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Petrichor is the scent of rain on dry earth. The word is constructed from Greek, petra, meaning stone + ichor, the fluid that flows in the veins of the gods in Greek mythology.
The term was coined in 1964 by two Australian researchers, Bear and Thomas, for an article in the journal Nature. Source
Actinomycetes, a type of filamentous bacteria, grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produces 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 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, although you'll notice it to some degree after most rainstorms. Source
Geosmin, which literally translates to "earth smell", is an organic compound with a distinct earthy flavour and aroma.
Geosmin is produced by several classes of microbes, including cyanobacteria (blue-green algae) and actinobacteria (especially Streptomyces), and released when these microbes die. Communities whose water supplies depend on surface water can periodically experience episodes of unpleasant-tasting water when a sharp drop in the population of these bacteria releases geosmin into the local water supply. Source
Originally posted by mileysubet now that you have broken it down to scientific facts...It is much less romantic.