THE MYSTICAL RAIN SMELL AND ITS SIGNIFICANCE

There are several incidents which occur during Rainy season. Filling up of empty ponds, a rise in the water level in dams are few incidents which occur during rainfall. There is another incident which occurs when the rainfall hits the dry ground for the first time after a long break. Yes, it’s the smell which occurs during the start of rain. But what is the real reason behind that Smell? Is it from the rain or from the soil? This post explains elaborately about the reasons for Rain-smell and its significance.

Rain is accompanied with Petrichor for every first time in a while

THE SMELL OF THE RAIN - PETRICHOR

A Distinctive odour is produced during the fall of rain-droplets after a very long time on the dry land. It is very pleasant-smelling odour and is sometime referred as the “Earth’s smell”. People tend to call that as the Earth’s smell or Rain smell but the real scientific name given to the pleasant smell which occurs when rain falls on a land after a long time is termed as Petrichor. studies began in the 1960s about the smell of the rain and now it has become well advanced as the scientists and researchers were able to uncover much secrets on this distinct rain smell – Petrichor.   

Petrichor smell can either come from the rain or from the land it falls. Clearly, we know that the rain is made of water droplets which are tasteless and odourless. Even though there are several gases which gets dissolved in rain droplets during their journey from the sky to the land, nothing adds up smell to the rain. Thus, we can easily conclude that the smell of rain or Petrichor is due to the compounds present in the land.

The Petrichor smell is made of several compounds which are present in the soil, plants etc. This smell is formed once the land gets humidified after a very long time – mainly during the onset of rain after a lengthy summer or arid climate. One of the primary compounds for the production of Petrichor smell is “Geosmin” which belongs to the alcohol group. Geosmin stands for “Earth smell”. Generally, the compounds with alcohol group like Menthol, Carvacrol will have pleasant smell. But the thing that makes Geosmin unique is that even in a minute concentration (0.0000000005% or 5parts per trillion), it can be sensed by human nose. That is why we can clearly distinguish the rain smell or Petrichor very quickly at the beginning of the rainfall.

GEOSMIN 

Geosmin is clearly an alcoholic group-based compound. It is synthesized by blue-green algae (cyanobacteria), Actinomyces and some other bacteria. Geosmin can be used to predict the quality of water as the increased amount of Geosmin in the sea water indicates the presence of large groups of blue-green algae. The Actinomyces group are normally occur in the soil. They release Geosmin when they degrade the organic matter (get nutrition from degradation of organic matter). During summer or hot climate, the degradation rate will be slow due to reduced moisture content in sand. So, when the rain-droplets fall on the ground, it increases the moisture content and thus the Actinomyces have all the primary factors needed for its metabolism and the degradation rate will be higher along with increased Geosmin secretion. 

Geosmin - Primary compound for Petrichor

SIGNIFICANCES

 The Rain smell or Petrichor not only attracts the human, but also few insects which lives in the soil. A recent study has claimed that ‘Springtail’ a soil living insect is very much attracted to the Geosmin smell and it uses the smell to prey on the bacteria that produces these smells. Most people have a feeling that the Springtail insects are harmful pests as people still fear that these insects will destroy crops and harm human. But these insects do not harm any human beings or plants while it helps to get rid of some unwanted creatures in the soil. 

But the story is not over when the insect preys on the bacteria – it has just started. All the bacteria which come under Streptomyces form spores to produce new bacteria. As soon as the actinomyces produce Geosmin, they also produce spores on their exterior side. So, they just need a vector for transmitting the spores and the Soil living insects come to their aid. They propagate the bacteria with the spores they consume or with the spores that are adhered to their body. 

Mechanism of Spore transmission by Springtail. Thanks to Geosmin for attracting a Vector.

Actinomyces and Streptomyces are genus that belong to the family actinobacteria, and helps in the production of several vaccines and also helps to keep the environment clean by degrading dead organic matter. Thus, the smell of the rain not just gives us a pleasant feel but also helps in the propagation of these prestigious bacterial spores. 

Also, in case of heavy rain accompanied with strong wind currents, the Petrichor or rain smell can travel some distance along with the wind thereby forecasting the people about the upcoming rain. That is why sometimes we smell the petrichor or rain smell before the fall of rain droplets.

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