What causes the smell after rain?

There’s a particular smell in the air after rain has fallen on dry ground, a distinctive fresh sort of scent called petrichor. But what is it about the interaction of the rain and the earth that causes it? Two scientists at M.I.T seem to have discovered the answer: bubbles.

(Image: Luigi Mengato).
(Image: “rain” by Luigi Mengato is licensed under CC BY 2.0).

Cullen R. Buie and Youngsoo Jong found that a drop of water falling at a certain speed can trap tiny air bubbles beneath it as it hits the ground. These bubbles capture molecules from the soil and as the water drop flattens, becoming less of a drop and more of a tiny puddle, these molecules are released up into the air. This causes the scent we call petrichor.

The best conditions for its creation seem to be when light to moderate rain falls on dry soil. The reasons for this are simple. If the soil is already damp or wet then the molecules will be too heavy to be captured by the bubbles; if the soil is dry but the rain is too heavy the molecules will be forced back to the surface by the falling drops.

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