Why Does Rain Smell? The Surprising Science of Petrichor
TL;DR: Petrichor is the smell of rain falling on dry ground. The word was coined by two CSIRO scientists in 1964, from the Greek for stone and the fluid said to flow in the veins of the gods. The scent comes from plant oils and a soil compound called geosmin, which the human nose can detect at one hundred parts per trillion. MIT filmed how it escapes: each raindrop traps tiny air bubbles that burst out of it like champagne.
Last updated: 15 September 2026
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Key takeaways
- Petrichor is the name for the smell of rain on dry ground. It was coined by CSIRO scientists Isabel Joy Bear and Richard Thomas in the journal Nature on 7 March 1964.
- The scent is a blend of oils released by plants and chemicals made by soil bacteria, above all a compound called geosmin.
- The human nose can detect geosmin at one hundred parts per trillion. The John Innes Centre puts it plainly: we sense geosmin better than sharks detect blood.
- Light and moderate rain release far more scent than a downpour, because a drop needs time to trap and fire air bubbles.
- The bacteria may make the smell to attract springtails, tiny soil animals that spread their spores, a partnership thought to be around 450 million years old.
What is petrichor?
Petrichor is the earthy smell that rises when rain falls on dry ground. Two scientists at Australia’s CSIRO, Isabel Joy Bear and Richard Thomas, named it in a paper called Nature of Argillaceous Odour, published in the journal Nature on 7 March 1964. They built the word from the Greek petra, meaning stone, and ichor, the fluid said to flow in the veins of the gods.
So petrichor is, quite literally, the blood of the stone. The smell itself was not new. Bear and Thomas opened their paper by noting that dry clays and soils give off a peculiar, characteristic odour when breathed on or moistened, something the old mineralogy textbooks already recognised. What they did was trace it to an oil that soaks into dry rock and soil and is released again by water.
CSIRO’s own account of the discovery, the smell of rain: how our scientists invented a new word, notes that the odour was especially familiar in arid regions, where it was widely associated with the first rain after a dry spell.
Why does light rain smell stronger than a downpour?
Because a gentle raindrop has time to trap air, and a fast one does not. In 2015, engineers at the Massachusetts Institute of Technology filmed raindrops with high speed cameras and found that a drop landing on porous ground traps tiny air bubbles at the point of contact. The bubbles rise through the drop and burst out of its surface in a fizz of aerosols, just like a glass of champagne.
According to MIT’s report of the study, the team ran roughly 600 experiments on 28 surfaces, 12 engineered materials and 16 soils. They saw many more aerosols released during light and moderate rain, and far fewer in heavy rain. A heavy drop hits too fast for bubbles to form inside it. One drop can throw off hundreds of tiny droplets within a few microseconds, which is how the scent of petrichor gets into the air you breathe.
The ground matters too. The researchers found the effect strongest when the surface is neither too wet nor too dry, with sandy and clay soils producing plenty of aerosols. That fits what most people notice: the smell is strongest in the first gentle shower after a warm, dry spell.
What is the smell actually made of?
Mostly two things. The first is a mix of oils that plants release during dry weather, which collect in soil and on rock. The second is geosmin, a compound made by soil bacteria called Streptomyces. Geosmin is the earthy note in petrichor, the part that makes a garden smell like a garden after rain.
The human nose is astonishingly sensitive to it. The John Innes Centre says we can detect geosmin at one hundred parts per trillion, and that we sense it better than sharks can detect blood. At that level, a few molecules among a trillion is enough for the brain to say rain.
That sensitivity may be why so many people say they can smell rain coming. CSIRO describes petrichor as an oil released from the earth into the air before the rain begins to fall, and a nose tuned to parts per trillion picks it up long before a rain gauge does.
Why do soil bacteria make the smell at all?
To attract help. Research published in Nature Microbiology in 2020, by scientists in Sweden and the UK, found that geosmin and a second compound, 2-methylisoborneol, attract springtails, tiny six legged soil animals. The springtails feed on Streptomyces and carry its spores away, stuck to their bodies and in their droppings.
In other words, part of what we call petrichor is an advert. The bacteria broadcast a scent, the springtails follow it, and the bacteria get their spores spread through the soil. The John Innes Centre describes this chemical conversation as one that has lasted around 450 million years.
We are simply eavesdropping on it. Every time you step outside after a summer shower and breathe in, you are smelling a signal that was never meant for you.
What does the smell tell you about the ground?
That it has been dry. Petrichor is strongest when oils and geosmin have built up in dry soil and a light shower releases them all at once. After a long dry spell the ground is also slow to absorb water, which is why the first heavy storm at the end of a hot summer can run straight off the surface instead of soaking in.
Our explainer on flash flooding after drought covers that side of the story, and our piece on groundwater recharge explains why a drought does not end when it rains, but when the aquifer refills. The smell is the pleasant part of the same process.
If you enjoy the physics of rain, our water facts on the real shape of a raindrop and how much a cloud weighs are good places to go next.
Frequently asked questions
What is petrichor?
Petrichor is the smell of rain falling on dry ground. The word was coined in 1964 by CSIRO scientists Isabel Joy Bear and Richard Thomas, from the Greek petra, stone, and ichor, the fluid of the gods.
What causes the smell of rain?
Oils released by plants in dry weather and geosmin, a compound made by soil bacteria. Raindrops release them as tiny aerosols when they hit porous ground and trap air bubbles that burst out of the drop.
Why does rain smell stronger after a dry spell?
Because oils and geosmin build up in dry soil and are released together by the first light rain. Heavy rain releases fewer aerosols, because the drops land too fast for bubbles to form inside them.
How sensitive is the human nose to geosmin?
The John Innes Centre says humans can detect geosmin at one hundred parts per trillion, more sensitive than a shark’s ability to detect blood.
Can you smell rain before it arrives?
Many people say they can. CSIRO describes petrichor as an oil released from the earth into the air before the rain begins to fall, and the nose detects geosmin at extremely low concentrations.
One last thought
Petrichor is a reminder that water is never just water. It carries the chemistry of every surface it touches, and even a single raindrop can do something as elegant as a glass of champagne.
We watch water all day. It still surprises us. You can find the rest of the series, and the monitoring work behind it, across our water quality monitoring pages.