The earthy smell that rises just before a rainstorm is not the rain at all. Researchers have traced it to oily compounds stored in dry soil and rock, and to bacteria living beneath the surface, a phenomenon two Australian scientists named petrichor in 1964.
In Taiwan, where the plum rain season in May and June and near daily summer thunderstorms keep the air heavy for months, that scent is a familiar warning that a downpour is minutes away. The sensation is not imagined. What reaches the nose is a mixture of compounds the ground has been stockpiling, loosened by moisture and thrown upward by the first drops to land.
Australian Scientists Named The Smell Petrichor In 1964
Isabel Joy Bear and Richard Thomas published a paper in the journal Nature in 1964 examining what happens when water meets dry clay, rock, and soil. They coined the term petrichor from the Greek petra, meaning stone, and ichor, the fluid said in Greek mythology to run through the veins of the gods.
Their work showed that during dry spells, oily material accumulates on mineral surfaces and inside the tiny pores of rock and soil. Add moisture and those compounds can escape. The odor most people file under "dirt," in other words, is largely assembled on the ground during the weeks when it is not raining at all.
Streptomyces Bacteria Produce The Compound Geosmin
A second ingredient is alive. Geosmin, an organic compound with a pronounced earthy odor, is manufactured by soil microbes, including bacteria in the genus Streptomyces. Research published in Nature Chemical Biology identified the bifunctional enzyme that allows Streptomyces coelicolor to build the molecule, and confirmed geosmin as a defining note of damp ground.
That makes petrichor a blend rather than a single chemical. Plant-derived oils, mineral residues, and microbial byproducts such as geosmin all contribute, which is one reason the smell differs between a limestone hillside, a rice paddy, and a city sidewalk.
Rising Humidity Releases Soil Oils Before Rainfall Begins
The scent often arrives ahead of the weather. Australia's Commonwealth Scientific and Industrial Research Organisation, the national science agency where the 1964 research was carried out, notes that humidity climbs well before a storm reaches the ground. Water vapor works into the pore spaces of rock and soil and can begin pushing the stored oils out.
Britain's Met Office makes a similar point, saying people can sometimes detect the odor before the first drops fall because the shift in atmospheric moisture is already acting on the surface. The feeling of smelling rain on the way tracks a measurable change in conditions, not a premonition.
MIT Cameras Captured Raindrops Firing Aerosols Into Air
What happens next was documented in 2015 by Young Soo Joung and Cullen Buie at the Massachusetts Institute of Technology. Using high-speed photography, the pair filmed individual drops striking soil and other porous surfaces and published the results in Nature Communications.
The footage showed a drop trapping minute air bubbles at the moment of impact. Those bubbles rise through the droplet and burst at its surface, spraying a fine mist of far smaller droplets into the air as an aerosol. Any odor compounds resting on the ground can ride upward with them, which explains why the smell spikes in the first minute of a shower.
Light Rain Lofts More Scent Than Downpours
The MIT team also varied drop speed and surface type, and found the effect is not proportional to how hard it rains. Light to moderate rainfall generated aerosols readily. Under heavy, fast-falling rain, the researchers observed fewer of them.
That runs against intuition but matches ordinary experience. The first scattered drops of a Taipei afternoon storm often smell far stronger than the sheet of water that follows 10 minutes later, because the gentler impacts are the ones that launch the scent.
The shorthand phrase "the smell of rain" gets the sequence backward. Rain is the trigger. The smell belongs to the ground, and what the first drops do is hand it to the air.
References:
- Nature: I. J. Bear and R. G. Thomas, "Nature of Argillaceous Odour," 1964.
- Nature Communications: Young Soo Joung and Cullen R. Buie, "Aerosol generation by raindrop impact on soil," 2015.
- MIT News: "Rainfall can release aerosols, study finds."
- CSIRO: "The smell of rain: how our scientists invented a new word."
- Nature Chemical Biology: "Biosynthesis of the earthy odorant geosmin by a bifunctional Streptomyces coelicolor enzyme."
- Met Office: "Rain has a smell."
