Pandemic Diary 15: A Rainy Day

Quarantine considerations: Outdoors or in?

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A 1964 article in Nature with the euphonious title, “Nature of argillaceous odour,” gave the world the euphonious-sounding word, “petrichor.” In the article, two chemical researchers attempted to scientifically break down what it is we smell when we smell the world after a rain shower and to give the phenomenon a name.

The two authors coined the word, “petrichor,” which I have been mispronouncing in my head since I first encountered it in 2015, when an article on the Huffington Post started to make its social media rounds. It has a long “I,” so say it like this: “petra,” then “eye-core,” which is not how I hear it in my head, with a short “i.”

It is one of the most familiar of smells, pungent, a little clayey, the very essence of spring and summer, yet until 1964 and even more recent experiments, no one knew exactly what that smell is. It didn’t even have a name.

In 1964 and ’65, the two researchers, one Australian and one British, wrote a pair of articles for the science journal Nature in which they outlined a theory about that post-rain odor. A hint is in their title, “argillaceous,” which simply means “things related to or having to do with clay.” (This will not stop me from saying the word the next time I eat something that I really enjoy. It sounds like a compliment. “You’ve outdone yourself. This was truly argill-” and then someone will kick my shin to prevent me from the insult.) Before 1964, the word “petrichor” did not exist, and every single writer who attempted to evoke that post-rain scent discovered that they were stuck with the phrase “post-rain scent.”

It had long been a scent that evokes sensations without a word to evoke the scent.

The researchers theorized that the scent is a bunch of compounds that are never otherwise exposed to our olfactory equipment during any other weather condition. This may strike one as a little obvious, but it isn’t. In dry weather, the authors wrote, certain plants, certain trees, excrete oils that are absorbed by the more clayey soils around them. These oils have a job: they help slow seed germination during dry seasons, when new plants might face a harsh start to life. During a rain shower, the soil yields up some of these oils, now no longer needed to protect seeds, along with another substance, geosmin, which is a waste product of certain bacteria in soil. This specific combination of molecules is what we smell, they wrote.

“Petr-” means rock or stone (petrology). “Ichor” is the blood of the gods in Greek mythology. Thus, petrichor is quite a poetic term; if “the blood of the gods released from stone” is what Isabel Bear and Roderick Thomas wanted us to hear when we hear or write that word, well, that smell indeed is rare and fine to most human noses.

The results of an MIT study were released in 2015, which further confirmed some thoughts about petrichor but also offered some surprises. A high-speed camera was used to photograph simulated rain against different soils. The Huffington Post article has “Crazy Slo-Mo Video Explains Why Rain Has That Distinctive Smell” as its headline, which is a bit more eye-catching than the MIT news release about the study: “Rainfall can release aerosols, study finds.” The study’s authors set up some six hundred experiments on twenty-eight different surfaces and they discovered that raindrops “fizz” on impact. (That is my word, not theirs.) This was something that no one had even theorized.

The images they produced revealed a mechanism that had not previously been detected: As a raindrop hits a surface, it starts to flatten; simultaneously, tiny bubbles rise up from the surface, and through the droplet, before bursting out into the air.

Until 2015, these phenomena—what rain looks like and even what it does when it hits the ground, when it hits different types of soil—had not been seen in such detail, even though almost every human nose has registered it since our ancestors first encountered rain.

The researchers further verified something that every human being who has stood outdoors in a light rain has noticed but never verified with “crazy slo-mo” tools of any kind: that a light rain (there are probably different, precise definitions for “light rain,” but most of us know it when we experience it), when it strikes clayey soils, it releases a lot more bubbles per raindrop. It fizzes. A heavy rain drowns each raindrop, pushes it into the dirt like a bully, a nerd, and the nerd’s locker; a light rain, like the kind I most fondly recall in the woods out behind my house when I was a child, a light rain that taps the leaves and branches of trees, which further slows their impact, that rain produces the strongest petrichor of all and is the one that renders me into an eight-year-old noticing the world for the first time.

The lightest of rain after the driest of spells leads to the most argillaceous type of petrichor, which is the kind that humans smell as relief, thought that things will start growing again, the very aroma of outdoors..

The WordPress Daily Prompt for April 15 asks us to reflect on the word, “Scent.”

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  1. My little corner · April 15, 2020

    Your essay reminds me of the late March/ early April/Spring smell in Vermont. I attended Middlebury College. When the piles and piles of manure at the many dairy farms thawed out – now that was a smell.

    Liked by 1 person

  2. Maria Dass The World · April 16, 2020

    Interesting! I wrote about it in this today and referred to it as ‘after rain’ scent. Now I know it has a term 😊

    Liked by 1 person

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