petrichor

Petrichor is a scientific coinage that became a popular word. Isabel Joy Bear and Richard G. Thomas, chemists at Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Melbourne, introduced it in a paper titled ‘Nature of Argillaceous Odour,’ published in the journal Nature in March 1964. They needed a name for the smell that follows rain on dry ground, which they described as widespread and familiar but chemically unexplained. Their term joined Greek petra, ‘stone,’ and ichor, the fluid said to flow in the veins of the gods, suggesting an essence drawn from rock.

Bear and Thomas found the source by steam-distilling rocks and clay that had been exposed to warm, dry conditions. The process produced a yellowish oil with the characteristic scent. They concluded that during dry periods plants release oils that collect on the surfaces of soil and stone, and that moisture frees them into the air. A second ingredient is geosmin, a compound made by soil bacteria of the genus Streptomyces. Geosmin, whose name means ‘earth smell,’ was identified in 1965, and the human nose is extremely sensitive to it, detecting it at concentrations of a few parts per trillion.

How the smell reaches the nose was explained more recently. In 2015 researchers at the Massachusetts Institute of Technology, Cullen Buie and Youngsoo Joung, used high-speed cameras to film raindrops hitting porous surfaces. They found that a drop traps tiny pockets of air, which rise as bubbles and burst, throwing a fine mist of aerosols upward, much like the fizz from a glass of champagne. Light and moderate rain produces the most aerosols, which helps explain why a gentle shower smells stronger than a heavy downpour.

Why humans like the smell is less certain. Some researchers suggest that sensitivity to geosmin helped ancestors find water, but this remains a hypothesis. Other animals clearly respond to it. A 2020 study in Nature Microbiology showed that tiny soil animals called springtails are attracted to geosmin, feed on Streptomyces colonies and spread their spores. Humans are less fond of geosmin in food and water: it is the source of the muddy taste sometimes found in freshwater fish and in tap water after algal blooms.

Long before the word existed, people valued the scent. In Kannauj, a perfume-making town in northern India, distillers produce mitti attar, an oil made by distilling baked clay so that it smells of the first monsoon rain. Petrichor is also distinct from the sharp, clean smell sometimes noticed before a thunderstorm, which comes from ozone carried down by storm winds. Since the 2000s the word has spread widely online, appearing in lists of beautiful English words and in the names of perfumes, cafés and songs.