The Fascinating Science Behind the Smell of Rain

If you love the scent that always follows a rainfall, thank chemistry.

The little things in life often are what make us the happiest. A hug from a loved one. The taste of homemade chocolate chip cookies. Wearing warm socks right after they come out of the dryer. And, of course, the amazing smell of rain after a long-awaited downpour.

It’s not all in your head, either. That pleasant, earthy smell of rain, known as petrichor, is caused by the release of specific chemicals when the rain reaches the ground. It’s as if the earth is rewarding everyone for putting up with terrifying thunderstorms and sudden deluges sans umbrella.

Soil-dwelling bacteria called Streptomyces secrete a molecule called geosmin, BBC reports. When rain hits the soil, the raindrops trap air bubbles containing geosmin. The bubbles move through the raindrop and burst out of it as aerosols, even smaller particles dispersed through the air. Once the geosmin gets off the ground and into the air, we’re able to smell it distinctly because human noses are extremely sensitive to it. According to Smithsonian Magazine, some people can smell it even when the concentration is as low as five parts per trillion.

Another contributing factor to petrichor is a combination of plant oils. Australian researchers Isabel Joy Bear and R. G. Thomas, who coined the term petrichor in 1964, discovered that some plants secrete oils during times of drought. When it finally rains, the oils that had been accumulating are released into the air in the same way geosmin is.

And if the smell of rain becomes especially strong—and oddly clean—after a thunderstorm, you can thank ozone. A bolt of lightning can split oxygen and nitrogen, which may recombine to form nitric oxide, one of the compounds needed to create ozone. That molecule is known for its pungent, chlorine-like odour.

So while the science behind petrichor isn’t all that romantic—who’d have guessed we’re actually smelling bacteria secretions and lightning?—the end result certainly is.

Next, find out what happens when you get struck by lightning.

Reader's Digest
Originally Published on Reader's Digest