Why does rain smell? The bubbles, the bacteria and the word petrichor

One drop, slowed down

What happens when a raindrop lands on dry soil?#

In a study published in 2015, two engineers at MIT, Young Soo Joung and Cullen Buie, filmed drops of water and a few other liquids landing on 16 soils and 12 other porous surfaces with a high-speed camera, in about 600 experiments.

A drop flattens faster than the soil can soak it up, and air from the pores is caught under the water as tiny bubbles. As the water seeps down it pushes more air up out of the pores, and the bubbles swell. When a bubble reaches the top of the drop it bursts and throws up a thin jet, which breaks into droplets far smaller than the drop.

On one of their test surfaces, hundreds of jets left a single drop at about 10 metres a second, all within about 20 millionths of a second.

The spray carried a dye from the pores of a test plate into the air. The engineers did not measure the smell itself; they suggest this is how it gets out of the soil.

Try it

Which rain, on which ground, throws up the spray?#

Choose the rain and the ground. The drop lands at the speed a real raindrop of that size reaches by the time it hits the ground.

The team saw the spray in light and moderate rain on soils that soak up water as slowly as clay and sandy clay. On sand, and in heavy rain, they saw none.

Light rain, in their paper, lands at under 4 metres a second, which means drops smaller than about 1 mm. Heavy rain lands at over 7 metres a second: drops larger than about 2.3 mm.

The first drops

Why does the smell come with the first drops?#

In one test Joung and Buie let drops fall on the same spot again and again. When they came 10 seconds apart, the spray stopped after the second drop. When they came 5 minutes apart, every drop threw one up. Once the pores are full of water, there is no air left to make bubbles.

Watch the ground darken where the rain has landed: the gold spray rises only from the patches that are still dry. It fits the old link between the smell and rain after dry weather.

What is in the spray

What exactly do you smell?#

The earthy note has a name: geosmin, from the Greek for “earth smell”. It was first isolated in 1965, from soil bacteria of the group called actinomycetes, and nearly every species studied of the best-known of them, Streptomyces, carries the gene for it.

Human noses are very sensitive to it. In a British test of 1996, people smelled it in water at about 3.8 billionths of a gram per litre: a single gram would be enough to make about 100 Olympic pools of water smell of earth. In a Japanese test in air, the threshold was 6.5 parts per trillion, about one molecule of geosmin among 150 billion molecules of air. Why human noses are so sensitive to it is not known.

Geosmin is not the only ingredient. In 1964 two Australian chemists, Isabel Joy Bear and Richard Thomas, left volcanic rock and weathered granite out in warm air, sheltered from the rain, then steam-distilled them and collected an oil that carried the smell. Their favoured explanation: over days and months of dry weather, rocks and clay soak up substances from the air, and rain sets them free.

Before a thunderstorm there is often another smell, sweet and slightly acidic: ozone, made by lightning and carried down by the gusts that run ahead of the rain. In clean air, people can detect it from about 7 parts per billion.

Who the smell is for

Why do soil bacteria make geosmin?#

In 2020 a team mostly from Sweden and Britain, led by Paul Becher and Klas Flärdh, found who it calls: springtails, tiny soil animals that eat the bacteria. The springtails’ antennae respond to geosmin, and they walk towards it; a single nanogram was enough to draw them in a test where they could choose between two paths.

Streptomyces make geosmin when they are making spores. The springtails eat the colony and carry the spores away, stuck to their skin and alive in their droppings.

Fruit flies read the same smell the other way round. They have a nerve circuit that responds only to geosmin, and it makes them stay away.

The word

Where does the word petrichor come from?#

Bear and Thomas gave the smell its name in the journal Nature in March 1964: petrichor, from the Greek petra, stone, and ichor, the fluid said to run in the veins of the gods.

In the same paper they wrote that the smell was already bottled. Near Kannauj, in northern India, perfumers left discs of baked clay out in the sun in May and June, before the monsoon, then steam-distilled them into sandalwood oil. The perfume is called matti ka attar, earth perfume.

A year later the two chemists found that small amounts of the oil slow down the sprouting of seeds. Romanian dictionaries do not list the word, not even spelled the Romanian way, petricor.