It doesn't have to be the first rain of the season to enjoy this aroma we all know and most love. Geosmin are little blue-green algae-like critters in the dirt which mingle with yellow plant oils. When it rains, these compounds combine to form petrichor, the spring-rain smell of earth.
The purpose is thought to retard seed growth during dry spells, but I know better.
It's for your enjoyment.