pe·tri·chor | /ˈpɛtrɪˌkɔːr/
Petrichor is that distinctive, clean and yet earthy scent we associate with rain-sodden ground after a long dry spell. It’s a relatively new word for a long-recognised phenomenon, first used by Australian scientists Isabel Bear and Dick Thomas in a 1964 article for Nature magazine. A combination the Ancient Greek pétra (rock) with ikhor (the divine blood of the gods), petrichor has all but replaced the rather dry term argillaceous odour previously used in scientific literature.
The compounds largely responsible for this alluring fragrance are growth-inhibiting fatty acids produced by plants during dry spells and geosmin – a molecule released after rain by a genus of bacteria called Streptomyces found in most healthy soils. Humans are acutely sensitive to geosmin and can detect it in concentrations as low as a few parts per billion. Of course, scent doesn’t only rise from the soil when it rains, as Nan Sheppard draws our attention to in her seminal book
The Living Mountain:
“Birch needs rain to release its odour. It is a scent with body to it, fruity like old brandy,
and on a wet warm day one can be as good as drunk with it.”
Using high-speed cameras to record rain landing, MIT researchers uncovered the mechanism for releasing petrichor. When it rains on a porous surface, tiny air bubbles form in the pores and then rise to the water’s surface, releasing the scent. This delicate process works best during a mizzle (light rain) rather than after plothering (a heavy downpour). The presence of sweet-smelling ozone (associated with thunderstorms) may also enhance the petrichor, which Bear and Thomas described as “tenuous essence” rather than “a fixed chemical entity”.
Introduction Aeolian Alpenglow
Benthos Crepuscular Crispate Crown shyness
Desire lines Dreich Endragoned Edgelands
Frondescence Fumarole Gluggaveður Gossamer
Karst Komorebi Lawrence Long acre
Machair Monkey’s wedding Moonglade
Psithurism Quartz Rakuyou Roaring forties
Snag Soft estate Specular, diffuse and pellucid
Spoondrift Steam fog Swash zone Sylvan