(n.) the earthy odour smelled when rain falls after a lengthy period of dryness
There’s been a run of heatwaves and thunderstorms around here at HH HQ recently, which brought to mind the word petrichor. And that led us to ask this over on Twitter today:
Reassuringly, almost three-quarters of you lovely word nerds already knew that one. but as for its origins...? A slough of curious emails and comments followed that survey, asking HH to explain where such a lovely word comes from. And lo, your wish is our command.
Petrichor (which, for the 28% of people in that survey, is the name of the rich, earthy aroma you can smell in the air after it’s rained) pulls together two Ancient Greek roots, one considerably more familiar than the other.
On the one hand, you have petra, meaning “rock” (which is also the origin also of petrify, a word that literally means “to turn to stone”). On the other hand, you have ichor, which in its original Ancient Greek was the name of a mysterious, ethereal fluid supposed to flow like blood in the veins of the gods.
Ichor has since been picked up in English and reapplied in a number of more concrete senses. Medically, for instance, it’s an old word for lymph, the watery fluid that secretes from a healing wound, while to a geologist ichor is the name of a type of magma containing vacuoles of superheated water, and which is supposed to form granite more readily than other types of magma. Outside of fairly specific fields like those, however, ichor remains a fairly abstract, figurative term—and it’s in that abstract sense that it’s found in the word petrichor: put those two elements together, and you essentially have a word that means “the life blood of rocks”.
As a phenomenon, this “life blood” was first described in a March 1964 edition of the scientific journal Nature, which alluded to an “apparently unique odour which can be regarded as an ‘ichor’ or ‘tenuous essence’ derived from rock or stone.” According to the same article, seemingly this smell had already been identified and classed as an “argillaceous odour”—the fantastic adjective argillaceous, should you ever need it, meaning “of or relating to clay”. Keen to reinforce the fact that this smell does not occur uniquely in clay-soil environments, the authors of the article in question proposed the word petrichor in its place.
As for precisely what causes it? No one is entirely sure, but supposedly—and especially after long periods of drought—rainfall can throw enough microscopic particles of rock and earth into the air to create a perceptible smell. And at least now we know that that smell has a name.