Science

The mystery behind the smell during rain

The distinct pleasant odor during the first raindrop on the soil is truly mesmerizing.

After the sweltering heatwave, the raindrops of the rainy season havetheir own charm. The specific scent during light rainfall is called “petrichor”. This word is derived from two Greek words; “petros” (a stone) and “ikhor”(according to the Greek mythology this is referred to the special fluid, flowing in the veins of their God) from and it is  Apart from the highly polluted area, rainfall does not contain any specific smell. So what is the reason behind such beautiful earthly smell during light rainfall? How does the rainfall add extra fragrance to the environment? Actually, chemistry is solely responsible for such a scent. Let’s find out.

Rainfall and soil contain a different set of chemicals. After a long dry period, when the first raindrops come in contact with soil, it leads to some interesting chemical reactions promoted by some bacteria and plants. The soil harbors a particular spore-forming bacteria called actinomycetes, which produce a chemical called “Geosmin”. The word “Geosmin” comes from two Greek words “ge” (the earth) and “osme” (the odor or scent). This chemical imparts the earthly scent. Geosmin is produced by those bacteria during spore formation or when they die. The chemical geosmin has such strong smell that humans can detect it at a very low concentration of 5 parts per trillion. It is observed that light rainfall events with little breeze make this scent more intense. Actually, during heavy rainfall, there is a complete washout of aerosols which leads to loss of flavor. Although the geosmin molecule possesses a beautiful scent, it does not taste good.

Keeping this aside, another theory is also available behind this smell. When the weather is stormy, the lightning breaks down the nitrogen gas molecules into oxygen and nitrogen in the environment. These oxygen molecules react with atmospheric oxygen and form ozone gas. This ozone gas comes downward along with or before the rainfall event that imparts a fresh scent.

Another theory suggests that aromatic plant oil is the reason behind such a musty scent. It is believed that plants secrete some oil-like molecules during the dry period, which gets absorbed by the surrounding clay. Actually, those aromatic substances are secreted as a survival mechanism, since the dry period is unsuitable for plant growth, these oily substances reduce the seed germination process. The smell is also dependent on the duration of dry period, as it increases the storage of oily molecules in the soil and during the advent of the rainy season those chemicals are spread throughout the area.

Interestingly, even before the characterization of such smell, it had been used in Indian scent as “mittika attar” or the earth perfume along with sandalwood oil. So, from now on, enjoy the rainfall and be aware of the scientific reasons behind this. Aren’t these scientific facts highly interesting?

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