Is there a distinctive smell associated with rain?

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Dear Tom,

Sometimes there is a very distinctive smell associated with rain. Am I imagining it, or is it real?

Matt R, Chicago

Dear Matt,

It is not your imagination. “Petrichor” is the name given to the phenomenon of the smell sometimes released during a rain. In studies conducted in the last three years at the Massachusetts Institute of Technology, Professor Cullen Buie and graduate students working with him have identified the mechanism by which raindrops cause aerosols to be released into the air. They used high-speed cameras to record what happens when a raindrop hits a porous surface. The raindrop starts to flatten. Simultaneously, tiny bubbles of air are trapped at the point of contact. The bubbles then shoot upward, bursting from the drop in a fizz of aerosols. The aerosols, released during light or moderate rains, not so much during heavier rains, account for the familiar “smell of rain.”