This distinctive aroma is scientifically called “petrichor.” In Greek, “petra” means “stone,” and “ichor” refers to “the fluid that flows in the veins of the gods.” The name was coined in 1964 by Australian researchers Isabel Bear and Roderick Thomas.
So, what do stones have to do with it, you may ask? Well, the smell of rain doesn’t come from the water falling from the sky itself, but from the stones and moist soil. It consists of several components.
The first ingredient is a mixture of special oils and fatty acids, particularly stearic and palmitic acids, which are released by various plant species during dry periods. These substances slow down the germination of seeds and the development of young sprouts. In this way, older plants signal the younger ones not to waste resources growing under unfavorable conditions and to wait for a change in weather. When rain dissolves these oils, they produce distinct scents.
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 something called geosmin, an organic compound that gives soil its characteristic smell. It’s composed of carbon, hydrogen, and oxygen and is a byproduct of the metabolism of various microorganisms, mainly actinomycetes and cyanobacteria. Through geosmin, bacteria attract tiny arthropods—springtails—which help spread them in the soil, aiding reproduction.
The third element of the smell of rain is 2-methylisoborneol, produced by blue-green algae (cyanobacteria) and filamentous actinomycete bacteria. This compound gives stagnant water, ponds, and marshy silt their unpleasant, musty smell.
Finally, another substance at play is ozone, a molecule made up of three oxygen atoms (O₃, also known as trioxygen). It contributes to the fresh scent during rain, although ozone can only mix with other smells during a thunderstorm. It forms when lightning splits oxygen molecules (O₂) in the air, which then recombine into O₃. If you smell a storm approaching before the first raindrops fall, that’s the ozone.
So, when water drops from the sky or when you water your garden, plant oils, geosmin, and 2-methylisoborneol rise from the soil into the air as aerosols and mix together. And there you have the distinct smell of rain—more precisely, wet earth.
This phenomenon was captured by specialists at the Massachusetts Institute of Technology using high-speed cameras. Their footage shows how aerosols are formed when raindrops hit the ground.
But why does this mix of plant acids and bacterial byproducts smell so good? Evolution is to blame.
Our animal ancestors developed the ability to detect the smell of wet earth to find drinking water. They would literally sniff out water sources to avoid dehydration.
The human nose is highly sensitive to geosmin and can detect it at a concentration of five molecules per trillion. 2-methylisoborneol is also easily detected by our sense of smell; even if there’s only 0.002 micrograms of it in a liter of water, you’ll still notice it.
The ability to smell wet soil from a distance was crucial for survival. So much so that even today, when we no longer drink from streams and puddles, the scent still feels pleasant to us. Perfumers have taken note of this, and now you can easily find perfumes with petrichor in stores.
