You know that sharp, light, floral odor of the first rain? It has a name: petrichor. Actually petrichor is an exudation of certain plants during dry spells. This chemical leaches into rocks and soil, which then give it off, along with a compound known as geosmin [literally, “earth smell,” that full, brown, slightly crunchy, moist smell of, well, earthy things, like beets]. Petrichor + geosmin = “smells like rain.”
Incidentally, Petrichor and Geosmin are perfectly conceivable, under the current ridiculous drug-naming schemata, as names for medication. Petrichor is for drooling idiots with rocks in their heads…it comes in little brown pills. Geosmin, which comes in swirly blue and green tablets, aims to counteract LGS [Loss of Gravity Syndrome], when excess air in the cranial cavity causes a person to float away. Neither of them are really effective, but they sure sound imposing.