Why Does Rain on Dry Soil Release an Earthy Smell?

When rain first reaches land after a dry spell, the air often acquires a distinctive earthy smell. It is not the smell of rainwater itself. Volatile substances already held in soil and rock are being carried into the air.

The scent is often called petrichor. In 1964, Australian researchers Isabel Joy Bear and Richard Thomas used the term while describing oily material that accumulated in porous surfaces during dry periods and was released when wetted. Geosmin produced by soil microorganisms is another familiar source of earthy odour. Human noses are highly sensitive to it, so only a small airborne amount may be noticeable.

How does the scent leave the ground? High-speed imaging has shown that when a raindrop strikes a porous surface, it can trap a tiny pocket of air beneath the drop. The bubble rises through the water and bursts at the surface, ejecting microscopic aerosol particles. These particles can carry volatile compounds from the soil into the surrounding air and eventually towards the nose.

This mechanism also helps explain why light or moderate rain can sometimes produce a stronger scent. The drop must strike a wettable porous surface at a suitable speed for bubbles and aerosols to form. Heavier rain is not automatically more fragrant: forceful impacts or a surface already covered by water change the conditions. The “smell of rain” is therefore a result of water, soil, microbial products and droplet impact working together.

https://doi.org/10.1038/201993a0
https://doi.org/10.1038/ncomms7083