I was walking my dog when the rain came. Early evening on a hot day, and suddenly the road is steaming. Although this must happen elsewhere, for me it is as much a part of an Australian summer as is the song of cicadas: the sheer delight of standing in the rain and getting wet as an earthy, musty smell – petrichor – rises from the road.

The word petrichor was termed back in 1964 by two Australian scientists studying the smells of wet weather. As one does. It’s a combination of two Greek words: petra - stone - and ichor - the fluid flowing through the veins of the Greek Gods.

A few things contribute to this smell of summer. Oils secreted by various plants during dry periods are released into the air when it rains. Also, a chemical reaction by a group of soil bacteria (called actinomycetes or actinobacteria, which help decompose decaying organic matter) with the falling rain releases an organic compound called geosmin. This aromatic compound is a type of alcohol, which can be detected at just a few parts per trillion air molecules. (It’s also present in some wines – an impressive term to describe an earthy smell at your next wine tasting!)

Then there’s the smell of coming rain, the scent on the breeze coming from the approaching storm clouds. Rain itself has no scent, but lightening can split both oxygen and nitrogen molecules, which then combine to nitric oxide, which further reacts to form ozone, which is believed to be the smell of coming rain. Also, as rain drops hit the ground, porous surfaces – such as the asphalt where I was walking – causes rain drops to splatter. This results in petrichor compounds being released as aerosols and carried in the wind.

I simply stood and enjoyed the rain while it fell. When I got home, a frog was singing contendly in one of my downpipes.

And, naturally, I’d left the washing on the line.

My backyard in the rain

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