Ask Smithsonian: Why Does Rain Have A Distinctive Smell?

Summer is the time of year when we are most acutely attuned to the smell of rain, and not just for poetic reasons. It turns out that humidity—omnipresent in some areas of the country during the summer months—helps prep our noses to smell better, and it’s also responsible for dispersing odors throughout the air.

“Our noses work better when they are warm and humidified,” says Pamela Dalton, an olfactory sensory scientist at the Monell Chemical Senses Center in Philadelphia. And, the warmer the environment, “the more likely any odor molecules are going to be more volatile,” says Dalton, who researches how cognitive and emotional processes modify the way we perceive odor and sensory irritation from volatile chemicals. 

“Think about a trash strike in the middle of the summer,” she says, noting that the odor will be a lot more redolent than a neglected garbage pile in a cold month.

Warmer months, of course, also coincide with lots of thunderstorm activity. Lightning inside of clouds produces ozone—that’s the smell that tells you that a storm is on the way. Ozone is made up of three oxygen atoms, and has sort of a light chlorine smell, says Dalton. Some people might describe it as fresh, others as a little sharp.

If it’s a torrential downpour, there might not be many smells associated with the rain, at least not until afterwards, when the sun comes out and hits the ground, or the humidity returns to start releasing odors again. The odor is far more prevalent with a light rain when a subtle chemical reaction loosens various scents into the atmosphere.

That process was first described in 1964 by two Australian scientists, Isabel Joy Bear and R. G. Thomas, who says that the smell that most people associate with rain is due to the water hitting the ground and mixing with plant oils. They called their discovery petrichor, a combination of the Greek words petra (stone) and ichor (the blood of mythological gods). 

The smell that rises up is never the same in any one place, and differs depending on where on the planet the rain is hitting, says Dalton. When raindrops land on a blanket of needles in a forest, the scent of pine is released. The acidity on a pine forest floor is different than what’s found in a rainforest, which creates a another odor experience. That’s partly because the composition of soil varies by location.

And the smells wafting up from an asphalt-covered urban area will not be the same as what floats up from a suddenly-wet desert gulch. Concrete and asphalt are known for storing all kinds of odors, including urine, which explains why dogs “kind of go crazy with the scents” after a rain, she says.

And while the description of a smell can be generalized, not everyone perceives that smell in the same way, said Dalton. “We have a lot of variability in the act of smelling something,” she says. “The first source of variation is what is actually released that could be in our breathing zone,” she says. “The rest is what we perceive based on genetics, nasal anatomy and past experience.”

The physical layout of the nasal passage and the smell receptors we are born with account for some of our smell perception. Then there’s a Proustian effect—for many of us, the smell of rain evokes memories, says Dalton.

Until earlier this year, scientists had not been able to pin down exactly how rain liberated smells, that is, how petrichor actually happened. But in January, researchers at the Massachusetts Institute of Technology reported that they had figured it out, using a high-speed camera.

They conducted a series of experiments allowing rain to hit a porous surface. They found that as a raindrop hits, it starts to flatten; simultaneously, tiny bubbles rise up through the droplet, and then burst out into the air. That causes the smells to spread. It might also cause the spread of microscopic viruses and bacteria, said the MIT scientists, who published their findings in the journal Nature Communications.

And, through hundreds of tests on different surfaces and with different soils, they confirmed what has already been pretty well known: that more aerosols were produced in light and moderate rain, while far fewer were released during heavy rain.

“They visualized the aerosolization, which hasn’t been done before,” says Dalton.  

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Alicia Ault | | READ MORE

Alicia Ault is an independent journalist whose work has appeared in the New York Times, the Washington Post, Medscape Medical News, WebMD and Wired. She splits her time between St. Petersburg, Florida, Washington, D.C., and New Orleans, exploring the rich variety of food, music, art and culture on offer in those cities.