Curious about…The Smell of Rain.
This morning I woke up here in Chicago and went outside: last night there were epic thunderstorm and today, broody clouds were scowling on the horizon. And then I smelt…that smell: the smell of rain, the scent of crackling electricity in the air, the sweet freshness tempered by the slightly acrid wet concrete - the smell…of Ozone.
Scientifically speaking…“The smell of rain is caused by ozone (O3). It is commonly produced through dissociation of molecular oxygen (02) into two unstable atomic oxygens (2 O), each of which then recombine with molecular oxygen to make ozone. This dissociation can occur either by high-voltage electrical discharge or by bombardment with ultraviolet light. The high voltages which occur in thunderstorms create ozone within the cloud (even when lightning is not actively occuring), and this ozone is carried toward the ground by the downdraft in the thundercloud and blown out ahead of the storm, where you smell it and can tell rain is coming.”
In wooded areas, small bacterial spores called Actinomycetes are kicked up by thewater droplets and dispersed in the same way aerosol air fresheners work. These have a distinctive earthy smell. In addition, plants release all kinds of oils and congeners in response to their environment, including rain. Acid rain also plays a role as well. Acids commonly react with other chemical, and our built environment is full of chemicals, including gasoline and diesel, motor and food oils, paints and plastics, and lots more. These reactions release gases that can smell strongly.
For many, the most evocative ozonic is wet sidewalk concrete. Some rocks such as limestone also smell like this when wet. Science tells us it is the slow evaporation of calcium; small particles are released into the air which enter your nose, and there’s a gaseous calcium-containing molecule produced by interaction between carbon dioxide, water, and a calcium compound.
Fragrance company Demeter makes a single-note ozonic fragrance called “Thunderstorm.” Their website describes it (and the whole phenomena) beautifully: “Have you ever considered the olfactory side of rain? You know it’s coming, you can smell it in the air. It does seem that on summer days when it is hot and dry, with a thunderstorm brewing just over the next hill, you can `smell the rain.’ Well, you can smell something, but rain? Have you ever tried to smell this same rain in January when the ground is frozen solid? Not a chance, but when the ground and plants are warmer, you can smell something. What you really smell comes not from the air, but the ground! Plants release oils that enter the soil and blend with the other earthy odors. These odors are released into the air when the relative humidity at ground level exceeds 75 percent. Moist humid air will transmit odors far better than dry air. In these moist humid conditions we notice these odors more readily. And since rain is so often connected with moist humid air, we tend to associate one with the other. The Demeter Fragrance Thunderstorm captures this complex sensory moment perfectly. Like poetry, Thunderstorm is subtle and difficult to define, but real, with Thunderstorm reflecting the deep and violent nature of a summer storm.”
What Of It? There is something universal about this - I was in China a few months ago and noticed, after a particularly intense bout of rain, the familiar electric crackle-smell in the air; asking people about it, they all talked nostalgically about nature, happier times and the memories that it evoked. Fragrance manufacturers may try hard to recreate ozone, but there is something so indefatigable about it, hard to describe, deeply personal.
I Am Curious about nature and its ability to evoke profound sense memory, about universal experiences that each of us can deeply personalise, about nostalgic places, smells, and emotions.
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