As it turns out, the smells people associate with rainstorms can be caused by a number of things, including raindrops themselves. (More on that later.) One of the more pleasant rain smells — the one we often notice in the woods — is actually caused by bacteria. Actinomycetes, a type of filamentous bacteria, grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produce spores in the soil.
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. These spores have a distinctive, earthy smell we often associate with rainfall.
The "rain smell" is caused by a chemical in the bacteria called geosin, which is released by the bacteria as they die. Geosin is a type of alcohol molecule with a very strong scent. The bacteria are 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 thrive in moist soil but release the spores once the soil dries out, the smell is most acute after a rain that follows a dry spell. That said, you'll notice it to some degree after most rainstorms.
Breakthrough Raindrop Study
In a study conducted in 2015, researchers at the Massachusetts Institute of Technology (MIT) uncovered a mechanism that may explain how raindrops release aerosols and potentially carry aromatic elements, bacteria and viruses from the soil into the environment. Using high-speed cameras, the scientists observed that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact, much like bubbles in champagne.
These bubbles then shoot upward and burst from the raindrop, creating a cloud of aerosols. The team was able to predict the amount of aerosols released based on the raindrop's velocity and the permeability of the surface. Researchers believe that in natural environments, these aerosols may transport aromatic compounds, as well as microorganisms and chemicals from the soil. Light or moderate rainfall may trigger this aerosol release, which can then be dispersed by wind.
This discovery has potential implications for understanding how soil-based diseases spread and how various compounds are distributed in the environment.