(Intro)

Lots of people love the smell of rain. It's an aroma we associate with freshness and cleanness and, uh, wetness. But water doesn't really smell like anything. So what exactly is that wet aroma?

Well, it turns out that even though rain often smells clean to us, much of its scent actually comes from dirt and rocks.

The smell of rain has its own scientific name: petrichor, from the Greek word for rock, petra, and ichor, the blood-like substance that was said to flow through the veins of the gods.

But the source of the smell isn't rocks themselves. It mostly comes from plants. Especially during long, dry periods, some plants release oils that are rich in fatty acids, some of which you might recognize as food ingredients, like palmitic and stearic acids. Plants release these acids when water is scarce, because they block other seeds in the ground from sprouting, which reduces competition for water.

And, over time, these oils build up on soil and rocks. And when rain falls, it kicks them up in the air and causes them to release fragrant volatile compounds, that, to us, smell fresh, vegetal and altogether pleasant.

But that's only one component of rain smell. Petrichor also contains a chemical that's released by soil bacteria called geosmin, or "earth smell," and it's the compound that gives soil the smell we think of as earthy. 

Geosmin is actually a pungent kind of alcohol. It's what gives some vegetables and wines their loamy, dirty flavor. 
Certain soil bacteria release geosmin when they die or when they go dormant,especially in hot, dry conditions.

And when rain finally comes along, it disperses the chemical in the air in the form of a fine mist. 

So a lot of what you detect when you smell rain are actually compounds that living things have produced to get through dry spells.

This would explain why the smell is particularly strong if it hasn't rained in a while. But there's one more source of rain scent that you can smell even before it rains: ozone.

It's just a molecule of oxygen. But it has such a distinctive aroma that its name also comes from the Greek word ozein, to smell. Ozone ,or O₃, is produced when lightning ionizes a molecule of oxygen, or O₂, in the air, separating the atoms. When they recombine, some of them form groups of three, creating ozone, which has a sharp, bracing scent, that gives petrichor its invigorating quality.

So the next time it rains where you are, take a moment to stop... and smell the chemistry. 

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