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.
This article explores the science behind petrichor and the main compound responsible for the smell – geosmin. It also looks at the mixed reactions people have to the scent and why we may find it enjoyable during rainy weather.
Discovering ‘Petrichor’
Mineralogist used to call this rainy day smell “argillaceous odour” because they believed it came from rain mixing with clay and and clay-like components.1
In 1964, Bear and Thomas, proved otherwise. They showed that “a wide diversity of rocks and mineral aggregates” could produce the distinct smell and proposed “petrichor” as an alternative term. The term petrichor originates from two Greek words, ‘petra’ meaning rock, and ‘ikhor’ meaning blood of the gods.2 The name change, they argued, would prevent the false assumption that the phenomenon is limited only to clays.3
Bear and Thomas were getting closer to understanding the earthy smell, but hadn’t yet 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 conducting research on the smell of soil when they discovered a compound they named geosmin. This name comes from two Greek words, ‘geo’ meaning earth, and ‘osme’ meaning odor.5 The bacteria Streptomyces griseus, which is commonly found in soil and freshwater environments, was found to be the source of geosmin.6
We now understand that geosmin is mainly responsible for the earthy smell after rainfall. But, why bacteria produce it was a mystery until recently.
Why Do Bacteria Produce Geosmin?
Scientists have hypothesized that geosmin may help deter predators and attract organisms that spread bacterial spores.7 Recently, they were able to demonstrate that geosmin deters fruit flies from eating the materials that Streptomyces grows on.8
Additionally, a new study out of Sweden shows that springtails, tiny arthropods, are attracted to geosmin and another compound called 2-methylisoborneol.9 After being drawn in by the smell, bacterial spores stick to springtails, which spread them through their movement and digestion.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
Our Love Hate Relationship With Geomin
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 Why humans are so sensitive to it and why we love it is unclear. 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
To complicate matters, we don’t always love the smell of geosmin. When geosmin is present in drinking water, we find it overwhelmingly unpleasant. It’s a big problem for municipal water supplies, as geosmin is not removed by conventional water treatment methods.15 The taste and odour of geosmin can vary depending on the concentration, but it is generally described as musty, mouldy, or earthy.
The reason for this conflicting response may lie in the connection between scent, emotion, and memory.16 Our experiences can shape our preferences for certain scents, creating positive or negative associations.17 When a scent conjures up a certain experience, time, or place, we refer to it as a “Proustian moment.”18 Understanding the link between scent, emotion, and memory may shed some light on why we find certain smells appealing or repulsive. If you enjoy the smell of rain, it may remind you of autumn hikes, playing in puddles as a child, or rainy day cabin retreats. Over time, you create a positive association with the smell, but not when it unexpectedly finds its way into your cup of water.
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.”19 Ozone molecules are then carried down by the rain. The smell of ozone has been described as chlorine-like, metallic, and clean.20
To Summarize
The smell of petrichor has captivated humans for centuries, but it wasn’t until recent years that scientists started unravelling the mystery behind it. With the discovery of geosmin, the identification of bacteria, plant oils, and ozone, we now have a better understanding of what causes that earthy, fresh scent.
Whether we find it soothing or nostalgic, the smell of petrichor is a reminder of the natural world and its complex processes. So, the next time you’re caught in a rainstorm, take a deep breath and enjoy the smell.
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From Grass to Gardens: Creating Habitat and Biodiversity With Native Plant Gardens
Why Do Birds Sing in the Early Morning?
Acadian Forest: History, Species, and Biodiversity
Sources
1 I Bear and R Thomas. 1966. “Genesis of petrichor.” Geochimica et Cosmochimica Acta.
2 C. Barnett. 2015. “Making Perfume From the Rain: Indian villagers have found a way to bottle the fragrance of monsoons.” The Atlantic.
3 Bear and Thomas. 1966.
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.
8 Ibid.
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.
10 Ibid.
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.
15 Halifax Water. “My Water Has a Taste and Odour.”
16 James Taylor. 2020. “What the Nose Knows.” Dana Foundation.
17 Rachel Herz. 2001. “Ah, Sweet Skunk! Why We Like or Dislike What We Smell.” Dana Foundation.
18 Taylor. 2020.
19 M. Prunier. PDF.
20 C. Vanvuren. 2017. “What Does Ozone Smell Like?” Molekule.