When it rains, you can smell a particular smell: petrichor. That pleasant smell of rain actually comes not from the deluge itself, but from chemical compounds associated with oils made by plants.
After three weeks without rain in Paris, and several days of strong heat, the deluge occurred. On the evening of June 11, 2023, downpours poured over the capital, flooding traffic lanes and metro stations. The same day, heavy rains also surprised the inhabitants of Toulouse or the fans of the singer Beyoncé in concert in Marseille.
From the first moments of this rain, perhaps you smelled a particular and familiar smell in the air. This is petrichor.
Definition: what is the name of the smell of rain?
Petrichor is the smell of rain. The word refers more precisely to an odor that emerges when the rain falls on the ground, after a dry period.
The etymology of petrichor is Greek: petrichor comes from “petra” (meaning “stone”) and “ichor” (meaning “blood of the gods”). The term was first used in 1964, in the journal Nature, by two scientists from CSIRO, the Australian government agency for scientific research. The authors then demonstrate that rocks exposed to hot and dry conditions, subjected to a steam distillation process, release an oil. This substance trapped in rocks and soil is responsible for the smell.
Does rain really have a smell?
In reality, the rain itself has no smell. Nevertheless, one can have the impression of “smelling” the moment when it is going to rain. This is explained as follows: with a higher humidity level just before the rain, the pores of the rocks and the soil are “trapped” by the humidity and partly release their oils into the air.
But it’s when the rain really starts to fall that the strongest smell is felt. Especially if it falls on dusty, clayey ground: tiny air bubbles form there, rising to the surface (like champagne bubbles) and bursting, releasing aerosols into the air. The presence of wind helps to quickly diffuse the smell.
What does petrichor smell like?
Petrichor is a fairly pleasant smell when you can smell it: its scent is fresh and reminiscent of musk.
Where does the smell of rain come from?
The mechanism of the petrichor was not unraveled until decades after the initial work in 1964, by researchers at the Massachusetts Institute of Technology in 2010. The petrichor is “ a combination of fragrant chemical compounds from oils made by plants “, sums up The Conversation.
Two elements are involved in petrichor: the secretion of oil by plants in dry weather as well as chemicals released by bacteria in the soil.
At the base of the petrichor, there are indeed microorganisms, actinobacteria. Their activity, which transforms dead organic matter into nutrients, produces an organic compound, geosmin, which combines with petrichor to give it its odor. Geosmin is not toxic to humans, our noses are simply very sensitive to it: we can smell it even in small quantities (and therefore even in town when it rains).
Just before a rain, the activity of actinobacteria accelerates and they produce more geosmin. When the rain stops and the soil dries out, the actinobacteria become inactive again.
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