WHAT CAUSE THAT EARTHY SMELL AFTER THE RAIN?
Rainy days are the favorite of many, especially when you can spend them at home in the warmth, perhaps with a cup of tea and cookies. And it is certainly one of the most pleasant things, that of listening to the rain pouring down from the window and it is equally pleasant also to feel, when everything is more calm, that characteristic smell in the air.
But have you ever wondered what it’s really called? Many call it the typical “smell of rain” but in reality it has a specific name and is that of petrichor.
THE SMELL AFTER THE RAIN: THE PETRICHOR
This term comes from the greek and its etymology is nothing short of epic: it comes from petra which means stone, and there isn’t anything special here, and ichor, who in Greek mythology was the blood of the gods and is said to possess immortality. So this Ichor was the mythical fluid that gushed from the bodies of the gods and immortals the very rare times they were wounded by some unfortunate fool, and it was like poison for all mortals who had the courage to get so close to a wounded divinity.
On a fantastic level then, the fragrance we feel after the rain would be the celestial blood of the stones. And it’s a very curious way to define it, don’t you think?
On a scientific level, however, petrichor is nothing more than a very particular natural aerosol.
The scent of rain derives from the fact that, when the precipitation of the raindrops on the ground occurs, very small air bubbles remain “suspended” on the surface, trapping at the same time solid particles of various types coming from the soil. The production of these aerosol particles is greater the slower the drop rate is, which is why this odor is typical of light rains in particular.
And it is precisely from the ground therefore that the substances that we feel in the atmosphere come after raining.
GEOSMIN: THE FAMOUS “SCENT OF WET EARTH”
Particular chemical components generate this aroma of wet earth, defined as geosmin, which are released by particular bacteria and microbes, including cyanobacteria or blue algae, present on soil and which react precisely in the presence of humidity.
Geosmin is one of the main components of petrichor in that it gives it the characteristic “earthy odor” defined precisely in the meaning of the term, an odor that we all remember with pleasure, and which therefore is not formed as one might perhaps think from a single odorant element, but by several elements that we distinguish as a single smell.
The interesting thing is that we as human species are biologically inclined to identify this compound at great distances and with great ease.
Consider that man’s sensitivity to geosmin and therefore also to petrichor reaches low concentrations of 5 ppt (parts per trillion) which is a lot considering that we humans are not very developed in this sense, and in terms of olfactory evolution many animals outnumber us; despite this however, perhaps thanks to the particular conformation of these aromas or even better for a fortuitous biological convenience, this smell is perceived more than others. In this regard, it is in fact thought that men as species have specifically adapted to this aroma because the smell of petrichor was related to the presence of water resources in the area, especially in arid lands where this became even necessary for the survival.
This makes us think about how much of what we feel and perceive nowadays had a meaning and importance completely different i to our ancestors than for ourselves now, and aspects of life that we often neglect may have been fundamental factors in the past.
The earthly smell or geosmin is also part of one of the seven major categories of odors described in classification Amoore(which is only one of many olfactory records systems), together with camphor with the intense and penetrating smell that characterizes it, ethereal with the typical sweet smell of cleaning fluids, peppermint, musky, floral, putrid and pungent.
THE SMELL “BEFORE” THE RAIN
Now that we know where the smell released after a refreshing rain comes from, it’s time to find out what generates that specific smell sometimes present before some storm.
If by chance you are going through one right now in your homes, open the window if it does not rain yet, and breathe for a while the dense air before the approaching tempest rough on you with fury.
It is similar to petrichor but you will notice, if you sense carefully, that it is another essence: it is in fact the ozone, which for humans can seem similar, on an olfactory level.
The ozone in the stormy clouds is produced through the cleavage of molecules of oxygen and nitrogen present in the air due to the electrical discharges of lightning which initiate the reaction, producing nitrogen monoxide which recombined with other compounds produces the ozone, which has an odor that vaguely resembles that of chlorine.
Even at this odorant element we are particularly sensitive, that we are able to sense it in large distances, and it is for this reason that before the storm approaches with the rain and with the lightning, we are able to perceive the odor thanks to the ozone transported by the wind, produced by the lightning activity of she storm.
A perfect example of this are the Tesla coils, which if in operation, the generation of “artificial” lightning creates a phenomenon almost identical to that of storms, with the cleavage of atmospheric molecules and its recombination in nitrogen oxides and ozone with that pungent and characteristic smell.
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WHAT CAUSE THAT EARTHY SMELL AFTER THE RAIN? WHAT CAUSE THAT EARTHY SMELL AFTER THE RAIN? WHAT CAUSE THAT EARTHY SMELL AFTER THE RAIN?