Petrichor, the Smell of Rain, Has A Lot of Science Behind It

Key Takeaways about the Science Behind the Smell of Rain or Petrichor:

• The smell of rain is known as petrichor, a term derived from petro — meaning rock — and ichor — meaning tenuous essence.

• The smell is generated from an oncoming rainstorm. However, if the ground is too wet and saturated already, the sent could be overpowered. If there is lightning with the storm, the smell could intensify.

• Petrichor is made from the organic compound known as geosmin. This compound is created from certain bacteria as plant matter breaks down.

If your idea of bliss is sitting on the porch or beside an open window with a cup of your favorite hot drink, enjoying a rainy day, then you’re probably a pluviophile, someone who loves rain. But before the rain begins, something happens that delights even people who aren’t so enchanted with rain itself. It’s the arrival of the scent that heralds the onset of rain.

What is the Smell of Rain Called?

The smell of rain is known as petrichor. The word itself is evocative. Coined in a 1964 paper published in Nature, it’s a combination of “petro,” meaning stone, and “ichor,” meaning a “tenuous essence,” so, “a tenuous essence derived from rock or stone,” according to the paper’s authors.

However, the more romantically inclined among us might go with Merriam-Webster and define ichor as something like, “the fluid that flows through the veins of the gods.”

Read More: Old Wives’ Tales to Predict Weather: What’s Based in Science and What’s Just Folklore?

What Causes Petrichor?

More prosaically, but no less fascinating, the scent of petrichor comes from a chemical compound called geosmin. Geosmin has a chemical structure similar to alcohol, but its scent is more pleasant, explains Timothy Logan, an atmospheric scientist at Texas A&M University. In fact, its scent is often described as “earthy.” We’re so sensitive to geosmin that we can detect very, very low concentrations of it, perhaps as low as one hundred parts per trillion.

“It doesn’t take much to get it mobile, and once it starts traveling, it goes right to your nose,” Logan says.

The scent is powerful, but not very durable. It blooms when dry ground is moistened by the first few drops of rain or even in response to an increase in humidity that precedes rain. However, it’s easily diluted, says Logan.

“Those compounds are very soluble. So if it’s too wet, you’ll never be able to smell it, because they just get drowned out.”

On the other hand, an approaching storm can enhance the effect.

“If there’s lightning in the distance, the ozone will kind of pump up the potency of what you smell,” adds Logan, who specializes in studying lightning.

But where does geosmin come from? And why are we so attuned to it?

What is Geosmin and How Does it Create Petrichor?

Actinomycetes are a type of bacteria in soil that break down dead plant matter. And one particular actinomycete is responsible for producing geosmin: Streptomyces. If that word sounds vaguely familiar, it’s probably because streptomycetes have been used to make antibiotics, such as streptomycin, which is produced by the bacterium S. griseus. But one thing all streptomycetes have in common is that they produce geosmin. Every one of them.

This fact intrigued Mark Buttner and Matthew Bush, researchers at the John Innes Centre in Norwich, U.K., so they teamed up with a multinational team of researchers to find out what’s going on.

“The fact that they all make geosmin suggested that it confers a selective advantage on the bacteria, otherwise they wouldn’t do it,” Buttner said in a statement announcing the research, which was published in Nature in 2020.

Hypothesizing that the bacteria were using the geosmin as a signal, the team set traps baited with streptomycetes. The traps caught springtails, tiny animals that live in moist soils. Testing showed that springtails are very responsive to geosmin.

The springtails are attracted to the streptomycetes because the bacteria are a great food source. Although the antibiotic properties of streptomycetes kill nematodes and fruit flies, springtails have evolved enzymes that neutralize them. The selective advantage to the bacteria? The springtails spread the streptomycete spores. Both the springtails and the streptomycetes benefit.

Why are Humans So Sensitive to Petrichor?

Of course, none of this tells us why we’re so sensitive to the smell of rain or why we perceive it as so pleasant. That’s still a mystery. But there’s no doubt people love the scent. Some labs have tried to create a perfume that smells like petrichor. They’ve not been too successful, says Logan.

“You never can get it quite right. Nature knows the exact proportions.”

But undoubtedly, it can be useful to know when rain is on the way. When our ancestors got a whiff of geosmin, they knew it was time to get inside, get the animals in the barn, says Logan. Perhaps today we know it’s time to brew a cup of tea.

Read More: Rain May Have Helped Form the First Cells, Kick-Starting Life As We Know It

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