temperatures on Earth, the rock steady pattern of our planet's monsoon seasons is changing rapidly. For example, according to some climate models, the rainy season in the Northern Hemisphere will likely be lengthened because it’ll take longer for the continent to cool off, but shortened in the Southern Hemisphere because the landmass can't heat up in time. The wet and dry monsoon cycle has been running like clockwork for millions of years.

If that gets thrown out of whack, it could affect their ability to carry our atmospheric trash to places where it can do less harm. While we may not fully understand the implications of the changing pattern, it’s a critical area of study for environmental scientists. Our future may depend on knowing which way the wind blows.

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