What Makes Rain Smell So Good?

A mixture of plant oils, bacterial spores and ozone is responsible for the powerful scent of fresh rain. Image via Wikimedia Commons/Juni

Step outside after the first storm after a dry spell and it invariably hits you: the sweet, fresh, powerfully evocative smell of fresh rain.

If you've ever noticed this mysterious scent and wondered what’s responsible for it, you’re not alone.

Back in 1964, a pair of Australian scientists (Isabel Joy Bear and R. G. Thomas) began the scientific study of rain’s aroma in earnest with an article in Nature titled “Nature of Agrillaceous Odor.”  In it, they coined the term petrichor to help explain the phenomenon, combining a pair of Greek roots: petra (stone) and ichor  (the blood of gods in ancient myth).

In that study and subsequent research , they determined that one of the main causes of this distinctive smell is a blend of oils secreted by some plants during arid periods. When a rainstorm comes after a drought, compounds from the oils—which accumulate over time in dry rocks and soil—are mixed and released into the air. The duo also observed that the oils inhibit seed germination, and speculated that plants produce them to limit competition for scarce water supplies during dry times.

These airborne oils combine with other compounds to produce the smell. In moist, forested areas in particular, a common substance is geosmin , a chemical produced by a soil-dwelling bacteria known as actinomycetes . The bacteria secrete the compound when they produce spores, then the force of rain landing on the ground sends these spores up into the air, and the moist air conveys the chemical into our noses.

“It’s a very pleasant aroma, sort of a musky smell,” soil specialist Bill Ypsilantis told NPR during an interview on the topic. “You’ll also smell that when you are in your garden and you’re turning over your soil.”

Because these bacteria thrive in wet conditions and produce spores during dry spells, the smell of geosmin is often most pronounced when it rains for the first time in a while, because the largest supply of spores has collected in the soil. Studies have revealed that the human nose is extremely sensitive to geosmin in particular—some people can detect it at concentrations as low as 5 parts per trillion . (Coincidentally, it’s also responsible for the distinctively earthy taste in beets.)

Ozone —O3, the molecule made up of three oxygen atoms bonded together—also plays a role in the smell, especially after thunderstorms. A lightning bolt’s electrical charge can split oxygen and nitrogen molecules in the atmosphere, and they often recombine into nitric oxide (NO), which then interacts with other chemicals in the atmosphere to produce ozone. Sometimes, you can even smell ozone in the air (it has a sharp scent reminiscent of chlorine) before a storm arrives because it can be carried over long distances from high altitudes.

But apart from the specific chemicals responsible, there’s also the deeper question of why we find the smell of rain pleasant in the first place. Some scientists have speculated that it’s a product of evolution.

Anthropologist Diana Young of the University of Queensland in Australia, for example, who studied the culture of Western Australia’s Pitjantjatjara people , has observed that they associate the smell of rain with the color green, hinting at the deep-seated link between a season’s first rain and the expectation of growth and associated game animals, both crucial for their diet. She calls this “cultural synesthesia”—the blending of different sensory experiences on a society-wide scale due to evolutionary history.

It’s not a major leap to imagine how other cultures might similarly have positive associations of rain embedded in their collective consciousness—humans around the world, after all, require either plants or animals to eat, and both are more plentiful in rainy times than during drought. If this hypothesis is correct, then the next time you relish the scent of fresh rain, think of it as a cultural imprint, derived from your ancestors.


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Water is a curious molecule

Water is a curious molecule with extraordinary properties, which vary depending on temperature and pressure. Life depends on these anomalous characteristics, such as the unusually large heat capacity, high melting and boiling points, high thermal conductivity and surface tension, and shrinking on melting.  

Water is thus of great interest to biologists, chemists, physicists, as well as cosmologists. Despite its simple structure and its obvious importance, it is still poorly understood and many of its aspects, either as a pure substance or as a solvent, are controversial. For more than a century, the combined significance and peculiarity of water inspired scientists to construct conceptual models, which in themselves reproduce the behavior observed. The exploration of structural and binding properties of small water complexes provides a key to understanding bulk water in its liquid and solid phases and to comprehending solvation phenomena.

Do you smell carrot?

Company: Foul odor from Bridgeton Landfill to worsen before getting better

The foul odor coming from a landfill in Bridgeton will get worse before getting better, according to a release from the landfill's owners on Friday. The next phase of improvement involves reinforcing concrete pipe, and is aimed at reducing the overwhelming smell in the area. Construction on this phase will begin Monday and last until April 30, weather permitting. The work will "significantly increase the odor for employees, local businesses and residents," according to the release. In March, Mo. Attorney General Chris Koster announced he would sue Republic Services, the owner of the Bridgeton Landfill , after he said the company violated the state’s environmental protection laws. Matt LaVanchy, assistant fire chief in Pattonville, said he requested the work because it will cut down on hazards involving the landfill, and eventually reduce the odor from the landfill. "This action takes care of two major problems – first, it eliminates a safety hazard that could cause future accidents, and second, and most important to the residents, the removal of the RCPs creates a smooth subgrade to ensure the plastic cap doesn’t tear," said LaVanchy. "This work is absolutely required for the landfill to finish its efforts to dramatically reduce the odors.” Company officials say contractors will conduct air sampling during the upcoming construction phase.

Big Fine For Odour Despite Earnest Efforts By Compost Company

Universal Resource Recovery Inc . pleaded guilty to 2 counts of discharging odours into the environment and was fined $80,000 for each count (total $160,000 + 25% victim fine surcharge, = $200,000). The Welland company's composting facility was located in an area zoned for mixed residential and industrial land uses.It received organic food waste from municipal green bin programs, and began to have difficulty controlling odours soon after the facility opened in 2008. The guilty pleas were for odour discharges on three days in each of 2010 and 2011, which caused discomfort to neighbours and loss of enjoyment of their normal use of property.On several occasions, the facility had received over the 200 tonne per day limit for food, leaf, yard and wet waste set out in its Waste Environmental Compliance Approval (ECA), and had failed to notify the MOE of the waste exceedences or why these had occurred.It had also breached a provincial officer's order issued to address ongoing odour emissions, which restricted the amount of waste received at the site to 100 tonnes per day. The company tried hard to address the odours, including being responsive to odour complaints (staff frequently responded in person), holding community meetings and conducting proactive odour checks.It also worked closely with the MOE. Despite these efforts, the company voluntarily shut down operations for 5 months during 2010 and then permanently in 2011.By the time it recommenced operations after the 2010 shutdown, the company had invested $35 million in capital expenditures on the facility.

Smelling is believing

The new scentsation in search

Coming to your senses: go beyond type, talk, and touch for a new notation of sensation.
Your internet sommelier: expertly curated Knowledge Panels pair images, descriptions, and aromas.
Take a wiff: the Google Aromabase - 15M+ scentibytes.
Don't ask, don't smell: For when you're wary of your query - SafeSearch included.



https://www.google.com/intl/en/landing/nose/





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