A lot of people detect a unique fragrance in the air after it rains. But what is the source of this smell? It is not actually the rain itself that you are smelling, but rather dirt. The pungent, musky aroma that lingers in the air comes from a scented chemical found in the soil called “geosmin” (which literally translates to “earth smell”).

When the soil becomes dry, a type of filamentous bacteria called Actinomycetes produces geosmin spores. The wetness and force of the rain drives these tiny spores into the air, where the moisture acts like an aerosol (similar to an aerosol air freshener). The moist air easily spreads the spores to us, so we inhale them.

This bacteria is extremely widespread and can be found in many areas around the globe, which explains why this “after-the-rain” smell is so universal. Since the bacteria thrives in moist soil, but releases the spores once the soil dries out, the smell is most prominent after a rain that follows a dry period.

Another type of smell is caused by the acidity of rain. Due to chemicals in the atmosphere, rainwater tends to be somewhat acidic, especially in urban areas. When it comes into contact with organic debris or chemicals on the ground, it can lead to some particularly aromatic reactions. It breaks down soil and releases minerals that were trapped inside, and it reacts with chemicals like gasoline, giving them a stronger smell. These reactions often produce an unpleasant smell, unlike the bacteria spores, which are typically perceived as a pleasant, fresh smell.

Yet another after-the-rain smell comes from volatile oils that plants and trees release. The oil then accumulates 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 similar to the bacteria spores in that most people find it to be a pleasant, fresh smell. It has even been bottled and sold for its aromatic properties!

Now, aren’t you happy to know all of this? 🙂