The Chemistry Behind the Smell of Rain

The fresh, earthy aroma that permeates the air after a much-needed summer rain has a name: It's called petrichor, and a surprising amount of research has been done into the origins of the scent since it was first identified more than five decades ago.

According to the PBS video series Reactions, petrichor was first defined in the 1960s, when Australian scientists Isabel Bear and Richard Thomas discovered they could extract a yellow oil from dry rocks, clay, and soil. This substance, which mainly consists of fatty acids from plants, is the basis of petrichor.

Plants produce petrichor during dry spells, and the oil is then absorbed by rocks and soil. On their own, the fatty acids don't smell like much of anything, but as they break down in soil, they leave behind compounds that are much more pungent. When it rains, these smelly compounds are released into the air along with geosmin, the chemical compound that creates the smell of earth.

The reason plants release petrichor during dry spells is still a mystery to scientists. Bear and Thomas suspected that it could help plants grow, but after feeding the oil to plants, they found it actually caused them to wither. An alternative theory is that plants emit petrichor when water is scarce in order to hurt competing plant life.

Get the full story behind the familiar smell in the video below.