- What is that smell when it rains?
Most people notice a distinctive smell in the air after it rains. But what causes it? It’s not rain you’re smelling; it’s dirt. The piquant, musky odor that hangs in the air emanates from an odorous chemical buried in the soil called “geosmin” (literally, “earth smell”).
When the soil dries out, Actinomycetes, a type of filamentous bacteria, produces the geosmin spores. The wetness and force of rainfall kick these tiny spores up into the air where the moisture after a rain acts as an aerosol (just like an aerosol air freshener). The moist air easily carries the spores to us so we breathe them in.
The bacteria is extremely common and can be found in areas all over the world, which accounts for the universality of this sweet “after-the-rain” smell. Since the bacteria thrives in moist soil, but releases the spores once the soil dries out, the smell is most acute after a rain that follows a dry spell.
Another sort of smell is caused by the acidity of rain. Because of chemicals in the atmosphere, rainwater tends to be somewhat acidic, especially in urban environments. When it comes in contact with organic debris or chemicals on the ground, it can cause some particularly aromatic reactions. It breaks apart soil and releases minerals trapped inside, and it reacts with chemicals, such as gasoline, giving them a stronger smell. These reactions generally produce more unpleasant smell than bacteria spores, which is why this after-the-rain smell isn’t always a good one.
Another after-the-rain smell comes from volatile oils that plants and trees release. The oil then collects on surfaces such as rocks. The rain reacts with the oil on the rocks and carries it as a gas through the air. This scent is like the bacteria spores in that most people consider it a pleasant, fresh smell. It has even been bottled and sold for its aromatic qualities!
Now, aren’t you glad to know that!? 🙂