The first day of wet weather brings the familiar punctuated sound of drops against concrete, the sloshing of careless feet in muddy puddles, and the almost indescribable scent of something distinctly…rain-y. It is such a familiar scent, yet so difficult to put into words. Mellow, subtly sweet, almost piney. , At the same time, heavy, as if to give yet another reminder of the rain’s previous absence. And fleeting, just like the rain. But hopefully here in Cupertino, whose sky is perpetually empty of any cloud, this smell will decide to return in the coming months, if only in passing.
Each day without rain in this drought-stricken town will only intensify the smell when precipitation arrives. During periods of dry weather, plants secrete palmitic, stearic, and oleic acid to prevent seed germination, and possibly to reduce competition for water (1). Months pass and these chemicals settle in the parched soil and grooves of rocks, awaiting showers from above.
Also in the soil are actinobacteria. When there is little of the moisture that these bacteria thrive in, they produce spores and release the substance geosmin, which is another contributor to the smell of rain (2). Again, this is a game of biding time. Only when precipitation arrives is this chemical released into the air.
Interestingly, the sweet smell of rain is also the smell of death. It is ironic that a first rain, a source of rejuvenation and revitalization, is partly the smell of the molecular carcasses of decomposing organisms. But it would be unfair to plug one’s nose the next time it rains, and inaccurate to call this odor a stench, or fetor, or funk. These molecules recombine with elements in minerals and clay, with which they form hydrocarbons and alcohols (3). This is what we end up smelling. But there is a caveat: these hydrocarbons and alcohols, as well as the plant secretions and actinobacteria-produced geosmin, must accumulate in the soil at great enough concentrations for the receptors in our nose to detect them. For geosmin, that figure can be as low as five ppm, but as petrichor is a concoction of additional substances, there must be a prolonged period without rain for them to collect in the earth (4).
Only after a prolonged dry spell can these chemicals be thrusted up into the air and into our nostrils. Enough of these substances must be present for us to take notice. But exactly how are we able to take notice of these chemicals beneath our feet? Slow motion video footage has captured images of the process: raindrops trap air bubbles when they hit porous surfaces, and these bubbles then explode into the air as a sort of froth (5).
The result is what scientists call aerosols, which take with them the substances from plants and the soil. In the air, these aerosols interact to form what is known as petrichor, the smell of a first rain, or a temporary respite from constant aridity.
Cupertino, for that matter, has had few of such much-needed interruptions from drought. Excluding a very light sprinkle a few weeks back, it last rained many months ago, and even then, only in patches. Because of this, the smell of petrichor has been few and far between, but that is the nature of this odor. It arrives only to pique our olfactory senses for a day, then evaporate into the sky. Indeed, it is only a reminder of an absence – can it be called the redolent incarnation of nostalgia? Rain-y and nostalgic. Either way, Cupertino needs more of it, or possibly, less of it. Because dryness is necessary for the odor to arise, a return to more frequent and periodic rain would also lessen our days taking in the chemical compounds of petrichor. But it would also help improve the area’s environmental health, and then, perhaps, we would not be nostalgic for the smell at all.
(1) Jyllian Kemsley, “Scent of Rain, Strands of Honey.” Chemical & Engineering News, 91 (2013): 56, American Chemical Society, 25 September 2015, <http://cen.acs.org/articles/91/i18/Scent-Rain-Strands-Honey.html>.
(2) Shannon Hall, “Why Rain Gives Off That Fresh, Earthy Smell,” 22 January 2015, LiveScience, 25 September 2015 <http://www.livescience.com/37648-good-smells-rain-petrichor.html>.
(3) Daisy Yuhas, “Storm Scents: It’s True, You Can Smell Oncoming Summer Rain,” 18 July 2012, Scientific American, 12 October 2015 <http://www.scientificamerican.com/article/storm-scents-smell-rain/>.
(4) Andy Brunning, “The Chemical Compounds Behind The Smell Of Rain,” 14 May 2014, Compound Interest, 31 October 2015.
(5) Cullen R. Buie and Young Soo Joung, “Aerosol generation by raindrop impact on soil.” Nature Communications, 6 (2015): 1-9, Nature Publishing Group, 12 October 2015 <http://www.nature.com/ncomms/2015/150114/ncomms7083/abs/ncomms7083.html>.