The smell can come from lightning and bacteria.

Can you smell that it's going to rain?

ASK A RESEARCHER: “We can smell different things both before, during, and after it rains,” researcher Anja Røyne says.

Are you one of those who swear they can smell when the rain is coming?

It makes sense, according to research.

But what causes it?

Anja Røyne is a senior lecturer at the Department of Physics at the University of Oslo.

The smell of ozone

“We can smell different things both before, during, and after it rains,” researcher Anja Røyne says.

And there are interesting reasons for that.

“Before it rains, we humans can sense the smell of ozone. Especially if there are thunderstorms and lightning first,” she says.

When you hear ozone, you might think of the ozone layer far above the Earth.

It works a bit like the Earth's sunscreen and protects us humans from strong rays from the sun.

But there is ozone in the air we live in as well. And when lightning strikes, there's more of it.

“Lightning is so powerful that atoms in the air are torn apart and swirl around. When they collide again, ozone can form,” she says.

And that’s what we can smell.

“Some say it can remind you of a swimming pool or cleaning products,” Røyne says.

We humans have varying sense of smell. It varies how good people are at picking up the different scents of rain.

The smell after rain

The smell after rain comes from bacteria in the ground.

They produce a scent called geosmin. They do this all the time, but when it rains, they spread into the air.

“You might wonder why the bacteria in the soil bother to produce so much of that substance,” she says.

Researchers have studied this.

“It turns out that geosmin helps the bacteria attract other animals. Especially springtails,” she says.

Springtails are small creatures that crawl in the soil.

When springtails eat the bacteria, they become part of the journey.

And then the bacteria are spread out.

The ozone layer functions as Earth's sunscreen. Lightning strikes before rain can make us smell ozone.

Summer rain

The scent of geosmin is initially hidden in the soil.

To understand how the smell spreads, researchers have checked what actually happens when a raindrop hits the ground.

“They used high-speed cameras,” Røyne says.

When a raindrop hits dry soil, a tiny bit of air remains between the raindrop and the ground.

 “Air with geosmin gets trapped beneath the raindrop as it splashes down,” she says.

As the drop continues into the ground, the air bubble rises to the top of the droplet.

“And when the droplet bursts, it's like a mini explosion,” she says.

Lots of tiny droplets shoot up into the air.

“The geosmin from the ground joins these tiny droplets in the air, and when we breathe and they enter our noses, we smell it,” the researcher says.


Translated by Alette Bjordal Gjellesvik.

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