Have you ever stopped to think about that fresh, Earthy smell that permeates the air after heavy rainfall? It's rich and inviting, and we always notice it when we walk outside in the moments after the rain has stopped, but that smell doesn't just come from moisture. Someone--more specifically, a pair of someones, Australian scientists Isabel Joy Bear and R. G. Thomas--wondered why rain sometimes produces such a powerful smell, and set out to research the answer. That was decades ago, but Surprising Science's blog on their research is still illuminating today. The smell we so strongly associate with rain actually comes from oils secreted by plants; Thomas and Bear coined the term petrichor to give a name to this phenomenon.
The smell of rain comes from a number of chemicals plants produce during dry periods. Those oils are secreted and absorbed into soil, and when it rains, they're activated by the water and released into the air. After a long dry period, the smell is understandably stronger--there are more oils covering rocks and soil, and the buildup produces a more intense reaction. Surprising Science writes "The duo also observed that the oils inhibit seed germination, and speculated that plants produce them to limit competition for scarce water supplies during dry times."
These chemicals aren't alone in producing odors we experience after a rainstorm. Another, called "Geosmin," is described by Wikipedia as having "a distinct earthy flavor and aroma, and is responsible for the earthy taste of beets and a contributor to the strong scent (petrichor) that occurs in the air when rain falls after a dry spell of weather or when soil is disturbed."
The scientists discovered that geosmin is produced by bacteria in soil and is especially common in heavily forested areas. The bacteria release spores which are released up into the air when it rains, which is when we smell the geosmin. And apparently the human nose is extremely sensitive to geosmin--we can detect it in quantities as small as 5 parts per trillion.
Check out the rest of Surprising Science's story, which includes the smell of ozone produced by lighting, and a more nebulous psychological question--why do we find the smell of rain universally pleasing? According to at least one anthropologist, the answer may be human evolution.