Before stormy, usually in summer, we say that it "smells like rain". That shared sense, of course, is a scientific explanation; although it's worth explaining that start the rain does not have odor. Now, yes, he's called a petricor The combination of chemical compounds that have been separated from plants and soil by wetting the environment, before rain, which produces that typical aroma.
In fact, "odor" has rain as a main generator and actinobacteria, a type of bacteria that are very important for decomposition of an organic matter – such as cellulose and citin – which become nutrients for plants. That process appears and geosmin, which means "smell of the earth" in Greek.
In chemical terms, Geosmin is a type of alcohol, which makes it emit a strong smell, to the point, although there are very few molecules in the air, which we can find. Our sense of smells can find very few geosin molecules for every trillion of molecules that circulate in the atmosphere.
But why is it more common to see in summer or on drier plots? The activity of the actinobacteria is, in periods of drought it becomes slower and concentrated. Therefore, at the time of wetting the ground before rain, the aroma is more intense.
Also, if the soil from earth, the first drops will extend that sensory, as the water will scatter some particles of the name aerosols that will carry the geosmin.
This explanation, which has been developed for years in different studies, adds to the concept Smell is one of the most complex senses have the human, for the great range of aromas that we can find.