Asked by: Carl Dennis, Cumbria
The fresh, earthy smell that we associate with rain, known as ‘petrichor’, comes from a variety of sources. During dry conditions, certain plants release oils, which then become airborne under the impact of rain.
Falling raindrops also stir up the soil, propelling a musky-smelling chemical secreted by bacteria, called geosmin, into the air. Finally, lightning can split molecules in the atmosphere to form ozone (O3), which has a chlorine-like tang. Some scientists speculate that the importance of rain to our survival has led us to evolve to enjoy its smell.