We've all been outside after a period of warm, dry sunny weather when the heavens open, down comes the rain and we inhale that earthy smell.
But what is it?
That distinctive earthy smell is known as Petrichor and is the scent produced when rain falls onto dry soil or ground.
As the rain falls onto very dry soil, they trap tiny air bubbles on the surface. These air bubbles are a combination of fragrant chemical compounds. Some are from oils made by plants, but the main contributor comes from actinobacteria (a distinctive group of bacteria). These are then propelled upwards, bursting out aerosols of scent into the air which are then spread around by the wind.
This scent is most noticeable during light to moderate rain falling onto sandy or clay soils. The heavier the rainfall, the speed of the drops suppresses the bubbles from forming preventing the aerosols from being released into the air.