Why rain smells nice

Ahrim said his Spanish latte would taste better if it were raining. I think a big part of that is the smell. And the smell of rain turns out to be a surprisingly well-studied thing.

It has a name

Petrichor. Two Australian scientists, Isabel Joy Bear and Richard Thomas, coined it in a 1964 paper in Nature. They built it from Greek: petra (stone) + ichor, the golden fluid said to flow in the veins of the gods. So the name literally means "the blood of the gods, from the stones." A very dramatic name for a very calm smell.

The main ingredient: geosmin

Most of the earthy note comes from geosmin ("earth smell"), a compound made by soil bacteria, especially Streptomyces. Our noses are absurdly sensitive to it. People can detect it at a few parts per trillion, which beats sharks smelling blood. Geosmin is also why beetroot tastes "earthy," and why some fish taste muddy.

Some people think we evolved to notice it so well because it means water is nearby. That's a nice story, but it's still a hypothesis.

Plant oils that wait for rain

During dry spells, some plants release oils that soak into rocks and soil. When rain finally comes, those oils get released along with the geosmin. That's why the first rain after a long hot stretch smells the strongest.

The delivery system: tiny bubbles

This part is my favourite. In 2015, researchers at MIT filmed raindrops with high-speed cameras. When a drop lands on porous ground like soil or dry sand, it traps tiny air bubbles. The bubbles rise and burst out of the drop like a glass of soda, flinging out a fine mist of aerosols that carry the smell (and even bacteria) into the air.

The effect is strongest with light to moderate rain. Heavy rain hits too hard and fast for the bubbles to form well. So gerimis really does smell nicer than a downpour.

And the smell before the storm?

That's a different smell: a sharp, clean one, from ozone. Lightning splits oxygen and nitrogen molecules, some of them recombine into ozone, and the wind from the storm carries it ahead of the rain. If you smell it, the storm is close.

So: ozone says it's coming, geosmin and plant oils say it's here, and tiny bursting bubbles bring the smell to your window.