The chemical responsible for petrichor (that wonderful “smell” of rain or impending snow) is called geosmin. It’s given off by Streptomyces bacteria in the soil when it gets wet. Humans can detect geosmin at a concentration of 5 parts per trillion. This means that humans are 200,000 times more sensitive to smelling geosmin than sharks are at smelling blood.
Sylvia Shults, author 