What Causes the Smell of Rain

By ANDY BEHLEN

The Fayette County Record

Who didn’t enjoy the smell of the rain that fell across much of Fayette County this week? Did you know that smell has a name? Scientists call it “petrichlor.” And it doesn’t really come from the rain, at least not directly. The smell comes from bacteria in the soil called “actinomycetes” or actinobacteria. These bacteria are just one of the many microorganisms that break down organic matter into simple chemical compounds that plants use for food. As rain falls and filters through the soil, these actinobacteria release several fragrant compounds that make up the petrichlor smell. One of these compounds is called “geosmin.” Geosmin is actually a type of alcohol, which helps explain why our noses are so good at detecting it. Most people can smell alcohol from quite a long distance. The human nose can detect geosmin in concentrations of just 0.4 parts per billion. Some scientists think that camels find water in the desert by smelling for geosmin. Actinobacteria slow down their decomposition rate when it hasn’t rained for a prolonged period of time. Just before it rains, the air becomes humid and kicks the actinobacteria into action. They speed up their decomposition process and begin releasing geosmin and the other compounds that make up the petrichlor smell.

Drops of rain fall onto the soil. They splatter and mix with soil particles, ejecting the petrichlor compounds into the air. The scent travels downwinds and that’s why you can sometimes smell the rain even before it starts raining.

Healthy soils rich in microorganisms release more of the petrichlor smell when it rains. The smell may not be detectable when it rains on poor soils.

Geosmin possesses another curious property. Experiments have shown that geosmin actually inhibits seed germination. This fact seems counterintuitive, since seeds require moisture for germination. Researchers theorize that plants may have exploited this property as a way to ensure that their seeds don’t germinate until the soil is moist enough to sustain the young plant after germination.

Soils that have undergone prolonged dry periods seem to release more geosmin during a rain than soils that receive more periodic rainfall.

While almost everyone enjoys the smell caused by geosmin, most people don’t like the taste. Geosmin gives water a muddy taste, and public water utilities go through much effort to keep it out of water supplies.

Winemakers also try to keep it out of their products, as it gives wine an unpleasant off-taste.