What causes that earthy smell when it rains?

Science & Technology

UTICA, N.Y. (WUTR/WFXV/WPNY-TV) — Have you ever noticed a really earthy smell right before or during rain showers? You might have even heard people saying they know it’s going to rain when they smell it. There’s actually a name for this unique smell and it’s called Petrichor! It’s a term coined by Australian scientists in the 1960’s used to describe earthy smell associated with rain. 

What exactly gives Petrichor its unique smell? The answer may surprise you. It’s a mix of several different things: plant oils, ozone and lightning, and geosmin.

You might be wondering what in the world is geosmin? Let’s start with that. Geosmin is a compound that is secreted from a bacteria called Actinomycetes commonly found in soil. Geosmin is also found in beets! What makes geosmin extra peculiar is that humans are actually quite sensitive to it and the human nose can detect it in the air at less than 5ppt (parts per trillion)!

You might be wondering how ozone and lightning contribute to that earthy smell? Well, when lightning strikes, oxygen molecules in close vicinity are split and rearranged to create ozone, or O3. Those ozone molecules are then carried down to the ground by droplets of rain which contribute to the scent.

Lastly, we have plant oils and compounds which are secreted by plants during dry weather. These oils accumulate in between rocks and in the soil until it rains and they are released into the air. Those oils paired with ozone from lightning strikes and geosmin from Actinomycetes create that earthy smell of Petrichor. For more information, you can visit the ACS (American Chemical Society) here.

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