The Smell of Rain Has a Name
You know the smell. It's been hot and dry for weeks, and then the first fat drops of rain start to land on the footpath and the garden beds. And up comes that smell. Earthy, warm, a bit sweet. Unmistakable.
It's one of those smells that almost everyone recognises and almost nobody can describe.
But somebody did give it a name. And the story of where that smell comes from turns out to involve mineral chemistry, soil bacteria, tiny insect-like creatures and champagne bubbles.
Naming the smell
In 1964, two scientists at the CSIRO's Division of Mineral Chemistry in Melbourne, Isabel Joy Bear and Richard Thomas, published a paper in Nature. It had the wonderfully dry title "Nature of Argillaceous Odour". Argillaceous simply means clay-like. So: the nature of the smell of clay.
The smell itself wasn't news. Older mineralogy textbooks had already noted that certain dry clays and soils give off an odour when you breathe on them or wet them, and that it was especially common in dry regions and linked with the first rains after a drought. In India, a perfume industry had even captured the scent in sandalwood oil, calling it matti ka attar, or earth perfume. But what was actually producing the smell was unknown to science.
Bear and Thomas went looking. They took rocks that had been sitting out in warm, dry conditions and steam-distilled them. What came out was a yellowish oil.
And they gave it a name. Petrichor.
Blood of the gods, from stone
The word is built from two Greek pieces. Petra means rock or stone. Ichor, in Greek mythology, is the ethereal fluid that flows in the veins of the gods, in place of blood.
So petrichor is, roughly, the essence of the gods, from stone. According to the CSIRO's own account, the authors described it as an "ichor" or "tenuous essence" derived from rock or stone.
They were also careful about what the name meant. They were careful to say the name didn't mean petrichor was a single fixed chemical. In the paper, the name belongs to the smell itself, a blend rather than a single chemical, although the oil behind it has come to share the name.
Where the oil comes from
The oil builds up in rocks, clay and soil during dry spells. Materials rich in silica, or in metallic silicates, turned out to be especially strong sources. Certain plants also give off oils during dry periods, which clay soils and rocks soak up. In a follow-up paper a year later, Bear and Thomas reported that the oil slows down seed germination and early plant growth.
Then the weather turns. As humidity rises before rain, tiny amounts of water fill the pores of the rocks and soil and flush the oil out. When the rain actually arrives, it speeds that release up and spreads the scent around.
That's the first layer of the story. But it isn't the only ingredient in the smell.
The bacteria's advert
A big part of that earthy note comes from a compound called geosmin. The name is from the Greek for earth and smell, which is about as literal as names get.
Geosmin is made by soil microbes, mainly bacteria called Streptomyces. It's the same compound that gives beetroot its earthy taste, and it's the reason water or wine can sometimes taste a bit muddy. Human noses are extraordinarily sensitive to it, picking it up at concentrations as low as about 5 parts per trillion.
For a long time, though, nobody knew why the bacteria bothered to make it. Virtually all streptomycetes carry the gene for it, which suggested it must be doing something useful.
In 2020, a team of researchers from Sweden, the UK and Hungary published an answer in Nature Microbiology. The smell, it seems, is an advert.
They worked with springtails: tiny, insect-like animals found on every continent, many of which live in soil and eat microbes. In the field, traps baited with Streptomyces caught springtails. Electrodes on the springtails' antennae showed that they could detect geosmin. And when the springtails were given a choice of two paths in a Y-shaped glass tube, they headed towards the earthy smell.
What's in it for the bacteria? Streptomyces make geosmin as part of producing spores. Those spores stick to the springtails' bodies, and spores that get eaten can survive the trip through the gut and come out in their droppings. Either way, the springtails carry the spores to places the bacteria couldn't reach on their own.
The springtails get dinner. The bacteria get a delivery service. And we get a nice smell after the rain.
Bursting bubbles
So the scent is sitting there in the soil. How does it get up into your nose?
In 2015, engineers at MIT went looking with high-speed cameras. They ran around 600 experiments, dropping single drops of water onto 28 different surfaces, from engineered materials to soil samples collected around the MIT campus.
What the cameras showed was something nobody had spotted before. When a raindrop lands on a porous surface, it flattens and traps tiny air bubbles where it touches the ground. Those bubbles rise up through the drop and burst out of the top, flinging a fine mist of tiny droplets, called aerosols, into the air. The researchers compared it to the bubbles in a glass of champagne.
Here's the counter-intuitive bit. Light and moderate rain produced more of these aerosols than heavy rain. In a downpour, the drops hit so fast that there isn't enough time for the bubbles to form.
The researchers suggested this could be how the smell of rain gets carried into the air. In their dye experiments, the aerosols carried dye from the surface, which suggests they may carry other material too.
Not all one smell
It's worth saying that the "smell of rain" is really a blend. Petrichor is part of it. Geosmin is part of it.
Strictly speaking, though, these are different things. Petrichor, in Bear and Thomas's sense, is the odour that comes from the oily essence collecting in rocks and soil. Geosmin is a separate compound made by bacteria. They're related in that they both show up after rain, but they aren't the same ingredient, and it's easy to blur them together.
What the rain is saying
So the next time that smell rises off a hot footpath, it's worth knowing what's really going on.
Part of it is an oil that's been quietly building up in the rocks and soil during the dry weather. Part of it is bacteria, sending out an advert to the tiny animals that help them travel. And some of it may be flung up into the air by raindrops, fizzing like champagne.
And two scientists in Melbourne named it, very fittingly, after the blood of the gods.
Sources
I. J. Bear and R. G. Thomas, "Nature of Argillaceous Odour", Nature 201 (7 March 1964), pp. 993–995. https://www.nature.com/articles/201993a0 (landing page checked by Reed; full text paywalled; quotes via CSIRO and Wikipedia)
CSIRO, "The smell of rain: how our scientists invented a new word" (31 March 2015). https://www.csiro.au/en/news/all/articles/2015/march/the-smell-of-rain-how-our-scientists-invented-a-new-word (accessed 1 Nov 2026)
MIT News, "Rainfall can release aerosols, study finds" (14 January 2015). https://news.mit.edu/2015/rainfall-can-release-aerosols-0114 (accessed 1 Nov 2026)
P. G. Becher et al., "Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal", Nature Microbiology 5 (6 April 2020), pp. 821–829. https://www.nature.com/articles/s41564-020-0697-x (abstract checked by Reed, 1 Nov 2026)
Lund University, news article on the springtail and geosmin study (15 April 2020). https://www.lunduniversity.lu.se/article/story-behind-earthy-smell-spring (accessed 1 Nov 2026)
The Conversation, "Here's why soil smells so good after it rains" (April 2020). https://theconversation.com/heres-why-soil-smells-so-good-after-it-rains-135978 (accessed 1 Nov 2026)
I. J. Bear and R. G. Thomas, "Petrichor and Plant Growth", Nature 207 (1965), pp. 1415–1416 (citation supplied by Reed)
Wikipedia, "Petrichor" (cross-check only). https://en.wikipedia.org/wiki/Petrichor (accessed 1 Nov 2026)
