Have you ever noticed the special smell of the hot, dry soil when the first drops of summer rain fall? I have memories of my childhood of relatives of the field that boasted of being able to "smell the rain" just before a storm.
Of course, the rain itself has no smell. But moments before it starts to rain, a "ground" smell known as petricor pervades the air. People describe it as a musky, fresh and generally pleasant aroma.
This smell comes off in fact when the soil is moistened. Some Australian scientists documented for the first time the phenomenon of Petricor in 1964 and, later, in the decade of 2010, researchers of the Institute of Technology of Massachusetts studied its mechanics.
Petricor is a combination of fragrant chemical compounds. Some of them come from oils made by plants. The most contributing are the actinobacterias. These tiny microorganisms can be found in both rural and urban areas, as well as in marine environments. They break down the dead or deteriorated organic matter into simple compounds ready to turn into nutrients for growing plants and other organisms.
A byproduct of its activity is an organic compound called geosmin, which contributes to the aroma of Petricor. Geosmin is a type of alcohol, such as disinfecting. The alcohol molecules tend to give off a strong aroma, but the complex chemical structure of the geosmin makes it especially perceptible to humans, even in extremely low amounts. Our nose can detect only a few parts of geosmin for every trillion atmospheric molecules.
During a prolonged period of drought, when it takes several days without rain, the rate of decomposition activity of the actinobacterias slows down. Just before an episode of rain, the air and the terrain begin to moisten. This process helps accelerate the activity of the Actinobacterias and forms more geosmin.
When raindrops fall to the ground, especially on porous surfaces such as loose soil or rough cement, they splash and eject tiny particles called aerosols. Geosmin and other petricor compounds that may be present in the soil or dissolved in the Raindrop are released in aerosol form and transported by wind to surrounding areas. If the rain is strong enough, the Petricor aroma can travel quickly with the wind and alert humans that the rain is coming.
The aroma finally fades when the storm passes and the terrain begins to dry. The Actinobacterias are waiting, willing to help us know when it might rain again.