A good rain shower can make the whole world feel a little nicer. If you’re like most people, you open up all the windows in the house to let the fresh breeze drift in and then you head out to the porch to soak up that delicious earthy scent that always seems to come with the rain.
Where does that wonderful rainy scent come from, anyway? It turns out that there’s a name for it: Petrichor, which is a word made from the roots petra (Greek word for stone) and ichor, which is a word that represents the mystical essence of the Grecian gods. The name for the scent of rain, however, doesn’t tell the whole story. There’s nothing mystical about petrichor, and the scent doesn’t come from rocks.
It Starts With a Dry Spell
During dry weather, several things happen that eventually combine to form the smell of rain. Decomposed organic compounds produced by dead leaves and other materials blow around, landing on the ground where they combine with minerals in the soil. At the same time, plants know that there isn’t enough water to support more growth. They produce oils that signal both seeds and root systems to halt their growth.
Actinobacteria, which grows in moist soil, plays a part, too. As the soil dries out, actinobacteria stop growing and start creating spores so that they can reproduce once more after a good rain. And, as these bacteria die, they create a compound called geosmin. If you’ve ever eaten a beet, the earthy flavor you taste is geosmin. Similarly, the muddy flavor that a fresh catfish fillet sometimes has is caused by high levels of geosmin.
It’s these four factors—mineralized organics, oils from plants, actinobacteria spores, and geosmin—that mix together to create petrichor. When you smell it, that’s because the force of raindrops hitting the ground creates an aerosol of these particles, which you then pick up with your nose.
The Right Kind of Rain
In order to catch a whiff of petrichor, the conditions have to be just right. The dry spell is only the first component. After that, you need just enough rain to distribute the aerosol. If the floodgates open up and it rains all week long, the scent of petrichor will be strong after the first shower. However, after an entire day or week of storms, the compounds that create petrichor will wash away.
The type of rain is just as important as the duration of the rainy spell. Light drizzles and misty rains may not be enough to aerosolize the compounds in petrichor. Conversely, a pounding rain will knock the airborne particles right out of the air, leaving you without that rain-fresh scent afterward. This is why petrichor is strongest right after a gentle soaking rain.
What About the Smell that Comes Before the Rain?
If you’ve ever enjoyed the scent of petrichor, then you’ve probably smelled something similar just before the rain starts, too. This pre-rain smell has an entirely different origin. This scent is attributed to ozone, which is created when lightning within storm clouds splits oxygen apart into separate atoms. Some of those atoms recombine to form ozone instead of oxygen. As the storm winds blow, the ozone is blown out ahead of the storm, which is why you can sometimes smell of approaching thunderclouds.
In a way, you could say that petrichor is nature’s air freshener. When it comes to scents, it’s certainly one of the best, right alongside the smells of freshly cut grass and blooming flowers!
You can’t smell it, but it’s still relaxing! Have a look at the video below: