What Causes Petrichor, the Earthy Smell After Rain?

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Most people notice a distinctive smell in the air after it rains. It’s frequently linked with spring, as the smell of fresh-cut grass is associated with summer. You’ll find it in a lot of poetry and also on many inspirational lists of things to be happy about. The scientific name for it is petrichor, and it was first named by two Australian researchers in the 1960s, the BBC reported. It actually comes from the moistening of the earth. But what causes the scent?

As it turns out, the smells people associate with rainstorms can be caused by a number of things. One of the more pleasant rain smells, the one we often notice in the woods, is caused by bacteria. Actinomycetes, a type of filamentous bacteria, grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produce spores in the soil. The wetness and force of rainfall kick these tiny spores up into the air where the moisture after a 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 “rain smell” is caused by a chemical in the bacteria called geosin, which is released by the bacteria as they die. Geosin is a type of alcohol molecule and has a very strong scent. 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-rain” smell. Since the bacteria thrive in moist soil but release 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.

Other ‘Rain Smells’

Another sort of smell is caused by the acidity of rain. Because of chemicals in the atmosphere, rainwater tends to be somewhat acidic, especially in urban environments. When it comes in contact with organic debris or chemicals on the ground, it can cause some particularly aromatic reactions. It breaks apart soil and releases minerals trapped inside, which react with chemicals, such as gasoline, giving them a stronger smell. 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, they don’t have the same reaction with the rainwater.

Another after-the-rain smell comes from volatile oils that plants and trees release. The oil then 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 bacteria spores in that most people consider it a pleasant, fresh smell. It has even been bottled and sold for its aromatic qualities!

These are a few common rain smells, but there are also all sorts of other scents after it rains. There are lots of aromatic materials that the moisture and impact of rain can stir up, and the moist atmosphere following a downpour is particularly good at carrying 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 to it after a good rain.

Originally Published: Sep 29, 2000

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