When those first fat drops of summer rain fall to the hot, dry ground, have you ever noticed a characteristic odor? I have adolescence memories of family associates who were farmers anecdotic how they could always “smell rain” right before a storm.

Of course rain itself has no scent. But moments before a rain event, an “earthy” smell known as petrichor does charge the air. People call it musky, fresh – about pleasant.

This smell absolutely comes from the moistening of the ground. Australian scientists first accurate the action of petrichor accumulation in 1964 and scientists from the Massachusetts Institute of Technology added advised the mechanics of the action in the 2010s.

Petrichor is a aggregate of ambrosial actinic compounds. Some are from oils made by plants. The main contributor to petrichor are actinobacteria. These tiny microorganisms can be found in rural and urban areas as well as in marine environments. They decompose dead or corrupt amoebic matter into simple actinic compounds which can then become nutrients for developing plants and other organisms.


A byproduct of their action is an amoebic admixture called geosmin which contributes to the petrichor scent. Geosmin is a type of alcohol, like abrading alcohol. Booze molecules tend to have a strong scent, but the circuitous actinic anatomy of geosmin makes it abnormally apparent to people even at acutely low levels. Our noses can detect just a few parts of geosmin per abundance of air molecules.

During a abiding period of boredom when it has not rained for several days, the atomization action rate of the actinobacteria slows down. Just before a rain event, the air becomes more humid and the ground begins to moisten. This action helps to speed up the action of the actinobacteria and more geosmin is formed.

When raindrops fall on the ground, abnormally porous surfaces such as loose soil or rough concrete, they will beat and eject tiny particles called aerosols. The geosmin and other petrichor compounds that may be present on the ground or attenuated within the raindrop are appear in aerosol form and agitated by the wind to surrounding areas. If the condensate is heavy enough, the petrichor scent can travel rapidly downwind and alert people that rain is soon on the way.

The scent eventually goes away after the storm has passed and the ground begins to dry. This leaves the actinobacteria lying in wait – ready to help us know when it might rain again.The Conversation