How to Smell Rain

A wonderful guest article today by Milo Carnegie Mee.


Of the plethora of smells that greet our noses when we are outdoors, the redolence of fresh rain evokes a familiar sense of change in the surrounding landscape.

Rain slugging itself across our surroundings draws out a smell, petrichor, which seeps from dampened pores. As the odour of smoke reveals a fire, this earthy perfume tells us of water greeting parched soil nearby. The word ‘Petrichor’ was coined in a 1964 study published in Nature and comes from a blend of petro, ‘of rocks’, and ichor, ‘the blood of Greek gods’, because the smell was thought to be an essence derived from the ground.

With a little more knowledge of how the primary notes of this and other scents of water come about, we can use our noses next time it rains to investigate the recent or oncoming weather, and smell the signs of a drizzle, deluge, snow, or lightning storm – and appreciate the inextricable biology of our sensory world.

The world’s cloak of soil is laden with living organisms, but we often fail to consider those which lie beyond our macroscopic sight. The soil that familiar worms push and rework is itself teaming with microscopic life, and a single teaspoon can contain 1 billion organisms representing thousands of species from all three domains of taxonomy (Archaea, Bacteria, and Eukarya).

William Blake may have seen the world in a grain of sand, and heaven in a wild flower, but we can now see the biological universe in a gram of soil. Microbes further decompose plant and animal matter that larger soil-borne organisms leave behind, helping create the finely processed non-living organic component of soil known as humus (this does not necessarily contain chickpeas or tahini). All the essential elements necessary for life pass through various soil microorganisms in Earth’s grand overlapping biogeochemical cycles.

When we smell petrichor, we are sensing a sign of the many species of streptomyces, an actinobacteria, which live in extensive fungi-like mycelial colonies ubiquitous throughout soil. Streptomyces have a complex secondary metabolism, which means they synthesise extra chemicals which act as signals or defend them from other organisms – they even produce many of our antibiotics. What we perceive as the smell of petrichor is one of these secondary metabolites, an aromatic oil called geosmin (meaning earth-smell), mingled together with other miscellaneous oils previously shed by plants. When streptomyces are faced with unfavourable dry conditions they grow more spores in order to spread elsewhere, and the role of geosmin is to attract organisms who are likely to unwittingly disturb and carry these spores to new areas – completing the streptomyces life cycle. For this reason, geosmin accumulates as soils become drier – so why do we humans only really smell it when it rains?

The answer is effervescence. Raindrops trap tiny air bubbles as they hit the ground, which fizz up and out of the collapsing raindrop and produce fine droplets of water which are thought to carry any aromatic compounds disturbed in the soil into the air, where they remain suspended. These droplets float into our nostrils and dissolve through a wet lining, where our olfactory nerve detects the geosmin and plant oils, sending identifying signals to the olfactory cortex of our brain, finally allowing us to consciously experience the smell of petrichor. The olfactory cortex is closely connected to brain structures associated with emotion and memory, which is thought to be why we so strongly associate odours with past memories and feelings. Our sense of smell allows us to detect the chemical composition of the air around us, and we are more sensitive to some molecules than others. It may come as a surprise that the human nose is extremely sensitive to geosmin, and we can detect it at concentrations as low as 5 parts per trillion – that’s equivalent to detecting a teaspoon of geosmin in 200 Olympic-size swimming pools!

Our sensitivity to geosmin may not be an evolutionary mistake, and we share it with numerous species. For many large animals detecting geosmin in the air usually means one extremely valuable thing: new water on dry soil. And in the business of running a biological entity, it is particularly useful to be able to home in on water like this during arid times. Cultures all over the world find great importance in petrichor – Western Australia’s Pitjantatjara people associate the smell with the first rains after dry season and the flourishing of their native ecosystems. I like to see our species’ positive associations with petrichor as an inhered part of our evolutionary collective unconscious – so maybe next time you smell fresh rain, think of the joy this very scent has brought to our ancestors during times of drought.

We can use all this knowledge to infer a number of things about our immediate environment.

Because we know streptomyces produce more and more spores and geosmin as their soil dries, a stronger smell of petrichor after a shower tells you that there has been a dry spell this area recently. It has also been found that more aerosol droplets are produced by light or moderate rainfall compared with a heavy deluge (heavier rain prevents the aerosols from rising and being smelt) – this means that the smell of petrichor tends to be stronger the lighter the rainfall, and we can try to anticipate the strength of an oncoming shower if we smell it coming. If we are caught in a heavy downpour, we may even be able to smell the rain starting to ease as more aromatic aerosols make their way into the air. You may also encounter a petrichor-like smell on days without rain – and this is a sure sign that digging has gone on nearby that has disturbed geosmin in the soil.

