The air is thick and full, the sky is about to burst.
Your clothes stick to your skin.
But with the discomfort comes a most attractive scent – the smell of impending wet weather. This smell is not as you might think, the rain or thunderstorm rolling in.
The smell is caused by two distinct compounds – petrichor and geosmin. Living in the pores of rocks and soil, these compounds are flushed out by the humidity preceding rain.
Wind helps distribute the compounds causing the wafting, earthy scent associated with rain.
Petrichor is a yellow oil, first named in the 1960’s by Australian scientists Isabel (Joy) Bear and Richard Thomas. By steam distilling rocks that had been left in warm, dry conditions, they discovered an oil formed in the rocks pores. They named this oil Petrichor, from “petra” meaning stone, and “ichor” meaning blood of the gods. Petrichor, therefore, means blood of the stone. It seems you can get blood from stones, just not in the way you might have been imagining.
Geosmin is produced by soil-dwelling bacteria called Actinomycetes, and causes the ‘earthy’ smell of rain.
When you dig in the soil and that ‘good soil smell’ wafts up, you are smelling the by-product of Actinomycetes.
Using high-speed cameras, researchers at the Massachusetts Institute of Technology found that when raindrops hit the porous surface of soil or rock, tiny air bubbles are trapped at the point of contact. These bubbles then shoot up through the raindrop and explode out the top with a pop – somewhat like champagne bubbles. The pop at the top releases the petrichor and geosmin into the atmosphere, allowing the heady scent of rain to fill the air.
Image by skeeze