Why does it smell like rain?
Because raindrops launch tiny bubbles of scent from dry ground into the air you breathe. The earthy smell, called petrichor, is mostly oils from plants and a soil chemical called geosmin.
The first cool drops after a dry spell.
Scent carried into your nose by bursting droplets.
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What you're smelling
During dry spells, plants give off oils that collect on rocks and soil. Soil bacteria make a compound called geosmin, the same thing that gives beets their earthy taste, and human noses can detect it at astonishingly low levels. Two Australian scientists named the whole smell petrichor in 1964, from Greek words for stone and the fluid in the veins of the gods.
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How raindrops launch it
In 2015 MIT researchers filmed raindrops with high-speed cameras. When a drop lands on porous ground it traps small air bubbles, which rise and burst out of the top of the drop as a spray of even tinier droplets, carrying scent into the air. Light and moderate rain on sandy or clay soil threw up the most. That's why the smell is strongest at the start of a gentle rain.
The smell before the storm
Sometimes you can smell rain before a drop falls. Storm winds carry the scent of wet ground from where it's already raining. And the sharp, clean smell can be ozone: lightning splits oxygen and nitrogen molecules, and the ozone it makes can be carried ahead of the storm by its downdrafts.
Questions people ask
- Why does rain smell stronger after a long dry spell?
- Oils and geosmin build up on the ground while it's dry, so the first rain releases a bigger burst of scent.
- Can you smell a storm coming?
- Often, yes. Storm winds carry the smell of rain from where it's already falling, and lightning makes ozone, which smells sharp and clean.
- Why doesn't heavy rain smell as much?
- In high-speed footage, light and moderate drops launched the most scent. Heavy drops tend to hit too hard to trap and release the bubbles the same way.