It’s a sensation most of us familiar with though if you’re living in much of the Northern Hemisphere then like me, it might not be one you’ve been familiar with recently. The sweet smell of rain or Petrichor. The name was first coined by two Australian scientists Isabel Joy Bear and Richard Thomas in their 1964 article “Nature of Argillaceous Odour”, published in the journal Nature.
The word was coined from Greek petros, meaning “stone”, and ichor, meaning “the fluid that flows in the veins of the gods”.
It turns out it’s not just gratitude that makes rain smell so appealing after a long period of dry weather. There’s actually some chemistry involved too.
Bacteria, plants and even lightning can all play a role in the pleasant smell we experience after a thunderstorm; that of clean air and wet earth.
“These critters are abundant in soil,” explained Prof Mark Buttner, head of molecular microbiology at the John Innes Centre.
“So when you’re saying you smell damp soil, actually what you’re smelling is a molecule being made by a certain type of bacteria. That molecule, geosmin, is produced by Streptomyces. Present in most healthy soils, these bacteria are also used to create commercial antibiotics. Drops of water hitting the ground cause geosmin to be released into the air, making it much more abundant after a rain shower.”
Whilst we as humans are used to our senses being much less powerful compared to many other creatures in the world of nature, it turns out that though plenty of animals are sensitive to Petrichor, human beings are extremely sensitive to it.
Now, geosmin is becoming more common as a perfume ingredient. There is something in our nature so that even when it is diluted down to the parts per billion range, we humans can detect it,
Yet we also have an odd relationship with geosmin – while we are drawn to its scent, many of us dislike its taste.
Even though it is not toxic to humans, the tiniest amount can put people off mineral water or wine when it is present.
According to Prof Nielsen, research also indicates that geosmin could be related to terpenes – the source of scent in many plants.
Rain could bring these fragrances out, says Prof Philip Stevenson, a research leader at Royal Botanic Gardens, Kew.
“Often the plant chemicals that smell pleasant are produced in leaf hairs… and the rain may damage these, releasing the compounds. Rain may also break dry plant material releasing chemicals in a similar way to when you crush dried herbs – the smell becomes stronger, very dry periods may also slow down plant metabolism, with renewed rainfall giving it a kick start and causing plants to release a pleasant scent. Thunderstorms have their role to play too, creating the clean, sharp scent of ozone – caused by lightning and other electrical discharges in the atmosphere.
Prof Maribeth Stolzenburg of the University of Mississippi explains: “Besides the lightning, the thunderstorm and especially the rain will improve the air quality. Much of the dust, aerosols, and other particulates are rained out and the air clears.”
If you enjoyed this post then why not check out my writings on Apricity, the word given to the feeling of the warmth of winter sun.