Petrichor

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, which makes the word itself feel like a small gift: a label finally arriving for something the nose had been carrying around for years without permission to name.

What the nose is actually catching is a cocktail. Oils that plants exude during dry weather and that settle into clay and porous stone. Geosmin, an aromatic compound made by soil bacteria, which humans can detect at vanishingly small concentrations. A whisper of ozone if there is lightning nearby. Petrichor is the name for the experience, not for any single molecule.

— Bear and Thomas, 1964 —

The word 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. 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" — 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 along by twenty-first-century blogs and a thousand list articles about words for things you did not know had names.

"The blood of the stones — a small piece of mid-twentieth-century scientific poetry."

— The chemistry of geosmin —

Geosmin, 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. The same compound gives beetroot its earthy taste and gives a cheap bottle of wine that mossy off-note when it has gone wrong.

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. We may, in other words, be wired to love this smell because our ancestors needed it to stay alive.

— Why our noses notice —

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 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 only in the 2000s.

Petrichor is now used loosely for any earthy, post-rain freshness, which has made some chemists slightly grumpy. The word, like the smell, is fragile: keep it for the moment it was made for, and it stays useful. Stretch it too far and it dissolves into the same vague poetry as "that nice smell after the rain" — which is, of course, exactly what the word was invented to replace.

Meteorologists have a related observation. Petrichor often arrives on the wind several minutes before the rain itself, carried on the downdrafts of a distant storm. Dogs, horses and migraine sufferers tend to notice it first. The smell, in that sense, is a forecast: a small chemical letter from somewhere over the horizon, dropped through the open window before the clouds have finished arranging themselves overhead. The Australians who named the molecule did not quite intend that, but the poetry of the word holds up regardless. Whatever one makes of "the blood of the stones", it is hard to shake once you have read it on a hot afternoon and felt the first heavy drops land on a pavement that has not seen water for weeks.