It's a wonderful feeling to encounter a scientific explanation for something one has experienced but never quite understood - in this case the distinctive smell that is discernible when rain starts falling. City dwellers may be unfamiliar with this, but anyone who has been caught out in a summer sprinkle knows that there is a characteristic odor in the air. I had always attributed it to the disturbance of dust; today I found the answer:
Geosmin, which literally translates to "earth smell", is an organic compound with a distinct earthy flavour and aroma, and is responsible for the earthy taste of beets and a contributor to the strong scent that occurs in the air when rain falls after a dry spell of weather (petrichor). The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion.Petrichor is derived from the Greek petros "stone" + ichor "gods' blood":
Geosmin is also responsible for the muddy smell in bottom-dwelling freshwater fish such as carp and catfish… Geosmin breaks down in acid conditions; hence vinegar and other acidic ingredients in fish recipes help reduce the muddy flavor.
[Petrichor] derives from an oil exuded by certain plants during dry periods, whereupon it is adsorbed by clay-based soils and rocks. During rain, the oil is released into the air along with another compound, geosmin, producing the distinctive scent… the oil retards seed germination and early plant growth.You learn something every day.
The scent is generally regarded as pleasant and refreshing… In desert regions, the smell is especially strong during the first rain after a long dry spell… The oil yielding the scent can be collected from rocks and concentrated to produce perfume; however, it has yet to be synthesized, perhaps due to its complexity. It is composed of more than fifty distinct chemical substances.
Found at a Reddit thread - wherein it is also learned that the smell of newmown grass comes from cis-3-Hexen-1-ol. I can't wait to use that in a casual conversation...