petrichor
noun

- a distinctive, earthy, usually pleasant odour that is associated with rainfall especially when following a warm, dry period, arising from a combination of volatile plant oils and geosmin released from the soil into the air.

- the smell produced when rain falls on dry ground, usually experienced as being pleasant.

- the pleasant, distinctive smell frequently accompanying the first rain after a long period of warm, dry weather, caused by an oil produced by certain plants and absorbed by soil and rock.

Merriam-Webster, Cambridge Dictionary, Oxford English Dictionary

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WORD ORIGIN

The word petrichor is one of the youngest in the English language — yet its roots are ancient.

It was coined in 1964 by two Australian scientists, Isabel Joy Bear and Richard Grenfell Thomas, who published a paper in the journal Nature titled "Nature of Argillaceous Odour." They needed a name for a smell that had never had one — the earthy scent released when rain finally falls on long-dry ground.

For the two roots they reached back to Ancient Greek:
- πέτρα / pétra = rock, stone (also the root of petroleum and petrified)
- ἰχώρ / ikhṓr = the fluid flowing through the veins of the gods in Greek mythology; a thin, ethereal essence

Together: petrichor = the essence of stone — or, more poetically, the blood of the gods rising from the rock after rain.

The word entered mainstream dictionaries only in the 2010s, when it was picked up by science writers and social media and suddenly became — for a geological term — surprisingly fashionable.

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THE NOSE KNOWS

Humans can detect petrichor at around 5 parts per trillion—roughly the equivalent of one drop of liquid diluted into about 40–50 million liters of water, or around 15–20 Olympic swimming pools.

The chemistry behind the experience is less romantic than the feeling: soil bacteria called Streptomyces produce a compound called geosmin as they dry out. When the first raindrops hit parched ground, microscopic air bubbles form inside the pores of the soil, burst at the surface, and spray an aerosol of geosmin into the air. That aerosol is what you inhale when you say the rain smells good. You are, in effect, breathing in bacteria.

There is also an oil component. During dry spells, certain plants secrete oils that drain into the soil and bind to rock and clay. Rain releases them back into the air. This is why petrichor varies so much from place to place — it carries a local signature, a chemical fingerprint of whatever grows in that particular patch of earth.

Helga & Paul Smith

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SYNONYMS

after-rain aroma, after-rain smell, aqueous earth perfume, argillaceous odour, breath of the soil, damp earth smell, damp-earth fragrance, dust-meets-rain scent, earth after rain, earthen scent, earthy aroma, earthy fragrance, earthy perfume, earthy scent after rain, earthy smell, first-rain fragrance, first-rain scent, fresh-earth smell, fresh soil scent, geosmin, geosmin scent, ground-rain scent, harvest-rain smell, loam-breath, moist-earth fragrance, monsoon scent, PETRICHOR, pluvial aroma, post-rain fragrance, post-rain freshness, post-storm scent, pre-storm freshness, rain perfume, rain scent, rain-kissed soil, rain-on-dust smell, rain-on-parched-earth aroma, rain-on-stone aroma, rain-soaked ground scent, rain-washed air, scent of approaching rain, scent of rain, scent of the first drops, scent of wet ground, smell of damp soil, smell of rain, smell of the storm coming, smell of thirsty ground, smell of wet earth, smell of wetted stone, storm scent, storm-cleansed air, storm-freshness, summer-rain aroma, terroir of rain, the earth-smell, the freshness after rain, the gods' blood, the smell before the storm, the smell of rain coming, the smell of the earth drinking, wet-clay fragrance, wet-earth aroma, wet-soil scent, wet-stone smell

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SMUGGLE OWAD into a conversation today, say something like:

“Thinking about PETRICHOR,… I wonder how many other human sensations still need a word to describe them.”

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