I love the earthy smell that greets me on a rainy day. The familiar aroma fills the air when rain falls after long periods of warm, dry weather. It’s called petrichor and, it turns out, humans are very sensitive to it.
Mineralogists used to describe this rainy day smell as argillaceous odour. Named that way because the smell was believed to originate from rain interacting with clay and clay-like components (argillaceous minerals).1
In 1964, Bear and Thomas, proved otherwise. They demonstrated that “a wide diversity of rocks and mineral aggregates” had the capacity to produce the distinct smell. They proposed petrichor as an alternative term. This, they reason, would avoid the “unwarranted implication that the phenomenon” was “restricted to clays or argillaceous materials.”2
The word petrichor comes from the Greek words ‘petra’ (rock) and ‘ikhor’ (blood of the gods).3
Bear and Thomas were getting closer to understanding the earthy smell, but hadn’t figured it out. In a later paper, they hypothesized that the smell was produced by plant oils (stearic acid and palmitic acid) that accumulated in rock and soil during dry periods and were released into the air when rain fell.4
They were partly correct. Plant oils do contribute to that rainy day smell, but they aren’t the primary cause.
What causes this smell?
Elsewhere, in 1966, Gerber and Lechevalier, were studying the smell of soil and isolated a compound they called geosmin. Derived from the Greek words geo (earth) and osme (odour).5 Geosmin was caused by the bacteria, Streptomyces griseus.6
We now understand that geosmin is primarily responsible for the earthy smell after rainfall. But, why bacteria produce it has been a mystery until recently.
Why do bacteria produce geosmin?
Streptomyces release geosmin when they die. Scientists have hypothesized that geosmin may help deter predators and attract organisms that spread bacteria spores.7
We now know that geosmin deters fruit flies from eating the materials Streptomyces grows on.8
A recent study out of Sweden shows that springtails, tiny anthropods, are attracted to geosmin and another compound called 2-methylisoborneol.9 After being drawn in, the bacteria spores stick to springtails that spread them through their movement and by eating them and defecating.10 Researcher, Mark Buttner, says it’s “analogous to birds eating the fruits of plants. They get food but they also distribute the seeds, which benefits the plants.”11
We love the smell of petrichor
Humans are really sensitive to geosmin. Our noses can detect geosmin at less than 5 parts per trillion!12 That’s 200,000 times more sensitive than a shark is to blood in water.13
Long before we knew the science behind petrichor, the scent was being extracted from the earth for use as perfume in Kannauj, India.14 The makers of the perfume call it “mitti attar” or “Earth’s perfume.”15
The ozone and lightening connection
Finally, there’s one last thing you might be smelling on a rainy day and that’s ozone.
Prunier explains, “when lightening strikes, diatomic molecules of oxygen and nitrogen are split, and rearranged to create nitric oxide (NO) and ozone, or O3.”16 Ozone molecules are then carried down by the rain. The smell of ozone has been described as chlorine-like, metallic, and clean.17
The earthy aroma on a rainy day is produced by the bacteria Streptomyces. Plant oils that build up in soil and rock can also contribute to the smell and, if lightening is present, you might also smell ozone.
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1 I Bear and R Thomas. 1966. “Genesis of petrichor.” Geochimica et Cosmochimica Acta.
3 C. Barnett. 2015. “Making Perfume From the Rain: Indian villagers have found a way to bottle the fragrance of monsoons.” The Atlantic.
4 Bear and Thomas. 1966.
5 M. Bush, A. Smith, and E. Vikeli. 2017. “Meet the Molecules; Geosmin.“
6 N. Gerber and H. Lechevalier. 1965. “Geosmin, an earthly-smelling substance isolated from actinomycetes.” Appl Microbiol.
7 Bush, Smith, and Vikely. 2017.
9 P. Becher, V. Verschut, M. Bibb, M. Bush, B. Molnár, E. Barane, M. Al-Bassam, G. Chandra, L. Song, G. Challis, M. Buttner, and K. Flärdh. 2020. “Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal.” Nature Microbiology.
11 R. Haridy. 2020. “The 500-million-year-old reason behind the unique scent of rain.”
12 M. Prunier. “Petrichor: The Smell of Rain.” PDF.
13 H. Campbell. 2018. “Geosmin: Why We Like The Smell Of Air After A Storm.
14 C. Barnett. 2015.
16 M. Prunier. PDF.
17 C. Vanvuren. 2017. “What Does Ozone Smell Like?” Molekule.