PETRICHOR/ˈpɛtrɪkə/

The smell of rain on dry ground.

More specifically, petrichor is the pleasant smell that often accompanies the first rain after a long period of warm, dry weather in certain regions. It was named by two Australian researchers, who discovered that the smell is an oily essence emitted from rocks or soils. The oil is a complicated mixture of at least fifty different compounds, rather like a perfume.

The diverse nature of the host materials has led us to propose the name “petrichor” for this apparently unique odour which can be regarded as an “ichor” or “tenuous essence” derived from rock or stone. This name, unlike the general term “argillaceous odour”, avoids the unwarranted implication that the phenomenon is restricted to clays or argillaceous materials; it does not imply that petrichor is necessarily a fixed chemical entity but rather it denotes an integral odour.

I J Bear and R G Thomas, The Nature of Argillaceous Odour in Nature, 7 March 1964. (Argillaceous means “consisting of or containing clay”, from Latin argilla, clay.)

It turned out that the oils are given off by vegetation during dry spells and are adsorbed on to the surface of rocks and soil particles, to be released into the air again by the next rains. There has been some suggestion that the smell may trigger reproductive behaviour in kangaroos and other marsupials. I can’t find any record of anybody having tried to bottle and sell it, but can’t help thinking it would be a hot item (my agent’s fee will be the usual modest 10%).

The word comes from Greek petros, a stone, plus ichor, from the Greek word for the fluid that flows like blood in the veins of the gods. Petrichor is a poetic creation that means something like “essence of rock”. Alas, it is encountered only very rarely.

Besides the pleasant, dewy petrichor of the post-rain afternoon, I see no hope or way out of a four-hour ride with the enigmatic mumbler.

VeloNews, the Journal of Competitive Cycling, 5 April 2004.

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