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Speaker 1: Welcome to Brainstuff from house stuff works dot com, where

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Speaker 1: smart happens him worse brain With today's question, what causes

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Speaker 1: the smell after the rain? Most people notice a distinctive

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Speaker 1: smell in the air after it rains. It's frequently linked

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Speaker 1: with spring, as the smell of fresh cut grass is

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Speaker 1: associated with the summer. You'll find it in a lot

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Speaker 1: of poetry and also on many inspirational lists of things

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Speaker 1: to be happy about? But what causes this smell? As

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Speaker 1: it turns out, the smells people associate with rainstorms can

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Speaker 1: be caused by a number of different things. One of

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Speaker 1: the more pleasant rain smells, the one we often notice

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Speaker 1: in the woods, is caused by bacteria. These bacteria grow

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Speaker 1: in soil when conditions are damp and warm. When the

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Speaker 1: soil dries out, the bacteria produces spores in the soil.

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Speaker 1: The wetness and force of the rainfall kick these tiny

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Speaker 1: spores up into the air, where the moisture after a

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Speaker 1: rain acts as an aerosol. Just like an aerosol air freshener.

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Speaker 1: The moist air easily carries the spores to us, so

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Speaker 1: we breathe them in. These spores have a distinctive, earthy

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Speaker 1: smell we often associate with rainfall. The bacteria are extremely

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Speaker 1: common and can be found in areas all over the world,

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Speaker 1: which accounts for the universality of this sweet after the

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Speaker 1: rain smell. Since the bacteria thrives in moist soil but

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Speaker 1: releases the spores once the soil dries out, the smell

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Speaker 1: is most acute after a rain that follows a dry spell,

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Speaker 1: although you'll notice it to some degree after most rainstorms.

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Speaker 1: Another sort of smell is caused by the acidity of

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Speaker 1: the rain. Because of chemicals in the atmosphere, rainwater tends

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Speaker 1: to be somewhat acidic, especially in urban areas, when it

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Speaker 1: comes in contact with organic debris or chemicals on the ground.

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Speaker 1: This acid rain can cause some particularly aromatic reactions. It

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Speaker 1: breaks apart soil and releases minerals trapped inside, and it

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Speaker 1: reacts with chemicals such as gasoline, giving them a stronger smell.

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Speaker 1: These reactions generally produce more unpleasant smells than bacteria spores,

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Speaker 1: which is why the after the rain smell isn't always

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Speaker 1: a good one. Like the smell caused by the bacteria spores,

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Speaker 1: the smell of chemical reactions is most noticeable when it

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Speaker 1: rains following a dry spell. This is because once the

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Speaker 1: chemicals on the ground have been diluted by one downpour,

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Speaker 1: they don't have the same reaction with the rainwater the

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Speaker 1: second time. Another after the rain smell comes from volatile

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Speaker 1: oils that plants and trees naturally release. The oil collects

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Speaker 1: on surfaces such as rocks. The rain reacts with the

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Speaker 1: oil on the rocks and carries it as a gas

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Speaker 1: through the air. This scent is like the bacteria spores,

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Speaker 1: and that most people consider it to be a pleasant

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Speaker 1: fresh smell. It has even been bottled and sold for

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Speaker 1: its aromatic equalities. These are a few common rain smells,

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Speaker 1: but there are also all sorts of other sense. After

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Speaker 1: it rains, there's lots of aromatic material that the moisture

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Speaker 1: and impact of the rain can stir up, and the

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Speaker 1: moist atmosphere following a downpour is particularly good at carrying

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Speaker 1: these particles through the air. So when you talk about

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Speaker 1: the after the rain smell with a friend, you may

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Speaker 1: mean one thing while your friend is thinking of something else.

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Speaker 1: You'll both agree, however, that the air has a much

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Speaker 1: stronger aroma after a good rainfall. Do you have any

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Speaker 1: ideas or suggestions for this podcast. If so, please send

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Speaker 1: me an email at podcast at how stuff works dot com.

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Speaker 1: For more on this and thousands of other topics, go

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Speaker 1: to how stuff works dot com and be sure to

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Speaker 1: check out the brain Stuff blog on the how stuff

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