We all know the scent, that earthy fresh aroma that fills the air during those first few minutes of rain. But what causes the phenomenon? After all, rain is just water, and water has no odor, right?
Thankfully, the scientists at the Massachusetts Institute of Technology (MIT) conducted hundreds of experiments and found out why they believe rain brings forth that lovely perfume. By using high-speed cameras to observe raindrops as they hit various porous surfaces, they discovered that small air bubbles become trapped under the drops upon impact, rise up to the surface, and then escape into the surrounding air. It's in the released air that we'll find the root of the scent called petrichor, the smell we associate with rain.
Those raindrops spread more than just a scent, however. In a new study published in Nature Communications, scientists found that under the right conditions, those raindrops also can spread bacteria. Again using high-resolution cameras, watched rain fall on dry, bacteria-laden soil. According to the press release:
When falling at speeds mimicking those of a light rain, at temperatures similar to those in tropical regions, the drops released a spray of mist, or aerosols. Each aerosol carried up to several thousand bacteria from the soil. The researchers found the bacteria remained alive for more than an hour afterward.
Think of raindrops as little pockets of air and rain that act as a delivery service to make the bacteria and microbes airborne. If the wind picks up the particles, they may travel even farther before settling back on the ground and growing a new colony, says Cullen Buie, associate professor and the Esther and Harold E. Edergton Career Development Chair in the Department of Mechanical Engineering at MIT.
"Imagine you had a plant infected with a pathogen in a certain area, and that pathogen spread to the local soil," Buie says. "We've now found that rain could further disperse it. Manmade droplets from sprinkler systems could also lead to this type of dispersal. So this [study] has implications for how you might contain a pathogen."
A high-speed camera captured raindrops splashing on a porous surface and releasing thousands of aerosols. (Photo: MIT)
Not all rain is created equal
Cullen R. Buie, an assistant professor of mechanical engineering at MIT, said about the findings, "Rain happens every day — it's raining now, somewhere in the world. It's a very common phenomenon, and it was intriguing to us that no one had observed this mechanism before."
In a 2015 MIT study, single drops of rain were tested on 28 surfaces, some man-made and other natural, simulating various types of rainfall. Water released from shorter distances mimicked lighter rainfall and water released from higher up acted like more of a downpour.
Not all types of rain are created equal when it comes to delivering aerosols into the air. MIT found that light and moderate rains were best suited for the task, and that, the harder the rain hits the ground, the less likely air would be to rise up to the surface of the drops.
To see those little bubbles of air that contain the smell as well as bacteria, chemicals and microbes, watch MIT's short video below that slows down the process with those impressive high-speed cameras.
Editor's note: This story has been updated since it was originally published in January 2015.