No. 31Nature3 minviews 24
Published by GK Library Annex
An earthy smell appears just before it rains
Open the window and the smell of earth arrives before the raindrops do. Where does it come from?
Just before it rains, or as the first drops begin to fall, an earthy smell often wafts up. It isn't something the rain brings down from the sky; it comes from the ground. Scent compounds that built up in dry soil are kicked up by the first raindrops and spread through the air.
A smell made by soil bacteria
In 1964, two Australian scientists extracted an oily substance from dry soil and rocks and gave the smell a name: petrichor. Their explanation was that oils released by plants during dry spells soak into soil and stone, then get freed when rain arrives.
At the heart of the earthy smell is also a compound called geosmin. It's made by actinomycetes living in soil, especially the Streptomyces group, and the human nose is so sensitive to it that people can reportedly notice just a few nanograms in a liter of water.
Raindrops flick the smell upward
How the smell gets into the air was shown in 2015 by researchers at MIT using high-speed cameras. When a raindrop lands on porous ground like soil, the air inside rises as tiny bubbles that burst, flinging very small droplets into the air. Those droplets carry soil compounds and drift away on the wind.
This happens readily in light rain, while heavy rain produced fewer bubbles. That's why the smell often seems stronger when a few drops are falling than during a downpour.
The first rain after a dry spell smells strongest
The drier the ground has been for several days, the more scent compounds have built up, so the first rain smells stronger. When the ground stays wet, as in the rainy season, the smell is weaker, and it's faint on asphalt roads with no soil.
When you smell it before any rain falls, the wind may be carrying the scent from drops that already fell nearby. The sharp, metallic smell as a thunderstorm approaches isn't the earthy smell at all; it's ozone made by lightning, carried down on the wind.
The same smell in tap water
When tap water sometimes smells earthy in summer, it's often geosmin made by algal blooms. It's known not to be harmful, but our noses are so sensitive that even tiny amounts stand out. The earthy taste of beets comes from the same compound. Why our noses are so keenly tuned to this smell still isn't clearly understood.
Related reads
The smell of rain comes from the ground, not the sky. The first raindrops toss geosmin and plant oils that built up in dry soil into the air.
Notes and references · Bear and Thomas 1964, Nature; Joung and Buie 2015, Nature Communications. The study that first extracted the scent compounds from dry soil and rocks, and an experiment that filmed tiny droplets leaping up as raindrops hit soil. How strong the smell is depends on the type of soil and the weather.
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