Our noses detect trillions of different smells and scents throughout our life. Some of them even have a deep impression and are emotionally tied to our memories. If you were told to list the most pleasant smells that your nose knows, the sweet smell of rain would definitely be at or near the top of the list.
The moment when the raindrop meets the thirsty earth and interacts with dirt, it releases a fresh, musky odour which soothes your nose. Pluviophile meaning those who love rain, eagerly wait for this scent the entire year.
But is that even possible because rain is nothing but water, and water doesn’t have any distinctive scent? So now the question arises – From where does the earthy smell of rain come? What is the smell of rain called?
Let us find out the reason behind this phenomenon.
Content: The Smell of Rain
Petrichor
The smell of rain has a specific name – Petrichor. As the rain begins to fall on dry ground, the pleasant smell that springs up from the soil is ‘Petrichor’. The smell of Petrichor specifically accompanies the 1st rain after a long dry spell. This is the exact reason why some of us eagerly wait for rain, as it fills the dry weather with a delightful aroma.
The concept of Petrichor was first introduced in a research paper titled “The nature of Argillaceous odour” in the year 1964. Australian scientists Isabela Bear & Dick Thomas were the two who published this research work in Nature Journal. They experimented with a wide variety of rocks by heating them upto 600° C. They did this to ensure that the rock was free from any living microbial entity. Then they left the rock outside for months to a year and protected it from any kind of moisture. Later after a particular time, they were subjected to steamed distillation.
Steamed distillation: It is a common technique which is used in perfumery to extract the essence out of varied elements to make perfumes.
After the distillation finished, they performed a chemical analysis of the components that were released (In a similar manner as they might have been released after the natural rainfall). This theory was again studied and proved by scientists at the Massachusetts Institute of Technology in 2010. Here they researched the mechanism behind this process of petrichor formation.
Meaning of Petrichor
The term Petrichor originates from the Greek names – Petros and Ichor. In Greek, Petros means stone, while Ichor refers to the eternal essence of the blood of Greek gods.
Petrichor Scent
The reason behind the smell of rain is the combined outcome of biology and chemistry functioning at the right timing. According to the researchers, there are basically three main factors which contribute to the production of this smell of rain. So, we can say that the smell of rain is not just because of one element; instead, the Petrichor is the combination of different components. These components are nothing but fragrant chemical compounds produced by some chemical or biological procedure occurring at the same time.
Can you smell Rain coming?
Have you ever noticed that storm winds before the rain has a significant scent? At times when a thunderstorm is rolling, you might sense a medley of separate smells. But among them, the earthy smell of rain that precedes the storm can be easily traced.
Smell Before Rain
The smell before rain is because you are probably smelling ozone. So, by this smell, you easily affirm that the rain is coming. Ozone itself means “to smell” in Greek.
Smell of Ozone
Technically ozone is always present in the atmosphere, but you can only smell it during storms. This is because when the electric charge of lightning strikes, it splits the diatomic molecules of oxygen and rearranges them into ozone consisting of three oxygen molecules.
The advancing winds and droplets of rain carry the ozone to bring the scent down to your nose level.
Smell after Rain
Plants Oils
When there is no rain for a prolonged time, the plants growing in that dry weather starts secreting some chemical compounds, basically oils. These oils accumulate in between the rocks and soil. The main function of this oil is to signalling other plants to halt the growth of their roots and inhibition of seed germination to reduce the competition for water.
As soon as the thunderstorm comes after a dry spell, these oils are released into the air to contribute to the earthy smell of Petrichor. The most common plant oils that contribute to the smell of rain are long-chained hydrocarbons with a carboxylic group and methyl group of either end, respectively. Examples are Stearic acid and Palmitic acid.
Geosmin
Actinomycetes are the type of soil bacteria which release a complex chemical compound called Geosmin. These bacteria mainly contribute to the decomposition of organic debris present in the soil, and during this decomposition reaction, Geosmin is synthesized.
Geosmin is non-toxic to humans. It is a kind of alcohol, just like rubbing alcohol which tends to have a strong, pungent smell. The human nose can easily notice the Geosmin even at its low levels. It can even detect a few parts of Geosmin that remain hidden in trillions of air molecules.
Since the decomposition process requires moisture to occur, thus it ceases during dry environmental conditions. That means no decomposition, no Geosmin. But as soon as the rainy season comes, the soil becomes moist. Consequently, the decomposition rate of the actinomycetes tends to increase with the increase in humidity level. Thus, as the rainy season arrives, they begin to release more and more amount of Geosmin.
When the raindrops hit the soil surface, the Geosmin mixes with the plant oils. This combination of Geosmin and plant oil gives out the smell of Petrichor, which we receive after the rain.
Rain Smells
Not always the after-rain smell pleasant to smell; sometimes it can be an unpleasant odour too. This mainly occurs due to the acidity of rainwater. When the rainwater is acidic and comes in contact with organic matter and other chemicals, it stimulates some aromatic reactions. Gradually, it percolates the soil and reacts with the minerals trapped inside it like gasoline. These chemical interactions produce strong, unpleasant odours that might irritate your nose.
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