
Everyone’s familiar with the smell after a heavy rain shower in the summer – or, for that matter, the ominous scent of an oncoming thunderstorm. In the third part of the Aroma Chemistry series, this graphic examines the chemical compounds that are the major contributors to these smells, and how they arise.
Different compounds are involved to varying degrees in the smell of rain, depending on whether we’re talking about the post-rain or the pre-rain smell. The three major origins of these compounds are bacteria in the ground and soil, oils released by plants during periods of dry weather, and electrical charge in the vicinity of thunderstorms.
Firstly, let’s consider the bacteria, as the compound they produce is the most widely cited whenever the smell of rain is discussed. It’s a particular class of soil-dwelling bacteria, called actinomycetes, that produce the compound we’re interested in – geosmin. They secrete it into the surrounding soil, and it is then disturbed by rainfall, spreading in the air and allowing us to detect it . Incidentally, there doesn’t need to be much geosmin in the air at all for us to detect it. It’s been estimated humans can detect geosmin at concentrations as low as 5 parts per trillion, which is roughly equivalent to a teaspoon of geosmin in 200 Olympic swimming pools.
Geosmin is also present in beetroot, and responsible for its earth flavour. Additionally, the presence of geosmin in water can cause an unpleasant, muddy taste. However, as far as the smell of rain goes, it isn’t the sole culprit.
Another factor is the oils produced by plants, particularly when there is a relatively long spell of dry weather. Studies have suggested that plants produce a particular mix of oils during dry weather, designed to inhibit growth and minimise competition for water. These oils collect in the soil and in rocks; rain then causes a range of smaller, volatile compounds within them to be released into the air. Their combination with geosmin in the air causes ‘petrichor’ – the name given by scientists in 1964 to the smell after rain.
The smell before rain, however, has a different cause. Particularly, the smell before a thunderstorm is a consequence of the electrical charge present in the atmosphere. This causes the splitting of some oxygen molecules in the atmosphere into individual oxygen atoms, which can then combine with other oxygen molecules in the atmosphere to form ozone, O3. Ozone has a sharp odour, compared by some to that of chlorine, or of burnt wires. It’s unstable in the lower atmosphere, and is usually only found higher up – however, the downdrafts of wind produced by a storm can sweep it down from the higher atmosphere, making it possible for us to detect it, and giving the calm before a storm that ‘pre-rain’ smell.
The graphic in this article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Want to share it elsewhere? See the site’s content usage guidelines.
References & Further Reading
- “Fatty Acids from Exposed Rock Surfaces” – I J Bear & Z H Kranz
- “Genesis of Petrichor” – I J Bear & R G Thomas
- “Petrichor & Plant Growth” – I J Bear & R G Thomas
- “Geosmin, an Earthy Smelling Substance” – N N Gerber & H A Lechevalier
55 replies on “The Chemical Compounds Behind The Smell Of Rain”
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[…] Compound Interest blog (which is about chemistry, not economics) has a really nice graphic and post that breaks down the different parts of petrichor and explains where they come […]
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[…] Everyone's familiar with the smell after a heavy rain shower in the summer – or, for that matter, the ominous scent of an oncoming thunderstorm. In the third part of the Aroma Chemistry series, th… […]
[…] Everyone's familiar with the smell after a heavy rain shower in the summer – or, for that matter, the ominous scent of an oncoming thunderstorm. In the third part of the Aroma Chemistry series, th… […]
[…] The after-rain smell, which scientists call “petrichor,” comes from soil-dwelling bacteria and p… […]
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As a perfumer in training, I find your site very useful and I wish to thank you for sharing your information!
Thanks, and you’re welcome! I’m actually looking at doing a graphic specifically on perfumes in the not too distant future.
Can’t tell you how excited that makes me! Please let me know when it comes out and I’ll share it with my other classmates!!!
Chemtrails and chembombs are the cause of the bad chemical smell in the air. Stay indoors!
[…] petrichor. The word was coined by scientists Bear and Thomas in the 1960s. The smell itself is a mixture of compounds. Geosmin, given off by bacteria in soil, adds an earthy smell (it’s also found in beetroot). […]
[…] petrichor. The word was coined by scientists Bear and Thomas in the 1960s. The smell itself is a mixture of compounds. Geosmin, given off by bacteria in soil, adds an earthy smell (it’s also found in beetroot). […]
[…] petrichor. The word was coined by scientists Bear and Thomas in the 1960s. The smell itself is a mixture of compounds. Geosmin, given off by bacteria in soil, adds an earthy smell (it’s also found in beetroot). […]
[…] petrichor. The word was coined by scientists Bear and Thomas in the 1960s. The smell itself is a mixture of compounds. Geosmin, given off by bacteria in soil, adds an earthy smell (it’s also found in beetroot). […]
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[…] called petrichor. The word was coined by scientists in the 1960s and the smell itself is a mixture of compounds. One of those compounds is called geosmin and is given off by bacteria in soil, adding an earthy […]
[…] called petrichor. The word was coined by scientists in the 1960s and the smell itself is a mixture of compounds. One of those compounds is called geosmin and is given off by bacteria in soil, adding an earthy […]
[…] called petrichor. The word was coined by scientists in the 1960s and the smell itself is a mixture of compounds. One of those compounds is called geosmin and is given off by bacteria in soil, adding an earthy […]
[…] called petrichor. The word was coined by scientists in the 1960s and the smell itself is a mixture of compounds. One of those compounds is called geosmin and is given off by bacteria in soil, adding an earthy […]
[…] called petrichor. The word was coined by scientists in the 1960s and the smell itself is a mixture of compounds. One of those compounds is called geosmin and is given off by bacteria in soil, adding an earthy […]
[…] called petrichor. The word was coined by scientists in the 1960s and the smell itself is a mixture of compounds. One of those compounds is called geosmin and is given off by bacteria in soil, adding an earthy […]
[…] called petrichor. The word was coined by scientists in the 1960s and the smell itself is a mixture of compounds. One of those compounds is called geosmin and is given off by bacteria in soil, adding an earthy […]
[…] petrichor. The word was coined by scientists Bear and Thomas in the 1960s. The smell itself is a mixture of compounds. Geosmin, given off by bacteria in soil, adds an earthy smell (it’s also found in beetroot). […]
[…] petrichor. The word was coined by scientists Bear and Thomas in the 1960s. The smell itself is a mixture of compounds. Geosmin, given off by bacteria in soil, adds an earthy smell (it’s also found in beetroot). […]
[…] petrichor. The word was coined by scientists Bear and Thomas in the 1960s. The smell itself is a mixture of compounds. Geosmin, given off by bacteria in soil, adds an earthy smell (it’s also found in beetroot). […]
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[…] petrichor. The word was coined by scientists Bear and Thomas in the 1960s. The smell itself is a mixture of compounds. Geosmin, given off by bacteria in soil, adds an earthy smell (it’s also found in beetroot). […]
[…] petrichor. The word was coined by scientists Bear and Thomas in the 1960s. The smell itself is a mixture of compounds. Geosmin, given off by bacteria in soil, adds an earthy smell (it’s also found in beetroot). […]
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[…] called petrichor. The word was coined by scientists in the 1960s and the smell itself is a mixture of compounds. One of those compounds is called geosmin and is given off by bacteria in soil, adding an earthy […]
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