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Why Does Rain Smell So Good—and Can You Really Smell It Coming?

You know the smell. Rain hits warm, dry ground and suddenly the air has an earthy, fresh scent that seems inseparable from a summer storm.

It has a name: petrichor.

But petrichor isn't one particular substance, and the smell of rain doesn't actually come from the rain itself. What you're smelling is a mixture of compounds from soil, plants and microorganisms that become much easier for us to detect when rain arrives.

Even more interesting, sometimes you really can smell a storm before the first drop reaches you.

What Is Petrichor?

The word petrichor was coined by Australian scientists Isabel Joy Bear and Richard Thomas in the 1960s to describe the distinctive earthy smell associated with rain falling on dry ground.

The name comes from Greek roots: petra, meaning stone, and ichor, the mythical fluid said to flow through the veins of the Greek gods. It's an unusually poetic name for a very real bit of chemistry.

During dry periods, oils and other compounds from plants can accumulate on soil, rocks and other surfaces. When rain arrives, some of those substances are released into the air.

But one of the most recognizable ingredients in the smell of rain comes from something even smaller: microorganisms living in the soil.

Geosmin: The Earthy Part of the Smell

Much of that rich, earthy scent comes from a compound called geosmin.

Geosmin is produced by certain microorganisms in soil, including bacteria in a group called Streptomyces. When these organisms produce spores, geosmin can become associated with them and the surrounding soil.

Humans happen to be remarkably sensitive to the compound. We can detect geosmin at extremely low concentrations, which helps explain why the smell of wet earth can seem so strong even when only tiny amounts are present in the air.

Geosmin is also responsible for the earthy smell you notice when digging in garden soil. It can even contribute an earthy or muddy taste to drinking water and some foods.

Fun fact: The word geosmin comes from Greek words meaning, appropriately enough, “earth smell.”

How Does Rain Get That Smell Into the Air?

This is where the physics gets particularly interesting.

When a raindrop hits a porous surface such as soil, tiny air bubbles can become trapped beneath it. Those bubbles rise through the drop and burst at the surface, releasing extremely small airborne particles called aerosols.

Those aerosols can carry geosmin and other compounds from the ground into the air, where the wind can carry them to your nose.

Researchers using high-speed cameras have observed this process in remarkable detail. A single raindrop hitting the right kind of surface can generate numerous tiny aerosol droplets.

In other words, rain isn't creating the earthy smell so much as helping launch the substances responsible for it into the air.

Why Does Light Rain Sometimes Smell Stronger Than a Downpour?

You might expect heavier rain to create a stronger petrichor smell, but that isn't necessarily the case.

Gentle or moderate rain falling on porous soil can be particularly effective at trapping those tiny air bubbles and producing aerosols. When rain hits too hard, the splash dynamics are different and may be less favorable for this particular process.

The condition of the ground matters too. Petrichor is often especially noticeable when rain arrives after a long dry spell because compounds have had time to accumulate on dry surfaces.

That's why the first shower after weeks of hot, dry weather can smell so much more dramatic than the third rainy day in a row.

Can You Really Smell Rain Before It Arrives?

Sometimes, yes.

Wind associated with an approaching storm can carry airborne compounds from areas where rain has already begun falling. You may be standing on completely dry ground while rain a few miles away is releasing geosmin and other substances into the air.

If the wind is blowing from the storm toward you, the smell can arrive before the rain itself.

That familiar earthy scent may therefore be telling you something real: somewhere upwind, raindrops may already be hitting dry ground.

But there's another smell people sometimes notice before thunderstorms, and it comes from an entirely different source.

What Is That Sharp Smell Before a Thunderstorm?

Some people describe the air before a thunderstorm as smelling sharp, clean, metallic or slightly like chlorine.

One possible contributor is ozone.

Ozone is a molecule made of three oxygen atoms. Electrical activity associated with thunderstorms can contribute to chemical reactions that produce ozone, and storm downdrafts can help transport ozone-rich air from higher in the atmosphere toward ground level.

Humans can smell ozone at relatively low concentrations, so under the right conditions its distinctive odor may be noticeable before or during a storm.

That means “the smell of rain” can actually describe at least two different experiences: the earthy petrichor released when rain interacts with the ground and the sharper atmospheric smell that can accompany thunderstorms.

Why Do Plants Contribute to Petrichor?

Soil bacteria get much of the attention, but plants contribute to the smell of rain too.

During dry weather, plants release various oils and volatile organic compounds that can collect on leaves, soil and rocks. Rain can release some of these compounds into the air, adding to the mixture we recognize as petrichor.

The exact scent therefore varies from place to place. Rain falling in a pine forest doesn't smell exactly like rain in a grassy field, desert or backyard garden because the surrounding vegetation, soil and microorganisms are different.

Even the same place can smell different depending on the season and how long it has been dry.

Petrichor isn't really a single perfume. It's a local recipe.

Why Do Humans Like the Smell So Much?

This is harder to answer.

The chemistry of petrichor is well understood compared with the question of why so many people find it pleasant. There are ideas about possible evolutionary associations between the smell of rain and the availability of water, but explaining a universal human preference is difficult—and not everyone necessarily experiences the scent in the same way.

There is also a simpler possibility: association.

The smell may remind us of summer storms, gardens, forests, childhood, cooler air after a hot day or the relief of rain after a dry spell. Smell is closely connected with memory and emotion, so part of what makes petrichor appealing may come from the experiences we've attached to it.

The chemistry can explain what reaches our noses. Why we enjoy it so much is a more complicated question.

Not Every Rainstorm Smells the Same

Once you know what's creating the smell, it makes sense that some rainstorms have a powerful scent while others barely smell at all.

The length of the preceding dry spell matters. So do the type of soil, vegetation, rainfall intensity, wind direction and microorganisms living in the ground. A light shower on warm, dusty soil can create a very different smell from heavy rain falling on an already saturated landscape.

And sometimes what seems like the smell of an approaching rainstorm really is arriving ahead of the rain.

You aren't smelling water falling through the sky. You may be smelling soil bacteria, plant compounds and even atmospheric chemistry carried toward you on the wind.

For something we casually call “the smell of rain,” there's a surprising amount going on in every breath.

Learn more interesting facts about rain here:

 

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