Lately, it has been a warm spring, but also interspersed with periods of rain. The weather in the course of a single day can also be unpredictable. Yesterday, for example, was warm and sunny until 4 pm when the sharp fresh scent of a thunderstorm hit; it was followed quickly by the wet earthiness of rain. I am sure that our bodies tell us that the rain is approaching or it is raining, but we do not actually give much thought about such common occurrences. It makes me curious as to what makes up the smell of rain in the first place.
The smell of rain or ‘petrichor’, as it was coined by two researchers Isabel Joy Bear and Richard G. Thomas in 1964, is a combination of ozone (O3) and aromatic compounds. Ozone is formed by sunlight-mediated reactions at high altitudes and carried by stormy draughts to earth’s surface, where we smell it. Lightning also facilitates the production of ozone as it splits atmospheric oxygen and nitrogen molecules into atoms that can form nitric oxide (NO), one of the precursors of ozone. It signals the incoming rain with a fresh, sharp snap.
As the falling rain drops hit the ground, they displace odoriferous compounds on dry surfaces and send them into the air as aerosols that we can detect. These aromatic compounds come from vegetation, decaying organic matter, and metabolic by-products of bacteria and algae. Therefore, the refreshing scent of rain varies from one place to another. Garden rain smells fresh, green, herbal, floral and musty, whereas city rain carries the steaming concretes and asphalts. Rain near sea shores and harbours smells of brine and ammonia, and those with mudflats are also redolent of loam and clay. These wet potpourris, combined with ozone, refresh the spirit after a dry spell.
One of the odoriferous compounds is geosmin. It is produced by blue-green algae and soil-dwelling bacteria and is responsible for the damp, earthy scent of rain. But beyond the earthiness, geosmin may actually have an important biological function. Some cacti emit the earthy, musty scent of dehydrogeosmin to attract bats for pollination. It may even help camels search for oases in the vast desert, and the bacteria can deposit spores on them. Humans, with our ability to sniff out rain, could also interpret it as a sign of lush greenery, where food abounds. In a more modern context, geosmin is the scent that delights gardeners as they dig into the wet earth, but dismays vintners and public authorities when it taints their wine and drinking water with an ‘off’ smell. Indeed, geosmin is an interesting work of nature.
And with such a fascinating olfactory experience of rain, it is no wonder that those in the village of Kannauj in Uttar Pradesh, India, have been bottling the scent of rain for thousands of years. In a recent article of the Atlantic, Cynthia Barnett describes how the post-monsoon earth are collected, steam-distilled for six to seven hours, and fixed in sandalwood oil in order to produce mitti attar — Earth’s perfume. But, for USD125, one can also find a great example of how geosmin lends a seductive earthiness in modern perfumery in Bat (Zoologist Perfumes, 2015) by Dr Ellen Covey.
Is it not amazing what we can learn from paying attention to nature?
- Bear, I. J. & Thomas, R. G. Nature of argillaceous odour. Nature 201, 993–995 (1964), Joung, Y. S. & Buie, C. R. Aerosol generation by raindrop impact on soil. Nat. Commun. 6:6083 (2015)
- Schlumpberger, B. O., Jux, A., Kunert, M., Boland, W., & Wittmann, D. (2004). Musty-earthy scent in cactus flowers: Characteristics of floral scent production in dehydrogeosmin-producing cacti. International Journal of Plant Sciences, 165(6), 1007-1015
- Storm Scents: It’s True, You Can Smell Oncoming Summer Rain in the Scientific American, What Makes Rain Smell So Good? From the Smithsonian Institute.