Do you know petrichor, the smell of rain?

The pleasant smell that one feels when rain falls, rising from the ground after the downpour, has a very precise scientific explanation.

It is a most pleasant smell, and one that one would come to want to feel more often in these hot weathers: the smell of rain, when it falls on the cobblestones of an old street, or on the earthy ground of a path.

Petrichor: a name coined not so long ago

It remains engraved in our olfactory memory and we savor it like a perfume that we would find with pleasure. But did you know that this very pleasant emanation has a name? This phenomenon has been named petrichor by Australian scientists.

A baptismal name that is quite recent, dating from 1964. It was the mineralogist Roderick G. Thomas and the chemist Isabel Joy Bear who together came up with this neologism, which comes from Greek and means “stone blood”. A poetic way of naming their research into the smell of rain.

A secretion from plants

It is in the columns of the very famous magazine Nature that the two Australian researchers have for the first time described from a scientific point of view the process of “the odor derived from an oil exuded by certain plants during the dry season”. They were later able to explain how these aromatic oils generated during the downpour could delay both the germination of seeds and the growth of the plants themselves. This would mean that Plants secrete these oils in order to protect the seedsto prevent them from germinating during a drought.

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The aroma of the earth

But does rainwater really have a smell? In itself, no, when it falls from the sky. But a combination of factors means that showers have an unforgettable olfactory imprint. It is actually the thermal shock from contact of water droplets with dry soil which generates this phenomenon. The secretions of the plants under the downpour, a sort of vegetable oil, actually combine with a volatile molecule emanating from the bacteria present in the soil, called geosmin (Greek word meaning “aroma of the earth”).

Walking after the rain with your dog – © Tatyana Vyc

Journey to the land of smells
In this documentary-comic strip, Joanne, Vincent and Tiffany set off to discover the perfumes and smells of nature guided by a specialist in the sense of smell. A book to read with your nose in the wind!

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A smell that cannot be synthesized

The quasi-chemical reaction of this smell of rain does not stop there, because these two substances will in fact mix with a third, well-known one: ozone. It is this mixture of molecules that will then generate this strong, recognizable smell, which spreads in the air before fading as quickly as it appeared. Note that this petrichor phenomenon is so particular, and so complex, that it has never been able to be synthesized: this smell remains and will remain purely natural and ephemeral.

One question remains: why does this phenomenon seem so pleasant to us and become so engraved in our minds? We can imagine that it is due to the instinct of the species, to the collective memory going back to the earliest times of human beings: Rain is life. Without it, nothing grows and nothing would survive.

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