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 earthy smell that overwhelms the air after it rains.
If you love the scent that always follows a rainfall, thank chemistry.
Soil-dwelling bacteria called Streptomyces secrete a molecule called geosmin, BBCreports. 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 secret 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 scent 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 odor.
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.