What causes the smell after rain?

The smell of rain is often linked with spring, and it's been alluded to in poems, songs and stories for centuries. But what exactly causes this distinctive smell? Tune in and find out.

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Episode Transcript

Speaker 1 (00:00):
Welcome to Brainstuff from house stuff works dot com, where
smart happens him worse brain With today's question, what causes
the smell after the rain? Most people notice a distinctive
smell in the air after it rains. It's frequently linked

(00:23):
with spring, as the smell of fresh cut grass is
associated with the summer. You'll find it in a lot
of poetry and also on many inspirational lists of things
to be happy about? But what causes this smell? As
it turns out, the smells people associate with rainstorms can
be caused by a number of different things. One of

(00:44):
the more pleasant rain smells, the one we often notice
in the woods, is caused by bacteria. These bacteria grow
in soil when conditions are damp and warm. When the
soil dries out, the bacteria produces spores in the soil.
The wetness and force of the rainfall kick these tiny
spores up into the air, where the moisture after a

(01:04):
rain acts as an aerosol. Just like an aerosol air freshener.
The moist air easily carries the spores to us, so
we breathe them in. These spores have a distinctive, earthy
smell we often associate with rainfall. The bacteria are extremely
common and can be found in areas all over the world,
which accounts for the universality of this sweet after the

(01:28):
rain smell. Since the bacteria thrives in moist soil but
releases the spores once the soil dries out, the smell
is most acute after a rain that follows a dry spell,
although you'll notice it to some degree after most rainstorms.
Another sort of smell is caused by the acidity of
the rain. Because of chemicals in the atmosphere, rainwater tends

(01:50):
to be somewhat acidic, especially in urban areas, when it
comes in contact with organic debris or chemicals on the ground.
This acid rain can cause some particularly aromatic reactions. It
breaks apart soil and releases minerals trapped inside, and it
reacts with chemicals such as gasoline, giving them a stronger smell.

(02:11):
These reactions generally produce more unpleasant smells than bacteria spores,
which is why the after the rain smell isn't always
a good one. Like the smell caused by the bacteria spores,
the smell of chemical reactions is most noticeable when it
rains following a dry spell. This is because once the
chemicals on the ground have been diluted by one downpour,

(02:33):
they don't have the same reaction with the rainwater the
second time. Another after the rain smell comes from volatile
oils that plants and trees naturally release. The oil collects
on surfaces such as rocks. The rain reacts with the
oil on the rocks and carries it as a gas
through the air. This scent is like the bacteria spores,

(02:53):
and that most people consider it to be a pleasant
fresh smell. It has even been bottled and sold for
its aromatic equalities. These are a few common rain smells,
but there are also all sorts of other sense. After
it rains, there's lots of aromatic material that the moisture
and impact of the rain can stir up, and the
moist atmosphere following a downpour is particularly good at carrying

(03:17):
these particles through the air. So when you talk about
the after the rain smell with a friend, you may
mean one thing while your friend is thinking of something else.
You'll both agree, however, that the air has a much
stronger aroma after a good rainfall. Do you have any
ideas or suggestions for this podcast. If so, please send

(03:38):
me an email at podcast at how stuff works dot com.
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