Everyone, or at least almost everyone, will have smelt the smell of rain as a big thunderstorm starts to bucket it down after a hot summers day. Most of us don’t know why this smell appears, however, a few years ago, American researchers explained more about this natural phenomenon.
Even though a scent floats in the air when the rain comes, the rain itself does not smell. Why and how does this happen? This phenomenon was first studied in 1964 in Australia when the name “petrichor” was christened. However American researchers from the Massachusetts Institute of Technology (MIT) explained this process in more detail in a study published in 2015 in the journal Nature Communications.
Petrichor is a mixture of odorous chemical compounds generated by plants also known as actinobacteria. They are found everywhere, whether in marine, urban or rural areas. These microorganisms fix nitrogen from the air for plants, and make their nutrients by transforming the decomposing organic matter.
One of the organic compounds which is involved in this process is geosmin and it is this compound which gives this recognisable, petrified smell when the rain falls after hot weather. It is a very easy compound to detect because there only needs to be a few particles in air for our nose to detect it.
During the summer, periods of drought mean there is less actinobacteria activity. However when the rain comes, this decomposition activity accelerates and more geosmin is produced. When drops of rainwater falls, geosmin and other petrichor compounds are released which potential winds can help to diffuse. As geosmin contributes to the petrichor smell, this is where the smell of freshly sodden ground comes from after a period of dry weather!
Here is the video that was published at the time of the 2015 study by the MIT scientists: