Finally, a scientific explanation for ‘alimuom’

Every Pinoy is familiar with that warm, earthy aroma you get immediately after rain on a hot day. We even have a name for it: "alimuom".

 

Called "petrichor" in English, alimuom's distinctive smell finally has a scientific explanation.

 

Scientists at the Massachussetts Institute of Technology (MIT) pinned down the explanation with the help of high-speed cameras trained on raindrops falling on 28 types of surfaces in some 600 separate experiments. 

 

Researchers led by Cullen R. Buie and Youngsoo Joung simulated rainfall by applying drops of water on different types of soil, while the cameras were trained on the points of impact.

 

 

The footage they acquired brought to light a mechanism that had been largely overlooked for decades: since the 1960's, scientists had believed that alimuom came from oils and chemicals produced by raindrops hitting the ground. The recent study proved that the distinctive smell that wafts in the air, is in fact caused by bubbles.

 

When raindrops hit the soil, tiny bubbles are produced inside the droplets. These bubbles make their way up to the surface of the droplet, and release 'frenzied aerosols' when they pop. 

 

“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.”

 

Joung and Buie found that there is a direct correlation between how heavy the rainfall is and the permeability of the soil. Light to moderate rain produces more aerosols than heavy rain.

 

“When moderate or light rain hits sandy or clay soils, you can observe lots of aerosols, because sandy clay has medium wetting properties,” Joung says. “Heavy rain [has a high] impact speed, which means there’s not enough time to make bubbles inside the droplet.”

 

The team is currently conducting variations of the experiment to learn if bacteria and pathogens can be transmitted via rainfall. — Jessica Bartolome/TJD, GMA News