Speaker 1: Welcome to Brainstuff, a production of I Heart Radio. Hey
Speaker 1: rain Stuff Lauren Bolga Bam here. Most people notice a
Speaker 1: distinctive smell in the air after it rains. It's frequently
Speaker 1: linked with spring, as the smell of fresh cut grass
Speaker 1: is associated with summer. You'll find it in a lot
Speaker 1: of poetry and also on lots of inspirational lists of
Speaker 1: things to be happy about. The scientific name for it
Speaker 1: is petrochor, and it was first named by two Australian
Speaker 1: researchers in the nineteen sixties, and it actually comes from
Speaker 1: the moistening of the earth by rain. But what exactly
Speaker 1: causes the scent? As it turns out, the smells that
Speaker 1: people associate with rainstorms can be caused by a number
Speaker 1: of things. One of the more pleasant rain smells, the
Speaker 1: one we often notice in the woods, is caused by
Speaker 1: bacteria actino my seats, is a genus of filamentous bacteria
Speaker 1: that grow in soil when conditions are damp and warm.
Speaker 1: When the soil dries out, the bacteria produce spores around
Speaker 1: themselves for protection. The wetness and force of rainfall kick
Speaker 1: these tiny spores up into the air, where the moisture
Speaker 1: After a rain acts as an air sol like an
Speaker 1: air sool air freshener. The moist air easily carries the
Speaker 1: spores to us so that we breathe them in. These
Speaker 1: spores have a distinctive earthy smell that we often associate
Speaker 1: with rainfall. The actual compound responsible for the smell is
Speaker 1: called geosmin, which is released by the bacteria as they die.
Speaker 1: Geosmin is a type of alcohol molecule and has a
Speaker 1: very strong scent. The bacteria are extremely common and can
Speaker 1: be found in areas all over the world, which accounts
Speaker 1: for the universality of this sweet after the rain smell.
Speaker 1: Since the bacteria thrive in moist soil but release the
Speaker 1: spores once the soil dries out, the smell is most
Speaker 1: acute after a rain that follows a dry spell, although
Speaker 1: you'll notice it to some degree to most rainstorms. Another
Speaker 1: sort of smell is caused by the acidity of rain.
Speaker 1: Because of compounds in the atmosphere, rainwater tends to be
Speaker 1: somewhat acidic, especially in urban environments. When it comes in
Speaker 1: contact with organic debris or compounds on the ground, it
Speaker 1: can cause some particularly aromatic reactions. It breaks apart soil
Speaker 1: and releases minerals trapped inside, which react with chemicals like gasoline,
Speaker 1: giving them a stronger smell. These reactions generally produce more
Speaker 1: unpleasant smells than bacteria spores, which is why the after
Speaker 1: the rain smell isn't always a nice one like the
Speaker 1: smell caused by bacteria spores. The smell of chemical reactions
Speaker 1: is most noticeable when it rains following a dry spell.
Speaker 1: This is because once the chemicals on the ground have
Speaker 1: been diluted by one downpour, they don't have as strong
Speaker 1: of or as many reactions with new rainwater. Another after
Speaker 1: the rain smell comes from volatile oils that plants and
Speaker 1: trees release. The oil then collects on surfaces such as rocks.
Speaker 1: The rain reacts with the oil on the rocks and
Speaker 1: carries it again as a gas through the air. This
Speaker 1: scent is like bacterious bores in that most people consider
Speaker 1: it a pleasant and fresh smell. It's even been bottled
Speaker 1: and sold for its aromatic qualities. These are a few
Speaker 1: common rain smells, but there are also all sorts of
Speaker 1: other scents after it rains. There are lots of aromatic
Speaker 1: materials that the moisture and impact of rain can stir up,
Speaker 1: and the moist atmosphere following a downpour is particularly good
Speaker 1: at carrying these particles through the air and into our noses.
Speaker 1: So when you talk about the after the rain smell
Speaker 1: with a friend, you may mean one thing while they're
Speaker 1: thinking of something else entirely. Today's episode is based on
Speaker 1: the article what causes petroc or the earthy smell after
Speaker 1: rain on how stuffworks dot Com, written Katherine Whitburn. Brain
Speaker 1: Stuff is production by Heart Radio in partnership with how
Speaker 1: stuff works dot Com and is produced by Tyler Clain.
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