We often use the phrase ‘the smell of the first rain’. This isn’t poetic or something which is relied upon fantasy. During my childhood, amidst of the scorching heat when I heard of the first raindrop falling on our asbestos, even I would jump and say ‘The inaugural smell of joy’.

Of course rain itself has no distinct inherent odour. But when the drop amalgamates the Earth, a smell known as petrichor is diffused into the air. This smell is a result of a physical reaction between the dust particles present in the ground and the incoming fresh water droplets. This phenomenon of petrichor was first demonstrated by a group of Australian scientists in the year 1964. About half a century later, in the year 2010, a team of researchers from the Massachusetts Institute of Technology studied the kinetics of the diffusion of this phenomenon.

It was hypothesised that petrichor is a blend of fragrant chemical compounds including microbial secretion and liberation of fragrant oils from the plants. Out of these responsible factors, the major chemical responsible for this fragrance is called geosmin. Geosmin are the by product of a certain class of microorganisms known as the actinobacteria. The microorganisms are majorly the habitats of rural and urban grasslands. They convert the organic matter and the decomposed matter into simple chemical compounds which in turn become the nutrients for plants. Geosmin is a complex mixture of alcohol and ester. This mixture liberates a strong fragrance, which is noticeable to people like us even at very low levels (ppm level). The report says that our noses can detect only a few parts of geosmin per trillion air molecules.

During the period of scorching heat, the decomposition kinetics and the activity rate of the actinobacteria retards. As the rain begins to pour, the activity enhances being exposed to favourable conditions and the process automatically speeds up which accelerates the formation of geosmin.

As the raindrops hit the ground, the porous surfaces open up and eject minute particle called aerosols. These geosmin particles coupled with other petrichor compounds gets dissolved with the raindrop and are released in the form of aerosols during the process of diffusion. As the intensity of the rain increases, the intensity of the smell increases because the rate of the formation and the diffusion of aerosols go up.

The fragrance eventually fades away when the rain stops and the entire cycle gets repeated once again during the process of rainfall.