June 7, 2015 Petrichor
Petrichor (/ˈpɛtrɨkɔər/) is the earthy scent produced when rain falls on dry soil. (…) the smell derives from an oil exuded by certain plants during dry periods, whereupon it is absorbed by clay-based soils and rocks. During rain, the oil is released into the air along with another compound, geosmin, a metabolic by-product of certain Actinobacteria, which is emitted by wet soil, producing the distinctive scent.