Know-how

Ever wonder if the way the air smells after a summer rain has a name? It does. Petrichor.

Livia Gamper
21.07.2022
Translation: Christine Duranza

It comes at the end of a warm summer day when the first drops hit the ground...the unmistakable scent of rain. It’s called petrichor and occurs only in summer.

At the moment, Switzerland is withering under a heat wave. The earth is warm and dry. That’s why when long-awaited relief in the form of cooling rain finally arrives, the air will be filled with the unmistakable smell of fresh earth. This phenomenon is known as petrichor. Daniel Gerstgrasser, meteorologist at MeteoSwiss, explains that the scent is most intense at the onset of rainfall and not, as often assumed, afterwards.

According to Gerstgrasser, if it rains every day, the scent of petrichor is barely detectable; it’s at its strongest after long periods of dry weather, as is currently the case in Switzerland.

A yellow plant oil is at the root of the phenomenon

But what exactly causes petrichor? It’s not like rainwater itself has a scent. Gerstgrasser refers to an article from the German weather service (German only) in which the German forecasters explain that a certain yellow-coloured oil is responsible for the scent. This oil is produced by plants during dry weather and binds to clay minerals and other particles. As soon as it starts raining, this oil combines with a substance called geosmin, which is secreted by microorganisms found in the soil. It’s this geosmin that gives off an earthy smell.

If, after a dry period, rain falls and hits the ground at a high speed, the drops create tiny air bubbles in the surface layer of dust. The bubbles, which carry the scent, eventually burst and are released into the air through the violent impact. Light rain maximises the effect of petrichor. Heavy rain, on the other hand, tends to inhibit the release of the scent because the rapid soaking of the soil reduces bubble formation.

Named after the Greek gods

You may still be wondering how petrichor got its name. The German weather service (German only) explains that more than 50 years ago, two Australian researchers gave petrichor its name: «Petrichor is derived from the words petros – stone in Greek – and ichor – the fluid in the veins of the Greek gods.»

When I asked why the petrichor phenomenon doesn’t occur in winter, Gerstgrasser responded that it’s probably because that’s when plants are dormant. The full process can only be reached in the summer months – making it that much more extroardinary.

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Testing devices and gadgets is my thing. Some experiments lead to interesting insights, others to demolished phones. I’m hooked on series and can’t imagine life without Netflix. In summer, you’ll find me soaking up the sun by the lake or at a music festival.


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