THE SCENT OF RAIN EXPLAINED NOW

PETRICHOR

One of the best parts of spring showers is the fresh rain scent they impart. We are smelling petrichor, the unique soft earthy scent produced when rain hits the ground. Until recently, no one could figure out exactly how petrichor is produced.

But now, Young Soo Joung and Cullen R. Buie, researchers at MIT, identified the mechanism that releases petrichor into the air. 

Buie and Joung conducted over 600 experiments on 28 types of surfaces: 12 engineered materials and 16 soil samples. 

They set up a system of high-speed cameras to capture raindrops on impact. They observed, for the first time, that a raindrop hits a surface, it starts to flatten. 

Simultaneously, tiny air bubbles trapped in the soil rise up, through the droplet, then burst out into the air, producing an aerosol effect. They carry micronized aromatic elements, along with bacteria and viruses stored in soil. They combine to produce the scent of petrichor.

Depending on the speed of the droplets, and the properties of the surface, a cloud of “frenzied aerosols” rises up. Hundreds of tiny droplets can spray into the air within a few microseconds.

Raindrops that move at a slower rate tend to produce more aerosols; this serves as an explanation for why the petrichor is more easily detected after light rains.

The term petrichor was coined in 1964 by two researchers, Isabel Joy Bear and Roderick G. Thomas, who first identified the scent. They determined that it derives from an oil exuded by certain plants during dry periods.

When it rains, the oil is released into the air along with another compound, geosmin, a metabolic by-product of certain Actinobacteria, found in soil. It may also include ozone if the rain has been accompanied by lightning.

Beyond delivering a beautiful scent, Buie and Joung found that raindrop aerosols can also help to spread both beneficial and harmful bacteria and viruses, as they send them airborne and wind carries them away. They have published their results recently in the journal Nature Communications.

Read more about Beautiful Rain, as they relate to Arts/Design, Nature/Science, Food/Drink, Place/Time, Mind/Body, and Soul/Impact including 10 Most Beautiful Books on Rain.

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IMAGE CREDITS:

1. Photo: by John Britt. Splash!
2. Photo: by rltherichman. The Clouds Above Opened Up.
3. Photo: by Marcin Bajer. Sudden Assault of Rainwater.
4. Image: Courtesy of MIT. Raindrop Striking Ground.
5. Photo: Courtesy of Daffodil Photo Blog. Raindrop Hitting Ground.
6. Photo: Courtesy of Imgkid. Petrichor.
7. Photo: by Stevie Gill. Rain Drops.
8. Photo: by Reji. Raindrops.
9. Photo: by Britta Heise. Dowitchers in Downpour.
10. Photo: by Jon Rawlinson. Splash!
11. Photo: by LadyDragonflyCC. Birdbath Splash.
12. Photo: Caro. Raindrops.  
13. Photo: by JR. Raindrop.
14. Photo: by Christopher Charles. Raindrops Keep Splashing.