Other odours abound within weather and water, and some are worth the pause to appreciate the story they tell when we encounter them. The sharp chlorine-bleach-like scent of ozone, should inspire caution. We are familiar with the thunderous sonic boom of lightning splitting the air, but the more subtle scent of ozone is produced after lightning splits the bonds of O2 in the air – the loose O atoms can recombine with O2 producing ozone O3. For this reason, we can smell ozone after lightning strikes, and similarly to petrichor, trace amounts of ozone can be carried ahead by the downdrafts and gust front of a storm and alert us to their impending arrival (we can detect 10 parts per billion of ozone, which is three teaspoons of water in an Olympic-size swimming pool!). Ozone can also be a sign of human civilisation nearby, as it forms from the reaction of common urban pollutants (nitrogen oxides (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs)) in the presence of UV sunlight (rather than lightning). More ozone is formed in the hot and sunny summer months, which can explain the seasonal increase in pollution and smog.

The tangy aroma we associate with the sea is not that of salt, but a molecule called Dimethyl sulfide (DMS) produced by bacteria. DMS is produced as a by-product of bacteria feeding on single-celled phytoplankton and a catchily named compound they produce called Dimethylsulfoniopropionate (DMSP). A billion tonnes of DMSP are formed every year, and when the phytoplankton bloom there is an influx of the compound and DMS in that area. Various competing organisms from all along the food chain are sensitive to the smell of DMS and use it to sniff out the planktons themselves or prey that might be feeding on it, from bacteria and whale sharks, to plankton-eating fish, squid, seabirds, and seals. DMS carried into the atmosphere is thought to aid the formation of clouds, reacting with sunlight and acting as a condensation nucleus – a small particle which water can adhere to when transitioning from gas to liquid – triggering and encouraging the conglomeration of water droplets – so large plankton blooms can result in strong seaside smells and maybe even a cloudier sky.

Some say they can smell snow, but this is not to do with specific molecules, rather the climate in which you are smelling them. As temperature lowers so does the activity and dispersal of particles in the air – making smells less noticeable – so we are sometimes able to notice drops in temperature by this lack of smell. However, what characterises a cold day with impending snow is an increase in humidity before the fall (the atmosphere reacts to being saturated with moisture by pouring its harboured water back to earth). It is this humidity which dampens the inner lining of your nose and increases your olfactory sensitivity – giving you the unique sensation of improved smell in an unusually unsmelly environment. Additionally, the sensations of the coldness of air, the coolness mint, and the tingling warmth of chillies in your breath are brought by a special interpreter both intertwined with but discernibly separate from the nerves normally responsible for taste and smell – the trigeminal nerve. So, although snow does not have a molecular smell, we can sense its presence by association when this unique combination of sensations: fewer smells, heightened sensitivity, and a stimulated trigeminal nerve (a distinct ‘clean’ lack of smells and a cool, fresh, sensation), is triggered by the conditions it requires (cold and humid air).

Veins of information like these run beneath our sensual world unseen. In a universe of matter, factual relationships permeate every moment at every scale – and whether we understand them or not has no impact on their validity. The joy of tapping into these hidden truths of things can leave us blind to a more practical view of the world – when outdoors it is often more useful to know the character of an organism rather than its taxonomic category. But it is all the more wonderful when a deeper scientific understanding can actually help us develop clearer heuristics for anticipating the seemingly random and indecipherable world we often find ourselves in. Smells are floating chemical signs which our noses can decipher, telling us of causes and sources nearby. We can now appreciate the way microbes essential to life on Earth reach into our senses with gentle plumes of aromatic molecules released from the earth by falling rain. With a little chemistry we know that whiffs of ozone are often a product of fantastically energetic bolts of lightning ripping through the air nearby. The tangy ‘sea’ smell of the ocean produced by bacteria can conjure the orchestra of predators and prey always whirling towards phytoplankton blooms for sustenance. And as conditions align for snow, we can now consider how our senses are uniquely tickled to let us know.

Article written by Milo Carnegie Mee.

Instagram: @Milo__Mee

Thanks Milo!


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