You know that scent of the first rain after a very long dry spell? Two Australian researchers tracked it down and called it "petrichor." The name means something like "the essence of a stone." And you can buy it.

I've done entries in the past about what goes into making the scent after a rain. This scent, a combination of the ozone leftover from lightning strikes and the bacteria that get kicked into the air by water drops hitting loose, dry soil, is a scent you can get anywhere after just a bit of a dry spell. In arid regions, where there is no water for months on end, the scent is something else. It is breathed out of the stone itself.


Two Australian scientists, I. J. Bear and R. G. Thomas, studied this scent in the early 1960s. The researchers found that, during long dry spells, plants produce an oil that prevents seeds from sprouting. The oil, a complicated mix of fifty different compounds, prevents new seeds from sprouting and dying for lack of water. Over time the oil gets absorbed into clay, rock, and other dirt.

When the rain comes down, the rocks open up and give off the odor. (The researchers noted that silica and metallic silicates were especially good at releasing the scent.) This smell, called petrichor - poetically, the blood of stones - is the earthy smell that people breathe in so appreciatively during the first rain after a long, hot, dry summer. People are not alone in their appreciation. Bear and Thomas speculated that cattle were able to scent the petrichor on the wind and follow it to water. The oil given off by eucalyptus leaves during dry spells, and absorbed by dirt, gets washed into rivers during rains and is taken by the vertebrate inhabitants of those rivers as a signal to start breeding.


It might turn human minds to thoughts of breeding as well. In the paper, the scientists explain that, in India, people spend May and June collecting dried mud. They steam the mud, and let the steam infuse sandalwood oil with the scent. The combined essence is then sold as "matti ka attar" - earth scent. (That is what it's called in the paper. I can't find that fragrance online, but there is "Mitti attar" which is said to be the scent of the first rain, and is made exactly the way Bear and Thomas describe the matti ka attar being made.) So you might be able to walk around wearing the smell of rain.

[Via Nature of Agrillaceous Odor, Petrichor and Plant Odor, Petrichor.]