Did you know that the smell of rain on dry earth has a name? It is called petrichor

You have had days of sunshine, the path is dry and, suddenly, the first drops fall. Instantly an unmistakable smell fills the air: the smell of wet earth. That smell has a name: petrichor.

A name with a history

In 1964, Australian scientists Isabel Joy Bear and Richard Thomas published a study of this aroma in the journal Nature and named it by combining two Greek words: petra (stone) and ichor, the fluid that flowed in the veins of the gods in mythology. In other words: “the blood of the stones”.

Where the smell comes from

There are several ingredients:

1. Plant oils. In dry periods, some plants release oils that build up on rocks and soil.
2. Geosmin. This is the star of the smell. It is produced by soil bacteria (of the Streptomyces group) and is responsible for that “earthy” aroma. The human nose is extraordinarily sensitive to it: it detects it at levels of parts per trillion, like noticing a few drops in an Olympic swimming pool.
3. Ozone, sometimes, just before a storm: lightning splits oxygen molecules and the wind brings a fresh, sharp smell.

How the smell reaches you

In 2015, MIT researchers filmed raindrops in slow motion. When a drop hits porous soil, it traps small air bubbles that rise and burst, launching tiny aerosols loaded with scent (and microbes) into the air. That is why light rain on dry earth smells the most: heavy drops do not give those bubbles time to form.

Some scientists believe we are so sensitive to geosmin because, for our ancestors, smelling rain meant finding water.

A warning in the mountains

If on a summer hike you start smelling ozone or wet earth while the sky darkens, a storm may be very close. Learn to read the other signs in how to tell if a storm is coming.