1. Some of the Smell Arrives Before the Rain, and Some Comes From the Ground
"The smell of rain" is really at least two smells. The first is the sharp, fresh, slightly electric one that shows up as a storm approaches. A thunderstorm's downdrafts, the columns of air that rush downward, carry ozone from higher in the atmosphere to nose level, and lightning can also make ozone by splitting and recombining the oxygen and nitrogen in air[1]. Ozone is a form of oxygen with three atoms in each molecule instead of two.
The second smell is the soft, earthy one you notice when rain lands on ground that has been dry for a while. That is the one with its own name, and the rest of this article follows it.
2. A Yellowish Oil Trapped in Dry Rock and Clay Gave the Smell a Name in 1964
Until 1964, the earthy smell of rain on dry ground had no scientific name, even though people had noticed it for a very long time. Two Australian scientists, Isabel (Joy) Bear and Richard Thomas, worked at CSIRO, Australia's national science agency. They heated rocks that had sat in warm, dry weather and passed steam through them. Out came a yellowish oil that had been trapped in the rock and was released by moisture[2].
They published the result in the journal Nature on 7 March 1964, in a paper called "Nature of Argillaceous Odour." (Argillaceous means clay-like.) To name the smell, they made up a word from two Greek ones: petra, "stone," and ichor, the fluid that Greek myths said flowed in the veins of the gods. The word is petrichor[2]. In the CSIRO account, the smell comes about when rising humidity ahead of rain fills the tiny pores in stone and lets the oil escape into the air[2].
Where does the oil come from? Descriptions of the study say that plants release it during the dry season and that it seeps into rocks and clay-based soil[3]. Dry ground is, in effect, a storeroom, and the first rain opens the door.
3. Soil Bacteria Make Another Part of the Smell, and Springtails Seem to Follow It
Petrichor is a mixture, and one ingredient comes from living things in the soil. Geosmin, whose name means "earth smell," is made by Streptomyces bacteria. A 2020 study reported that nearly all Streptomyces species carry the genes to make geosmin, and that people and various animals can sense it at extremely low concentrations[4]. (If you have ever tasted a beet and thought "dirt," you have met it before.)
Why would a bacterium make a chemical that smells like soil? The same study offers an answer with a twist. The bacteria switch on their geosmin genes when they make spores, the tough seed-like cells they use to spread. Springtails, tiny soil animals, were attracted to the smell in field experiments, ate the colonies, and carried the spores away on their bodies and in their droppings[4]. So the earthy smell may work as a "come and eat me" signal that helps the bacteria travel. The study was about springtails; it does not show why humans can smell geosmin so well.
4. Bubbles Inside Each Raindrop Fling the Smell Into the Air
Oil in rock and chemicals in soil still have to get from the ground to your nose. In 2015, Cullen Buie and Youngsoo Joung at MIT filmed drops hitting different surfaces with high-speed cameras. They ran roughly 600 experiments on 28 surfaces, 12 engineered materials and 16 soil samples. On porous surfaces, the drop traps tiny pockets of air. As the MIT team put it, the bubbles then shoot upward "as in a glass of champagne" and burst out of the drop in a fizz of aerosols, hundreds of tiny droplets released within a few microseconds[5]. An aerosol here simply means a droplet small enough to hang in the air.
The team also found that light and moderate rain make more aerosols than heavy rain, because a fast, hard impact leaves too little time for bubbles to form[5]. A follow-up study on soil bacteria in 2017 found the best transfer at impact speeds of about 1.4 to 1.7 meters per second (3 to 4 miles per hour) and less at higher speeds. It also found that a single drop can lift living soil bacteria into the air[6]. Buie has said the bursting bubbles made him think of the smell of a quick rain shower[5]. Those experiments show how a drop launches material from the soil; they were not a direct test of the smell itself, so treat "gentle rain smells strongest" as a good guess, not a proven rule.
5. Perfume Makers in India Were Bottling the Smell Long Before Scientists Explained It
CSIRO's own account points out that a small perfume industry in India had already captured this scent in sandalwood oil, calling it matti ka attar, "earth perfume"[2]. The center of that craft is Kannauj, a town on the Ganges in Uttar Pradesh, often called India's perfume capital[7].
The traditional method, called deg bhapka, starts with half-baked clay shards from nearby villages. They go into a copper pot called a deg, with water, and the lid is sealed with mud. A fire burns underneath. The vapor travels through a bamboo pipe into a receiver, the bhapka, which holds sandalwood oil, and the oil soaks up the earthy scent[7]. Craftspeople have been making perfume in the town for centuries[7].
Back to the porch. The smell of rain is a team effort: ozone from the sky, plant oil stored in rock and clay, geosmin from bacteria, and a bubble-powered launch from each drop. Long before anyone drew up that list, people in Kannauj were already catching it in a bottle.
How to Test Rain Smell Yourself at Home
- Compare a sprinkle with a pour. Find a patch of dry soil, such as a flower pot or garden bed that has not been watered for a few days. Use a watering can with a sprinkler head, or a cup with small holes poked in the bottom, to sprinkle slowly from low down. Smell it. Then, on a different dry patch, tip the water out fast from higher up. Do the two smells differ? The research above suggests gentle drops may work better, but your nose is the judge. Wash your hands afterward.
- Smell the storm from indoors. The next time a storm is coming, open a window a little and notice whether the sharp smell shows up before the first drop. If you hear thunder, stay inside and keep watching from the window.
- Read the original. The CSIRO article on how its scientists invented the word "petrichor" is short and tells the story of the 1964 steam experiment.