"Every storm blows in on a scent, or leaves one behind. The metallic zing that can fill the air before a summer thunderstorm is from ozone, a molecule formed from the interaction of electrical discharges—in this case from lightning—with oxygen molecules. Likewise, the familiar, musty odor that rises from streets and storm ponds during a deluge comes from a compound called geosmin. A byproduct of bacteria, geosmin is what gives beets their earthy flavor. Rain also picks up odors from the molecules it meets. So its essence can come off as differently as all the flowers on all the continents—rose-obvious, barely there like a carnation, fleeting as a whiff of orange blossom as your car speeds past the grove. It depends on the type of storm, the part of the world where it falls, and the subjective memory of the nose behind the sniff."
"City rain smells of steaming asphalt, in contrast to the grassy sweetness of rain in the countryside. Ocean rain smells briny like Maine clam flats on a falling tide. In the desert of the southwestern United States, rare storms punch the atmosphere with creosote and sage. In the southeast, frequent squalls leave the damp freshness of a wet pine forest. “Clean but funky,” Thomas Wolfe called the exquisite scent of the American South." Source: theatlantic.com