Prior to a storm, usually in summer, we say that "it smells in rain." That shared feeling has, of course, a scientific explanation; although it is worth clarifying from the beginning that Rain has no smell. Now yes, it is known as petricor to the combination of chemical compounds that emerge from plants and soil when wetting the environment, before rain, which creates that characteristic fragrance.
In reality, the "smell" in rain has the main generator a the actinobacteria, a type of bacteria of great importance for the decomposition of organic matter – such as cellulose and quitin – which become nutrients for plants. That process goes out the geosmine, that in Greek means "aroma a tierra".
In terms of chemicals, Geosmine is a type of alcohol, which makes it a strong aroma, at such a point that, although there are very few molecules in the air, we are able to detect it. Our smell is able to perceive very few molecules of geosmine per billions of molecules that circulate into the atmosphere.
But why is it more common to perceive it in summer or dryer land? It is that, in periods of drought, the activity of the actinobacteria It becomes slower and concentrated. Therefore, at the time of moistening the soil before a rain the aroma is more intense.
Likewise, if the soil is ground, the first drops will extend that feeling, since the water will scatter some particles called spray they will transport the geosmine.
This explanation, developed for years in different studies, is added to the fact that The smell is one of the most complex senses which has the human being, for the great range of aromas that we can perceive.