Lots of people love the smell of rain. It’s an aroma with freshness, cleanness, and wetness. You’ve probably noticed a distinctive smell if you’ve ever taken a stroll outside after a storm. The smell of rain has its name. It’s called petrichor. But rain is only water, and water doesn’t have any smell. So what makes rain smell so nice? There is more than one kind of rain smell, and many factors can cause them.
What causes the smell after rain?
Petrichor is the smell of dust after rain. It’s a combination of plant oils, bacteria, and ozone. But this doesn’t tell the whole story. A recent study from researchers at MIT suggests an aerosol effect is the main cause of the smell of dust after a light rain.
Petrichor was coined by two scientists, Isabel Joy Bear and R. G. Thomas of Australia’s Commonwealth Scientific and Industrial Research Organization, in 1964. The word uses Greek terms, Petra-meaning stone and ichor, the special substance that flows in the veins of gods. The smell after rain is caused by three factors: Ozone, Bacteria, and Soil.
For ozone
One smell comes before the rain even hits the ground. It’s slightly sweet, sharp, and according to some people, reminiscent of chlorine. That’s ozone. Ozone’s a molecule of oxygen, but it has a distinctive aroma. It comes from the Greek word “Osin” to smell. Ozone is produced when lightning ionizers a molecule of oxygen in the air separating the atoms. When they recombine, some form groups of three, creating ozone, a sharp, bracing scent.
During a storm, lightning splits oxygen and nitrogen molecules. These can recombine into nitric oxide, reacting with other atmospheric chemicals to form ozone. The wind from an approaching storm can carry this ozone scent from the clouds straight to your nose. But that’s not the only aroma in this mix.
For bacteria
One of the more pleasant rain smells caused by bacteria. A type of bacteria called actinomycetes grows in soil when conditions are wet. When the soil dries, this bacteria produces spores. The wetness and force of rain kick these tiny spores into the air.
Patrica is the compound that gives the soil the smell. It contains a chemical released by soil bacteria called Gozman or Earth Smell. Gozman is intense alcohol. Certain soil bacteria release Gozman when they die or are dormant, especially in hot, dry conditions. When the rain finally comes along, it disperses the chemical into the air as a fine mist.
So a lot of what you detect when you smell rain are compounds that living things have produced to get through dry spells. It would explain why the smell is powerful if it hasn’t rained in a while. So we breathe them in and sense that distinctive, earthy scent so often associated with everything from a drizzle to a storm. It is because the spores are released when the soil dries out. The scent is strongest after rain that breaks a dry spell. This bacteria is extremely common, and people worldwide will recognize the aroma.
For soil chemical
The smell mostly comes from plants, especially during long, dry periods. Some plants release rich and fatty acids, some food ingredients like a Politiken. Starick acids plants release these acids when water is scarce because they block other seeds in the ground from sprouting, which reduces competition for water.
Over time, these oils build upon soil and rocks. When rain falls, it picks them up into the air and causes them to release fragrant, volatile compounds that, to us, smell like fresh vegetables and are altogether pleasant. But that’s only one component of rain smell.
Why does rain smell so good?
The mechanism behind Petrichor was a bit of a mystery until MIT scientists captured slow-motion images of falling water drops. The videos revealed that raindrops trap air bubbles. When they hit the ground, the bubbles capture small molecules from the soil. These molecules bubble up and release aerosols, not unlike the bubbles in a glass of champagne. Capturing this common occurrence took a little while.
The scientists performed over 600 experiments dropping water on 28 different surfaces: 12 artificial surfaces and 16 soil samples. They varied the height to recreate the different types of rainfall, light, heavy, and everything in between. The higher the drop, the faster it fell. They found a Goldilocks effect. Most aerosols were produced when the porous surface wasn’t too wet or too dry and when the rain wasn’t too fast or too slow.
If the rain falls too fast, there’s not enough time for the bubbles to form. So that’s how the smell gets in the air, but what is the smell made up of? Previous studies suggest that the smell comes from oil given off by plants during dry periods that fall on top of clay and dirt. Speaking of dirt, another part of petrichor comes from bacteria from the genus actinomycetes. The same bacteria are responsible for recent break-in antibiotics. When these bacteria die, they release an organic compound called Geosmin, which comes from the greek “earth smell.”
Geosmin creates an unpalatable taste in water and any fish in it, imparting an earthy flavor to beets. The smell is nicer, and it’s an important part of petrichor. Our noses can detect geosmin at concentrations less than ten parts per trillion. That’s around a teaspoonful in two hundred Olympic size swimming pools.
Human noses are extremely sensitive to the compound. It is 5 parts per trillion. It’s a harmless chemical, while we love it outside. It can reduce the quality of drinking water and wine. The smell of storms comes from the ozone. The word also has its roots in greek for “smell.”
How might the smell reach your nose? Raindrops fall on a porous surface. They contain water from the raindrops and chemicals from the soil. They’re forming an aerosol of tiny droplets suspended in the air, much smaller than the raindrops. It is because the aerosol drops are so tiny.
They can travel on the wind much more quickly than raindrops and carry the smell of petrichor to our noses. Heavy rain doesn’t create those bubbles. So a heavy downpour may not smell like much instead of carrying the poetic aroma of petrichor.
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Sources:
Phipson, T.L., Cause of the Odour Emitted by the Soil of a Garden after a Summer Shower”, The Chemical News, Vol.63, No.1638.
Phipson, T.L., “The Odor of the Soil after a Shower.” Scientific American. 64 (20): 308. JSTOR 26100386.
Specific Odour of Soil”, The Chemical News, Vol.63, No.1637.
“Comptes rendus hebdomadaires des séances de l’Académie des sciences”. 1891/01 (Tome 112): 598–599.
Logan, Tim. “Why You Can Smell Rain.” The Conversation.
