That familiar earthy smell after rain isn’t the smell of water itself. Part of it comes from a compound called geosmin, made by bacteria living in soil. When rain disturbs the ground, geosmin can enter the air and reach your nose. It’s one reason a passing shower can make a garden smell freshly turned.
The bacteria aren’t suddenly appearing when it rains. They were there all along, quietly producing geosmin beneath your feet. According to the American Chemical Society’s explanation of geosmin, the compound is also found in beets. If you’ve noticed a similar earthy note while cutting one, you’ve encountered a familiar scent in an unexpected place.
How does a smell in the soil get into the air? Researchers at MIT filmed raindrops hitting porous surfaces and found that a drop can trap tiny air bubbles. As those bubbles burst, they send up a fine spray of droplets called aerosols. The researchers proposed that this spray helps carry soil aromas into the air, especially during light or moderate rain. You can see the process described in MIT’s account of the raindrop research.
Geosmin is only part of the story. Plant-derived compounds in dry soil also contribute to the smell often called petrichor. That’s why “the smell of rain” is really a mix of scents from the ground, released and carried through the air when rain arrives. The exact impression may vary from place to place: a garden bed, a dirt path, and a stretch of pavement won’t offer rain the same materials to work with.
Next time a gentle shower passes, take a moment to notice that earthy scent. A small part of what you’re smelling is the work of soil bacteria, made noticeable by falling rain.