Why We Can Smell Rain Coming [VIDEO]

Do you ever wonder why we can smell rain coming? Well it turns out that that distinctive fresh, earthy aroma of an approaching rainstorm is due to the release of aerosol clouds that are carried by the wind, a new study says.

Researchers at MIT captured this phenomenon using high-speed cameras, comparing the effect to that of a glass of champagne. When a raindrop hits a surface such as soil or leaves, it traps tiny air bubbles that then shoot upward, ultimately bursting from the drop in a fizz of aerosols. These tiny bubbles carry miniscule amounts of aromatic particles - as well as bacteria and viruses from soil - that we can smell from miles away due to gusts of wind that carry them.

This effect, known as Petrichor, is nothing new. It was first described by Australian scientists in 1964, however, they could not explain what might cause it to occur.

"They talked about oils emitted by plants, and certain chemicals from bacteria, that lead to this smell you get after a rain following a long dry spell," Dr. Cullen Buie, an assistant professor of mechanical engineering who led the work, said in a statement. "Interestingly, they don't discuss the mechanism for how that smell gets into the air. One hypothesis we have is that that smell comes from this mechanism we've discovered."

So Petrichor, and not some bizarre sixth sense, is why humans can sometimes forecast a rainstorm long before it arrives, even when grounds have been dry for days. The researchers hope this revelation can even help to explain how certain soil-based diseases spread to humans. 

During the study, Buie and his co-author Youngsoo Joung conducted about 600 experiments on 28 types of surfaces, including 12 engineered materials and 16 soil samples. Though their efforts, capturing them on high-speed cameras, they found that the speed of the droplet, as well as the properties of the surface on which it fell, determined whether or not a cloud of "frenzied aerosols" was released.

"Frenzied means you can generate hundreds of aerosol droplets in a short time - a few microseconds," Joung explained. "And we found you can control the speed of aerosol generation with different porous media and impact conditions."

For example, light showers and moderate rain seem to trigger more aerosols while downpours associated with thunderstorms release far fewer aerosols.

"Rain happens every day - it's raining now, somewhere in the world," Buie said. "It's a very common phenomenon, and it was intriguing to us that no one had observed this mechanism before."

While the researchers revealed that rain can actually have a smell, thanks to aromatic particles, they found that raindrops may also carry contaminants such as soil-based bacteria and viruses that can lead to diseases in humans. The team is currently conducting similar experiments using pathogens like E. coli to see whether rain can carry, along with its fresh scent, certain harmful contaminants.

The phenomenon is described further in the journal Nature Communications.

[Credit: Massachusetts Institute of Technology (MIT)]

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