What Is Responsible For That Distinct Post-Rainfall Smell?
When a dry spell is broken by rainfall there is a distinct smell in the air after the storm. That smell, a rich earthy smell that strongly resembles the scent of fresh beets plucked from the soil, is called petrichor–a term coined by researchers in the 1960s to describe the post-rainfall scent released by freshly dampened dry earth.
The petrichor itself is caused by a very specific organic compound: Geosmin. Geosmin is the by product of the metabolic processes of many classes of microbes, including cyanobacteria and actinbacteria. When these organisms increase their metabolic activity after a heavy rainfall, the side effect is the release of Geosmin.
Geosmin is also found in beets and in bottom-dwelling fish like carp and catfish (and is responsible for the earthy/”muddy” flavor of bottom-dwelling fresh water fish). Humans are particularly sensitive to the compound and can smell it in concentrations as low as five parts per trillion.