WEBVTT - What Causes the Smell After a Rain?

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<v Speaker 1>Welcome to Brainstuff, a production of I Heart Radio. Hey

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<v Speaker 1>rain Stuff Lauren Bolga Bam here. Most people notice a

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<v Speaker 1>distinctive smell in the air after it rains. It's frequently

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<v Speaker 1>linked with spring, as the smell of fresh cut grass

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<v Speaker 1>is associated with summer. You'll find it in a lot

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<v Speaker 1>of poetry and also on lots of inspirational lists of

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<v Speaker 1>things to be happy about. The scientific name for it

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<v Speaker 1>is petrochor, and it was first named by two Australian

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<v Speaker 1>researchers in the nineteen sixties, and it actually comes from

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<v Speaker 1>the moistening of the earth by rain. But what exactly

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<v Speaker 1>causes the scent? As it turns out, the smells that

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<v Speaker 1>people associate with rainstorms can be caused by a number

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<v Speaker 1>of things. One of the more pleasant rain smells, the

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<v Speaker 1>one we often notice in the woods, is caused by

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<v Speaker 1>bacteria actino my seats, is a genus of filamentous bacteria

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<v Speaker 1>that grow in soil when conditions are damp and warm.

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<v Speaker 1>When the soil dries out, the bacteria produce spores around

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<v Speaker 1>themselves for protection. The wetness and force of rainfall kick

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<v Speaker 1>these tiny spores up into the air, where the moisture

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<v Speaker 1>After a rain acts as an air sol like an

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<v Speaker 1>air sool air freshener. The moist air easily carries the

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<v Speaker 1>spores to us so that we breathe them in. These

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<v Speaker 1>spores have a distinctive earthy smell that we often associate

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<v Speaker 1>with rainfall. The actual compound responsible for the smell is

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<v Speaker 1>called geosmin, which is released by the bacteria as they die.

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<v Speaker 1>Geosmin is a type of alcohol molecule and has a

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<v Speaker 1>very strong scent. The bacteria are extremely common and can

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<v Speaker 1>be found in areas all over the world, which accounts

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<v Speaker 1>for the universality of this sweet after the rain smell.

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<v Speaker 1>Since the bacteria thrive in moist soil but release the

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<v Speaker 1>spores once the soil dries out, the smell is most

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<v Speaker 1>acute after a rain that follows a dry spell, although

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<v Speaker 1>you'll notice it to some degree to most rainstorms. Another

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<v Speaker 1>sort of smell is caused by the acidity of rain.

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<v Speaker 1>Because of compounds in the atmosphere, rainwater tends to be

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<v Speaker 1>somewhat acidic, especially in urban environments. When it comes in

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<v Speaker 1>contact with organic debris or compounds on the ground, it

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<v Speaker 1>can cause some particularly aromatic reactions. It breaks apart soil

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<v Speaker 1>and releases minerals trapped inside, which react with chemicals like gasoline,

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<v Speaker 1>giving them a stronger smell. These reactions generally produce more

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<v Speaker 1>unpleasant smells than bacteria spores, which is why the after

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<v Speaker 1>the rain smell isn't always a nice one like the

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<v Speaker 1>smell caused by bacteria spores. The smell of chemical reactions

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<v Speaker 1>is most noticeable when it rains following a dry spell.

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<v Speaker 1>This is because once the chemicals on the ground have

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<v Speaker 1>been diluted by one downpour, they don't have as strong

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<v Speaker 1>of or as many reactions with new rainwater. Another after

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<v Speaker 1>the rain smell comes from volatile oils that plants and

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<v Speaker 1>trees release. The oil then collects on surfaces such as rocks.

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<v Speaker 1>The rain reacts with the oil on the rocks and

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<v Speaker 1>carries it again as a gas through the air. This

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<v Speaker 1>scent is like bacterious bores in that most people consider

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<v Speaker 1>it a pleasant and fresh smell. It's even been bottled

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<v Speaker 1>and sold for its aromatic qualities. These are a few

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<v Speaker 1>common rain smells, but there are also all sorts of

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<v Speaker 1>other scents after it rains. There are lots of aromatic

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<v Speaker 1>materials that the moisture and impact of rain can stir up,

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<v Speaker 1>and the moist atmosphere following a downpour is particularly good

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<v Speaker 1>at carrying these particles through the air and into our noses.

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<v Speaker 1>So when you talk about the after the rain smell

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<v Speaker 1>with a friend, you may mean one thing while they're

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<v Speaker 1>thinking of something else entirely. Today's episode is based on

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<v Speaker 1>the article what causes petroc or the earthy smell after

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<v Speaker 1>rain on how stuffworks dot Com, written Katherine Whitburn. Brain

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<v Speaker 1>Stuff is production by Heart Radio in partnership with how

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<v Speaker 1>stuff works dot Com and is produced by Tyler Clain.

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<v Speaker 1>For four more podcasts. For my heart Radio, visit the

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