Can You Smell the Rain?

ADDITIONAL CONTRIBUTORS Zach Schepis

By Zach Schepis

Photo courtesy of Nishanth Jois.

There seems to be a general division of camps between those who relish the experience of a rain storm and those who pray for nothing more than a larger umbrella. But the calm immediately following a downpour carries with it a universally soothing aura. It’s made possible through a smell that our noses have come to recognize.

The post-pour scent is so pleasant that corporations have even sought to commoditize its tranquil essence for consumers to bring into the household. Don’t have the patience to wait for the next summer shower? Fear not! Candles and detergents and air fresheners are available to mimic the natural fragrance.

That unique and almost primordial satisfaction we collectively derive is made possible by an aroma known as petrichor. Thanks to a couple of curious Aussies searching for a source to the smell back in 1964, we now know that petrichor is a result of certain plants that secrete oils during arid periods.

The longer the dry spell, the more oil seeps into the ground. Bacteria residing in the soil (known as Actinobacteria if you want to get technical) release chemicals that mix together with oils that need the rain to help spread their spores.

After a drought, the rain kicks these compounds up into the air, creating a musky earth-tinged fragrance.

That’s not all–thunderstorms also play a central role in this odorous dance. While the smell after a drizzle can stir peaceful tidings, the ominous approach of a storm carries along with it a different kind of redolence.

Through a badass display of power, electrical charges from lightning will literally split oxygen molecules to create what we know as ozone. Ozone itself has a pretty distinct odor–somewhat akin to chlorine or burnt rubber.

The ozone scent is definitely a lot more ominous than petrichor.

While all the smelly dynamics are undoubtedly interesting, scientists remained puzzled as to exactly how petrichor was released into the air.

All up until recently, that is. A few months ago the discovery was finally made by a group of scientists at MIT who uncovered the mechanism responsible for unleashing aromas into the environment.

Employing high-speed cameras capable of capturing movements to the microsecond, Assistant Professor Cullen R. Buie and his colleagues observed that when the right kind of raindrops collide with the right kind of soil at the right velocity, the interaction traps tiny air bubbles upon contact. These bubbles, in turn, travel to the top of the raindrop where they burst to release aerosols.

Seems like a lot of specific conditions just to create some measly bubbles. So what’s the big deal?

Photo courtesy of Edal Anton Lefterov.

BTR took the time to discuss the implications of these findings with James Bird, an Assistant Professor of Mechanical Engineering at Boston University. Bird has a PhD from Harvard and researches fluid dynamics, with a specific focus on the capillary dynamics of drops and bubbles.

That’s right; we found a bubble expert.

“I think the real interest behind this most recent study is that the droplet can carry with it things that were already in the soil, such as bacteria,” Bird tells BTR.

Don’t fret, however–not every rain drop is created equal. Only under very specific conditions can these aerosols be released. Bird explains that if the soil is too loose or too compact (like gravel) then either the water won’t permeate the soil or it will infiltrate too quickly and air bubbles won’t be created.

The same is true for the velocity and size of the droplets. Both the impact speed and composition will change the energy and inertia, explains Bird, so if the rain is coming down too fast or too slow then the effect too will be rendered null.

As it turns out, drizzles to moderate rainfall colliding with sandy or clay soils create the ideal conditions for maximum aerosols.

Let’s get back to the troubling bit behind all of this: bacteria. The spread of bacteria into the environment is already well documented. However, the spread of bacteria cells in aerosols that can act as cloud condensation nuclei is not as understood.

We’ll let Bird break down the jargon.

“When you have this kind of moisture in the air, it can condense into little particulates,” he says. “It could be sea salts; it could be bacteria. Clouds can develop on that scale.”

Believe it or not, but the mechanism means that human beings are capable of feeding clouds to create certain kinds of rain by employing different types of particulates in the soil. The cloud manipulation can affect global radiation balance and even influence the weather.

Since bacteria can be pathogenic, the discovery of this bubbly phenomenon could help combat diseases. Legionnaires Disease and Red Tide–both respiratory illnesses that stem from bacteria and toxins in the soil–could effectively be battled and better understood through rain’s aerosol releases.

“There is potential for future research, to better understand the transmission of microorganisms from nature to humans,” reflects Bird. “It’s all quite eye-opening.”

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