What is the smell of rain called? The answer is petrichor and it intrigues scientists

It's been a long, hot summer but this week’s thunderstorms and torrential downpours brought sweet relief - and with them the invigorating smell of fresh rain.

And after so long without a raindrop, many Britons are now noticing the scent with renewed vigour and find themselves wondering 'what is that smell'?

The phenomenon has been noted for centuries, and in 1964 it was given the name petrichor, derived from the Greek for stone “petra” and “ichor”, which refers to the golden fluid flowing through the veins of immortals.

It is produced both pre- and post-downpour, with the water forcing the release of aromatic chemicals trapped in rocks and soils.

Raindrops form air pockets on the rocks and soil and allow petrichor-scented compounds to be released upwards into the atmosphere as a gas, like a glass of Champagne.

But while the rocks are pivotal in releasing the smell, the odour itself is generated predominantly by a chemical called geosmin, which is made by bacteria in soil.

Highly pungent

In April, scientists found that geosmin does have a use as it is made by soil microbes to fend off hungry worms.

Dirt-dwelling bacteria, including toxic ones, produce the chemical to keep predators, such as worms, away and the compound builds up in the soil.

Geosmin is also a water contaminant, falling from the heavens when it rains and generating the petrichor scent we associate with a damp spring morning.

The chemical is highly pungent, with just five parts per trillion enough to be picked up by the human nose.

Historically, the scent would have been a harbinger of joy for our hunter-gatherer ancestors as it would have signalled to them that fresh water was imminent. This, some scientists believe, has led to an in-built delight when humans inhale it today.

Scientists from Concordia University in Canada investigated the role of geosmin and conducted various experiments to see how it affected nematode worms.

First, they observed the movement and behaviour of worms when surrounded by geosmin and found they desperately tried to escape its sphere.

However, mutant worms that were unable to smell or taste behaved normally around geosmin. It was also, in and of itself, harmless.

A separate experiment focused specifically on Streptomyces coelicolor bacteria, a toxic bacteria that makes geosmin.

The researchers saw the worms avoid their prey, the microbes, when they could taste the presence of geosmin. In contrast, the genetically modified mutants devoured the toxic bacteria and subsequently died.

“Through our study we found that geosmin in Streptomyces coelicolor, a bacteria that is toxic to nematodes, does not appear to have any role other than as a signal,” says Dr Brandon Findlay, the supervising author of the paper published in the journal Applied and Environmental Microbiology.

“It doesn’t help the cells grow, eat or divide. It doesn’t ward off predators directly. It just seems to be there as a warning.”

The conclusion that a bacteria produces a smelly chemical with the sole purpose of warning off would-be predators is unusual, and it is thought to be a unique case.