Mesocyclone

A very large area of rotation within a convective storm, from which a tornado may be formed. A local tornado warning is often issued once a mesocyclone signature is detected on radar. Thick-feeling humid air, and scents of harvested fields mixing with a hint of rain. It is not a realistic interpretation of an atmospheric condition, and is not a petrichor/rainy/aquatic fragrance at all, but heavy on greenery.