After a run of hot, dry days in a Georgia summer, the first fat raindrops on the driveway release a smell almost everyone recognizes. It is earthy, clean and a little sweet, and it seems to rise from the ground itself. For centuries people simply called it the smell of rain. Since the 1960s, it has had a proper name: petrichor.
A word invented in 1964
Two Australian scientists, Isabel Joy Bear and Richard Thomas, coined the term in a paper published in the journal Nature in 1964. They built it from two Greek words: petra, meaning stone, and ichor, the ethereal fluid said to flow in the veins of the gods in Greek mythology. The name suits a scent that seems to be drawn out of rock and soil.
Their research found that during dry periods, oils given off by plants collect on rocks and in soil. When rain finally arrives, those oils are released into the air along with other compounds, creating the familiar smell.
The earthy note: geosmin
A large part of the scent comes from a compound called geosmin, whose name means earth smell. It is produced by microbes that live in soil, including bacteria in the group known as Streptomyces. Human noses are remarkably sensitive to geosmin and can detect astonishingly small amounts of it, which helps explain why the smell of rain on dry ground can feel so strong and so sudden.
There is also a sharper, cleaner smell that sometimes arrives just before a thunderstorm. That one is often ozone, a form of oxygen created when lightning splits molecules in the air, which storm winds can carry ahead of the rain.
How the scent reaches us
In 2015, researchers at the Massachusetts Institute of Technology used high-speed cameras to watch raindrops land on porous surfaces such as soil. They found that a drop can trap tiny bubbles of air that rise and burst, flinging a fine mist of particles upward, a little like the fizz from a glass of soda. Those particles carry scent with them. Light and moderate rain released more of them than heavy downpours, which may be why a gentle first shower often smells strongest.
Bottling the smell of earth
In the town of Kannauj in northern India, long known for its traditional perfumes, artisans make a fragrance called mitti attar, or earth attar. Clay is baked and then distilled, and its scent is captured in sandalwood oil, creating a perfume that smells like the first monsoon rain on parched ground. It is a lovely example of people cherishing this smell enough to preserve it.
The next time a summer shower begins, step to an open door or a covered porch and take a slow breath. If there is thunder, enjoy the moment from inside, since lightning can strike well away from the storm itself. Notice the earthiness, the sweetness, the sudden cool. It is one of the oldest smells there is, and one of the few that seems to make everyone stop and breathe a little deeper.