Everyone recognises it and almost nobody can name it: the smell that arrives in the minutes before and after the first rain on dry ground. It has a name, it has a cause, and the cause is stranger than the smell.
The word is younger than you would think
Petrichor was coined in 1964 by two Australian researchers, Isabel Joy Bear and Richard Thomas, in a paper in Nature. They built it from the Greek: petra, stone, and ichor, the fluid said to run in the veins of the gods. The smell had existed for as long as there had been rain and noses. It simply had no term in English until sixty years ago.
What they identified was an oil that accumulates on rocks and in soil during dry periods, secreted by plants, and is released into the air when rain disturbs it.
Geosmin, and a nose tuned to almost nothing
The dominant note of that smell is a single compound: geosmin, produced by soil bacteria, chiefly Streptomyces. When the ground dries, the bacteria form spores. When rain lands, the impact throws those spores and the surrounding compounds into the air as a fine aerosol, and geosmin travels with them.
The human nose is astonishingly sensitive to it — detection thresholds are usually quoted in the range of a few parts per trillion, which puts it among the compounds we perceive most keenly. It is the same molecule that makes beetroot taste of earth, and the same one that ruins a bottle of water or a fish that has grown in a muddy pond.
The ground is not passive. It has been accumulating a smell all through the dry months, waiting for something to knock it loose.
Why it arrives before the rain does
The smell that reaches you in advance of the first drops is usually a different molecule: ozone, carried down from higher in the atmosphere on the downdraught ahead of a storm. So the full experience is three things arriving in sequence — ozone on the wind, then geosmin as the drops hit dry ground, then the green, resinous note of plants responding to water.
That sequence is why the smell of rain is so hard to reproduce with a single material, and why most attempts at it in perfumery smell like wet concrete and nothing else.
Distilling it
India solved this centuries before anybody named it. In Kannauj, discs of clay from the Ganges plain are baked, then loaded into a copper still and distilled into a receiver of sandalwood oil. What comes out is the smell of rain on hot earth, captured from the earth itself rather than assembled from parts.
It remains the one material in perfumery with no convincing synthetic substitute. A.001, Before the Rain, is built on it — and is named for the ozone part of the sequence rather than the geosmin, because the twenty minutes before a storm breaks is the part we wanted.