Word of the day
Petrichor
/ˈpɛt.rɪ.kɔːr/
noun
Petrichor is the smell that rises from dry earth when the first rain hits it after a long warm spell. It is sweet, metallic and a little dusty all at once. Most people know the sensation long before they meet the word. What the nose is actually catching is a cocktail: oils that plants exude during dry weather, plus geosmin, an aromatic compound made by soil bacteria that humans can detect at vanishingly small concentrations. Petrichor is the name for the experience, not for any single molecule.
Petrichor is the smell that rises from dry earth when the first rain hits it after a long warm spell. It is sweet, metallic and a little dusty all at once. Most people know the sensation long before they meet the word. What the nose is actually catching is a cocktail: oils tha...
The first storm of September brought a gust of petrichor through the open kitchen window and we both stopped what we were doing.
Examples
- The first storm of September brought a gust of petrichor through the open kitchen window and we both stopped what we were doing.
- She said the whole point of leaving the city was the petrichor, that you do not get it the same way over asphalt.
- Perfumers have spent decades trying to bottle petrichor and most of their attempts smell faintly of wet concrete instead.
Where it comes from
Petrichor was coined in 1964 by two Australian researchers, Isabel Joy Bear and Richard G. Thomas, in a paper for the journal Nature titled simply 'Nature of Argillaceous Odour'. They had isolated a yellowish oil that accumulates on rocks and clay during dry weather and showed that rainfall released it into the air as the smell people associate with the start of a storm. They needed a name for this oil and the smell it produced. They reached into Greek and put together petros, meaning stone, with ichor, the ethereal fluid that ran in the veins of the gods in Homer instead of mortal blood. Petrichor, then, literally means 'the blood of the stones', which is a small piece of mid-twentieth-century scientific poetry. The word was a technical term inside chemistry and geology for several decades before escaping into general English, helped by twenty-first-century blogs, weather apps and a thousand articles about words for things you did not know had names.
Stories around the word
The science behind petrichor is busier than the smell suggests. The geosmin component, identified by the Rutgers microbiologist Selman Waksman's school of soil chemistry in the 1960s, is produced by Streptomyces bacteria as they decay; the human nose can pick it up at four parts per trillion, which is more sensitive than a shark detecting blood in water. There is a working hypothesis, supported by behavioural studies on camels in the Negev and on Drosophila fruit flies in laboratories, that this hypersensitivity evolved because finding water meant finding life: animals that can smell distant rain at trace concentrations have a survival edge, especially in arid landscapes. India has a richer and older version of the story. The city of Kannauj, in Uttar Pradesh, has been distilling an attar called mitti attar for centuries, capturing the smell of dry earth meeting the monsoon by steaming baked clay discs in copper stills called degs and trapping the aroma in sandalwood oil. The technique is recognised as a protected craft tradition and is still practised by a handful of family-run distilleries. Western perfumers picked the idea up much later, with houses such as Demeter and Comme des Garçons releasing post-rain accords in the 2000s. Petrichor is also a popular shorthand among meteorologists for the way thunderstorms send odors miles ahead of themselves on downdrafts, which is why dogs and migraine sufferers often seem to know a storm is coming before any human eye catches the cloud. The word itself has now overflowed its original technical meaning and is sometimes used loosely for any earthy, post-rain freshness, even when no real rock-oil is involved, which has made some chemists slightly grumpy.
Sources and further reading
Common questions
- How do you pronounce petrichor?
- Three syllables: PET-ri-kor. Stress on the first syllable. The 'ch' is a hard 'k', not the 'ch' of church, because it comes from Greek chi. The final syllable rhymes with 'door'. Some American speakers shorten it to PET-ri-ker, which is acceptable but slightly less faithful to the Greek root.
- What actually makes the smell?
- Three things blend in. Plant oils that build up on dry soil and rocks during a heatwave; geosmin, a compound released by dying soil bacteria of the genus Streptomyces; and ozone, generated by lightning during nearby thunderstorms. Rain disturbs all three at once, kicking aerosols into the air. That is why petrichor is most intense after a long dry spell.
- Is petrichor the same as the smell of rain in general?
- Strictly, no. Petrichor specifically refers to the smell of rain hitting dry, sun-warmed ground. The fresh, ozonic smell before a storm is a related but separate phenomenon. The wet-asphalt smell of city rain shares some chemistry with petrichor but is dominated by different aromatics. Purists keep the word for the countryside version.
- Can you bottle petrichor?
- People try, with varied success. The Indian attar tradition, especially mitti attar from Kannauj, has been doing it since at least the eighteenth century by distilling baked earth into sandalwood oil. Modern perfumes such as Demeter's Dirt and Comme des Garçons' Series 1: Soil aim at related notes. None of them quite captures the moment, because the experience is partly anticipatory, tied to the sound of approaching rain.
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