What causes the smell after rain?
Quick answer
The earthy smell after rain is called petrichor. It comes mainly from geosmin, a chemical made by soil bacteria (Streptomyces), along with oils that plants release during dry spells. When raindrops hit dry ground, they trap these chemicals in tiny bubbles that burst and fling a fragrant aerosol into the air.
Where the word comes from
Australian scientists Isabel Joy Bear and Richard Thomas coined petrichor in a 1964 paper in Nature. They combined Greek petra (“stone”) and ichor, the golden fluid said to flow in the veins of the gods.
The three smells of a storm
| Smell | Source | When you notice it |
|---|---|---|
| Ozone (sharp, clean) | Lightning splits oxygen and nitrogen molecules | Just before a storm, carried by wind |
| Petrichor (earthy) | Plant oils and geosmin from soil | When rain first falls on dry ground |
| Geosmin (beetroot-like) | Streptomyces bacteria | Strongest after a dry period |
How raindrops release the scent
In 2015, researchers at MIT filmed raindrops hitting soil with high-speed cameras. They found that light and moderate rain traps air in the surface, forming tiny bubbles that rise and burst like fizz in a glass of soda. This shoots out aerosols carrying scent, and possibly bacteria, into the air. Heavy downpours release fewer aerosols, which is why a gentle shower often smells strongest.
Our noses are superb geosmin detectors
Humans can smell geosmin at extremely low levels, around a few parts per trillion, more sensitive than a shark detecting blood. One theory is that our ancestors benefited from sensing rain and water nearby.
Geosmin also gives beetroot its earthy taste, and it can make tap water or freshwater fish taste “muddy.”
Perfume from rain
In Kannauj, India, perfumers make mitti attar (“earth perfume”) by baking clay and distilling its scent into sandalwood oil, capturing the smell of the first monsoon rain.
For more on weather, see what cloud causes thunderstorms.
Still curious?
Jump to a random question from the archive.