ST. GEORGE — That first drop hits the dry earth, creating a dusty plume. Dark clouds roll over the mountainous landscape as the air cools to the sound of rain pitter-pattering against the red dirt.

It’s been too long since it rained in Southern Utah, and it smells incredible.

This earthy smell, called petrichor, fills the air, particularly during the first rain after a dry spell, according to the American Chemical Society. The word, constructed from ancient Greek, means “blood of stones.”

“Petrichor” was coined by Australian scientists Isabel Joy Bear and Richard Thomas in 1964 after extracting yellow oil from rocks, clay and soil that contained fatty acids from plants.

These volatile plant oils are one of several causes of petrichor, including ozone, created by lightning and carried to Earth via raindrops, and geosmin.

Professional geoscientist Becky Johnson told St. George News that while some may think “dirt is just dirt,” there’s more going on than meets the eye. Actinobacteria, a genus of bacteria present in most healthy soil, are among the organisms that produce terpenes called geosmin.

Terpenes are aromatic chemical compounds produced by bacteria and plants, such as sagebrush, pine trees, cedar trees and lavender.

Johnson said, “Most any plant that produces a strong smell — it’s a terpene of some kind.” Terpenes from actinobacteria and plants build up in the soil during dry periods.

When raindrops hit the dirt, they can trap air bubbles which “force their way to the surface, blasting out in a spray of tiny aerosolized droplets and spread on the wind,” the American Chemical Society states. This contributes to the familiar scent of petrichor.

The smell may be slightly different depending on where a person is as the species of bacteria within the genus could vary, Johnson said.

The scent may also be influenced by the concentration of bacteria within a particular area or by the amount of rain a location receives. For instance, those living in areas with high amounts of precipitation may experience a lighter scent than people living in more arid conditions, she said.

Geosmin-producing bacteria can be killed by certain herbicides, Johnson said, which could make it difficult for plants to grow.

“They break down seed pods so that new plants can germinate,” she said. “But the other thing that bacteria in the soil do is break down the minerals and the nutrients that are in the soil and make them bioavailable for plants.”

“If you have unhealthy soil without the presence of that bacteria, you’re not going to get that smell (petrichor) the way you would in an area where the bacteria is thriving,” Johnson continued. “And you probably wouldn’t have much vegetation either. I wouldn’t say you won’t have any, but you wouldn’t have as much.”

The chemical may also prevent plants from germinating during a drought, regulating the timing of plant growth, Johnson said.

“Bacteria rule the world. … We couldn’t digest food without the microbiome in our gut,” she added.

Southern Utahns may also detect other smells in the breeze on a stormy day. For instance, foliage on many deciduous trees contains chemicals called tannins, which are released when leaves fall to the ground.

“They lose all the chlorophyll before they fall, but there’s a lot of tannins in those leaves,” Johnson said. “If you’ve ever seen leaves that fall into a puddle of water and then the water turns dark — that’s the tannins leaching out of the leaves. And that can create a different smell and it’s not typically a pleasant one.”

Additionally, many desert plants, like creosote brush, four-wing saltbush and desert prickly pear cactus, also produce aromatic terpenes, which can be released in the rain.

In a 2022 study published in the International Journal of Environmental Research and Public Health, researchers found that 13 types of fragrant plant oils emitted from desert plants provide health benefits to humans.

The study also reports that some evidence indicates that people and other animals also respond positively to the smell of geosmin.

People can absorb these oils by breathing them in, and they register in the brain in as little as 22 seconds, releasing into the bloodstream in less than 90 seconds, the study states.

“Within half an hour’s time, they may be found present in every cell of the body and reach all the body’s organs,” according to the study. “It takes two and a half hours or less for most of therapeutical aerosol inhalation of volatile oils to be metabolized in ways that may potentially affect human health in a more lasting manner.”

The oils could have evolved to deter animals from eating the plants, the amount of water plants release into the atmosphere or to attract pollinators. Researchers found that these compounds may contribute to improving “sleep patterns, stabilizing emotional hormones, enhancing digestion, heightening mental clarity, and reducing depression or anxiety.”

The researchers wrote that additional study would be required to compare desert plants to the hinoki forests of Korea and Japan, where people are encouraged to “bathe” in the woodlands, breathing in fragrant terpenes, for improved health.

Why do we love the smell of rain?

Human noses are extremely sensitive to the smell of geosmin and can detect it at concentrations as low as 5 parts per trillion — the “equivalent of one teaspoon in 200 Olympic swimming pools,” according to a 2023 “mini review” in Environmental Microbiology.

The smell is also present in damp soil and waterbodies. And, while not harmful, geosmin tastes “off” when present in water, wine or freshwater fish products, among others, the review states.

Additionally, scientists are still figuring out why humans are sensitive to geosmin’s scent.

“Mammals and insects can detect geosmin at extremely low concentrations,” the review states. “While geosmin-sensitive receptors are known from insects, such a receptor has not been discovered in mammals so far, and hence we cannot reconstruct the evolutionary history of human sensitivity to this compound. It was proposed that geosmin could have allowed our ancestors and other animals to find water in arid environments.”

Johnson said that humans’ ability to smell geosmin could be tied to ancient hunter-gatherers’ dependency on rainfall for food.

“We don’t have olfactory senses like that for the most part,” she said. “Dogs certainly do, but humans don’t. … We can detect it because rainfall was so crucial to our survival.”

But as to why people love the smell of rain, Johnson said she didn’t have a good answer. But she was always grateful for the rain while growing up in Texas as those living West of the Mississippi experience long dry periods.

“It’s a big deal when it rains in the desert,” she said.

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