The smell of rain was named petrichor in 1964 by Australian researchers Isabel Joy Bear and Richard Thomas. It comes from multiple sources working together. Soil bacteria called actinomycetes produce a compound called geosmin, which humans can detect at extraordinarily low concentrations. Plants release oils during dry periods that accumulate on soil and surfaces. When raindrops hit dry ground they trap tiny air bubbles that burst and launch aerosols into the air, carrying geosmin and other compounds upward.
Daniel and Mom trace the name, the compounds, and the remarkable sensitivity of the human nose to geosmin. The episode explores why humans might be particularly attuned to the smell of rain and closes on a detail about geosmin detection that reframes the human sense of smell entirely.
DanielMom, I can smell the rain coming through the window. But, rain is just water, right? Water doesn't have a smell. So, what am I actually smelling?
MomDaniel, you are smelling at least three completely different things at once. And, none of them are the rain itself.
DanielReally? None of them?
MomYou know, water is actually odorless. What you're smelling is the earth, the plants, and, if there is lightning, the sky itself.
DanielOkay, what's the main one, the smell I always notice?
MomThat earthy, almost sweet smell when rain hits dry ground? That has a name. It is called petrichor. The word was coined in 1964 by two Australian scientists, Isabel Joy Bair and Richard Thomas, who published a paper in the journal Nature. They named it from two Greek words.
DanielNow, that's interesting. What were the two words?
MomPetra, meaning stone, and ichor, the fluid that flows in the veins of the gods in Greek mythology.
DanielSo, the smell of rain on stone has a name that means the blood of the gods?
MomYes, and it is one of the most beautiful word origins in science. And petrichor actually comes from two things mixing together. First, oils that plants release during dry periods. Those oils soak into rocks and soil. When rain hits, the impact releases them into the air.
DanielSo the ground has been storing plant oils, and rain unlocks them.
MomAnd the second thing is a compound produced by bacteria living in the soil. Certain bacteria called actinomycetes break down dead plant matter in the ground. As they work, they release a chemical called geosmin.
DanielGeo, earth. Osmin, smell?
MomExactly right. Earth smell. And here is the remarkable part. Humans can detect geosmin at concentrations as low as 5 parts per trillion.
DanielHow small is that?
MomImagine a teaspoon of geosmin dissolved into 200 Olympic swimming pools. You would still be able to smell it.
DanielThat is insane! Why are we so sensitive to something in dirt?
MomScientists think it may go back to our ancient ancestors, following the smell of rain to find water. Geosmin may have been a signal, water is coming, or water is near, and our noses evolved to be extraordinarily tuned to it. In fact, humans are about 200,000 times more sensitive to geosmin than sharks are to blood.
DanielWow, that's just insane! We can smell dirt better than sharks smell blood?
MomWhen it comes to this particular compound, yes.
DanielOkay, and what about that sharp electric smell before a big storm? That is different from petrichor.
MomCompletely different. That is ozone. When lightning strikes, it is so powerful it splits oxygen molecules in the air. Some of those oxygen atoms then combine with ordinary oxygen molecules to make ozone. Three oxygen atoms join together. And the storm's downdrafts carry it down to ground level ahead of the rain.
MomSo you can smell the lightning before it arrives.
DanielSo you're actually smelling the electricity in the air?
MomThe aftermath of it, yes. That sharp, clean, almost metallic smell. That is ozone, reaching you before the rain does.
DanielSo how does the smell actually get to my nose? Like, how does geosmin get from the soil into the air?
MomThis is the part I love. When a raindrop hits a porous surface, soil, rough concrete, the impact creates tiny bubbles of air trapped beneath the drop. Those bubbles shoot upward through the water and burst at the surface, releasing microscopic droplets into the air called aerosols.
MomAnd those aerosols carry geosmin and the plant oils with them.
DanielSo every raindrop is basically a tiny catapult, launching smell molecules into the air.
MomA tiny catapult! That is exactly what it is! So, Daniel, what is one thing you are taking with you today?
DanielLet's see. The smell of rain is called petrichor, and it comes mainly from plant oils and a chemical from soil bacteria called geosmin. And humans can detect geosmin at five parts per trillion, which makes us better at smelling it than sharks are at smelling blood.
MomNext time it rains, you will know exactly what you are smelling.
DanielThe blood of the gods and 200 Olympic swimming pools!
MomYou know, science is so much stranger than people realize.
