The earthy essence is strongest when rain quenches dehydrated ground... In the 1950s and 60s, a pair of Australian mineralogists, Isabel Joy Bear and Richard Grenfell Thomas, set out to discover the source of that piquant perfume...Ultimately, Bear and Thomas linked the scent to organic compounds that build up in the atmosphere, including heady-smelling terpenes secreted by plants. The major components in turpentine and resin, terpenes also put the essence in essential oil. They are the freshness in pine, the cool in peppermint, the spice in ginger. From the tallest conifers and from the tiniest mosses, hundreds of millions of tons are released into the atmosphere each year.
Unleashed, the terpenes make some remarkably diverse contributions. They give hops its bite and cannabis its smooth character. They help form the blue haze that hangs over the Blue Ridge Mountains of Virginia and the Blue Mountains of Australia. They also make some of the planet’s most intoxicating perfume... The aroma is more powerful in the wake of drought because the essential oils have had longer to build in the layers of rock.
Publishing in the journal Nature in 1964, Bear and Thomas proposed a name for the scent brought on by rain. They called it “petrichor”, a blend of the Greek words “petra” (rock) and “ikkor”, the ethereal fluid that flowed as blood in the veins of the gods.
But the scientists acknowledged that they were not the first to identify the stormy smell. They were not even the first to extract it. In fact, the element they dubbed petrichor was already the signature fragrance in an attar produced in an ancient perfumery found in the north Indian state of Uttar Pradesh, in the village of Kannauj.
~ from Rain: A Natural and Cultural History by Cynthia Barnett