If you browse popular lists like “20 Things That Make You Happier,” you’ll likely find the smell of rain among them. It’s written about in poetry and mentioned in books. But what makes it so appealing? Let’s take a look.
This characteristic aroma is scientifically called “petrichor.” The Greek word “petra” means “stone,” and “ichor” refers to “the fluid flowing in the veins of the gods.” The name was coined in 1964 by Australian researchers Isabella Bear and Roderick Thomas.
You might ask, what do stones have to do with it? Well, the smell of rain isn’t produced by the water falling from the sky itself, but rather by the stones and moist earth. It consists of several elements.
The first ingredient is special oils and fatty acids—particularly stearic and palmitic acid, which various plant species release during dry periods. These substances slow down seed germination and the development of young shoots. In this way, older plants communicate with younger ones, telling them not to waste resources on development during unfavorable conditions but to wait for the weather to change. When rain dissolves these oils, they produce distinctive aromas.
Yes, bushes and trees can communicate with each other through chemical signals, in case you didn’t know.
The second important component of petrichor is geosmin, an organic compound that gives earth its characteristic smell. It consists of carbon, hydrogen, and oxygen and is a metabolic byproduct of various classes of microorganisms, mainly actinomycetes and cyanobacteria. Bacteria use geosmin to attract tiny arthropods called springtails, which help distribute them through the soil and aid in reproduction.
The third component of rain’s scent is 2-methylisoborneol, produced by blue-green algae—cyanobacteria and filamentous actinomycete bacteria. This is what gives stagnant water, stale ponds, and swamp sludge their unpleasant odor.
And finally, there’s another substance at work—ozone, a molecule made of three oxygen atoms (O₃, also called trioxygen). It’s what gives rain its fresh scent. However, ozone only mixes with other smells during thunderstorms. It’s created when lightning splits molecular oxygen O₂ and the atoms recombine into O₃. If you smell an approaching storm before the first raindrops fall—that’s ozone.
So when water drops from the sky or you water your garden with a watering can, plant oils, geosmin, and 2-methylisoborneol rise from the soil into the air as aerosols and mix together. And there you have it—the specific smell of rain. Or more precisely, wet earth.
Specialists from the Massachusetts Institute of Technology created a video using high-speed cameras showing how aerosols form when water droplets hit the ground.
But why does this mixture of plant acids and bacterial waste products smell so good? Evolution is involved here.
The fact is, our animal ancestors developed their ability to sense the smell of wet earth in order to find drinking water. They literally sniffed out sources of liquid to protect themselves from thirst.
The human nose is very sensitive to geosmin and can detect it at a concentration of five molecules per trillion. And 2-methylisoborneol is also very easily picked up by our sense of smell: even if a liter of water contains just 0.002 micrograms of this compound, you’ll notice it.
The ability to detect the smell of wet soil from a great distance was extremely important for survival. So much so that even today, when we’ve stopped lapping water from streams and puddles, it still seems pleasant to us. Perfumers have taken note of this, and now you can easily find fragrances with petrichor in stores.
