With heavy rain lashing several parts of the country, there is a familiar, all-pervading smell that seems to envelope us; an earthy fragrance of nostalgia, it is what makes us love the monsoon more and more. Even as people tend to push travel plans and choose to wait out the rain instead, the whiff of soil, as persistent raindrops lay a wet film of water over it, makes this waiting time all the more memorable. The smell of rain has a name, the not-so-common ‘petrichor’.
The term ‘petrichor’ first came into use in 1964, when scientists Isabel Joy Bear and Richard Grenfell Thomas of Australia tried to explain the earthy smell associated with rain. It was mentioned in the journal Nature, in an article titled, “Nature of Argillaceous Odour”.
Petrichor occurs when rainwater falls on dry soil, hence releasing a mixture of chemicals and oils into the atmosphere. The distinctive smell from the moistening of earth is a result of bacteria, plants, other organic matter, and even lightning to a certain extent. Generally, the characteristic smell is caused by bacteria. Actinomycetes, a filamentous kind of bacteria, usually develops in the soil when the conditions are warm and damp. Eventually, when the soil dries, the bacteria create spores in it. When it rains, the wetness and force of the raindrops propel these tiny spores into the air, where the moisture from the rain performs the role of aerosol. The moist air efficiently carries the spores to us, which we breathe in, and derive the smell. This is a cyclical process.
Further, the smell of rain itself is caused by a chemical in Actinomycetes called Geosmin. However, this is released by the bacteria after their death. Geosmin, which has a very impactful scent, is a type of alcohol molecule. It is a very common type of bacteria, found across the world. Geosmin is produced by streptomyces, which is present in most healthy soils. They are also responsible for creating commercial antibiotics.
Another prominent reason for the smell is the oil that is released by plants into the air. This oil is secreted by plants during the dry period. The fauna of the region, which includes stearic and palmitic acids, add to the whiff of petrichor.
Another scent associated with rain is ozone. When a thunderstorm strikes, lightning can divide the oxygen and nitrogen molecules in the atmosphere, which recombine with nitric oxide. This substance associates with chemicals in the atmosphere to form ozone. This smell is faintly reminiscent of chlorine.
Humans have for long associated petrichor with the arrival of rain. Apart from the physiological reaction, it can also impact the autonomic nervous system. This part of our nervous system controls involuntary body movements like breathing. Petrichor results in decrease in ANS activity, leading to sensations of relaxation, calmness and wellbeing. While the petrichor originating from clay-based soils are earthier, petrichor from sandy soils can be sweeter and citrusy. Petrichor can also be a delight for chocolate lovers, as the aroma given out by rocky soils is likened to melted chocolate.
Meanwhile, it has been found that the rain-drenched soil has been captured in Uttar Pradesh for ages, and sold as a perfume called “Matti ka Attar”.
Although humans are drawn to the scent of Geosmin, the majority of them dislike its taste. Although it is non-toxic, when present in mineral water or wine, it can put people off.