Mark P. Henderson

“Blood out of stone” – the smell of rain

When a long, hot summer day ends with a light shower of rain, step out into the garden and inhale the aroma. You’ll detect the same odour when rain falls on warm silica-rich rock, or when damp ground dries under warm sun. Delightful? Most people find it so. Perhaps our remote ancestors recognised the smell as betokening the combination of warmth and moisture that induces growth, and therefore food, so we’ve inherited a positive response to it.

But what is the smell? What are its main chemical components and what produces them?

Those questions were investigated in Australia during the 1960s by Isabel Bear, Richard Thomas and their colleagues. They found that the aroma could be attributed to two classes of chemicals: geosmin, and some short chain fatty acids.

Geosmin is generated by certain fungus-like soil bacteria known as actinomycetes, which also produce most or our naturally-occurring antibiotics. ‘Geosmin’ is a user-friendly name for dimethylhydroxyoctahydronaphthalene, or at least one isomer of dimethylhydroxyoctahydronaphthalene. I don’t know whether any of the scores of other possible isomers affect our olfactory nerves as efficiently the one described when you look up ‘geosmin’ in scholarly sources, but it seems we can detect this form in concentrations of five parts per trillion, which is about half a drop in an Olympic swimming pool.

The short-chain fatty acids (technically, they’re predominantly eight, nine and ten carbon straight-chain aliphatic acids, plus a few others that are probably branched-chain) are formed by bacterial breakdown of an oil secreted by many species of plants during hot, dry conditions. When these oils are released, they inhibit the growth of other plants. Presumably this is an evolutionary device that enables the plant to discourage competition for water when water is scarce.

The mixture of geosmin and fatty acids that produces the ‘divine aroma’ enjoyed by so many of us is released in aerosol form from pores in the soil and rock when rain falls. And what’s the name for this odour? Bear and Thomas christened it petrichor, combining the Greek words for rock (petros) and for the blood that flowed in the veins of the gods (ichor), and the name has stuck, though it isn’t in common use. It should be. It’s a smell like no other, and I for one love it.

Blood (of the gods) from stone: the smell of rain.

Many people say they can smell the approach of rain, sensing the change in the atmosphere when the humidity rises above 75%. But this isn’t petrichor; it’s a different odour. It’s said to be dominated by atmospheric gases, particularly ozone, that are generated by electrical storms, but I personally can’t detect the sharp scent of ozone when rain is imminent unless there are thunderstorms in the vicinity. So I’m not convinced that the chemistry of the approaching-rain-smell is fully known.

But thanks to Bear and Thomas and their co-workers and successors, we know what’s responsible for the smell of rain that’s already fallen on sun-drenched soil and rock: petrichor.