Have you ever left home on a warm summer day or a cold, dry winter afternoon and thought you could smell rain? Apparently, not everyone can detect approaching bad weather via the nose – is there any science to back this up, or is it just another myth?
The claim that it is possible to smell the rain before it arrives seems to have divided people quite a lot. Just a quick glance at social media is enough to show that there are those who insist they can predict rainfall with their noses, while others seem much more skeptical.
But whether you believe it or not, there is some evidence to back up these claims. In fact, there are a few contributing factors to the possibility of smelling rain before it arrives, the most important of which has a lot to do with petrichor.
The word “Petrichor” is derived from the Greek word “Petros”, meaning stone, and “Ikor”, which is the fluid that ran in the veins of the ancient gods. This term refers to that familiar and strangely satisfying odor that the earth gives off after heavy rains, especially after a long period of drought. The term was first coined in 1964 by mineralogists Isabelle Joy Behr and Richard Thomas.
For many people, petrichor is among their favorite scents in the world, but until relatively recently no one was entirely sure why we find it so pleasant. Then, in 2020, a team of scientists found that the scent is also particularly attractive to other animals, too.
The reason for this is that the common soil bacteria, Streptomyces, produces a compound called geosmin. It’s the smell of geosmin that we are particularly drawn to.
In fact, the human nose is so good at detecting trace amounts of the compound that it surpasses sharks’ ability to smell blood in the water. The bacteria produce geosmin to lure critters — mostly insects and other invertebrates, but also larger animals — that coat themselves with their spores and then distribute them over wider distances.
So when it rains, researchers showed in 2015, water droplets hit the surface and then flatten, trapping pockets of air in the Earth’s pores. These pockets of water then explode into small sprays. In the process, they take with them traces of whatever was on Earth, including geosmin, which can then be stirred up by the air and carried great distances—even miles ahead of the rain clouds themselves.
It is likely that these are aerosol particles that people detect when they smell rain on the horizon. This phenomenon may also explain why bacteria are found high in the atmosphere, where the wind carries small microorganisms.
Another factor contributing to the smell of approaching rain is the presence of ozone in the wind. This chemical has a sweeter smell than the earthy smell associated with Petrichor.
Ozone is made up of three oxygen atoms, and its name is derived from the Greek word ozein, which means “to smell.” It is a gas that occurs naturally but can also be produced by artificial fertilizers or other pollutants.
It can be made from an electrical charge – whether from lightning or an artificial source – that separates atmospheric nitrogen from oxygen molecules. Some of these molecules will recombine to form nitric oxide, which can sometimes be converted to ozone, after reacting with other chemicals in the atmosphere.
The smell of ozone in the air can signal the approach of heavy rain, as the downdraft from a thunderstorm sends ozone to ground level where humans can detect it.
So, the next time you walk out the door on a new day and your nose catches the scent of something wet and interesting in the wind, you’ll know it’s time to wear an umbrella, just in case.
And once it rains, other smells come with it. Dripping water disturbs and dislodges odorous particles on surfaces, especially dry ones, carrying them into the air. If you are near vegetation, these particles may come from plants and trees. For city dwellers, these smells are heightened by concrete and asphalt. Some are fragrant, others foul.
There’s a name commonly attributed to this group of after-rain scents: petrichor. Petrichor was first described in 1964 by mineralogists Isabelle Joy Beer and R.G. Thomas of Australia’s Commonwealth Scientific and Industrial Research Organization.
By their definition, it occurs when airborne particles from decaying plant or animal matter stick to mineral or clay surfaces. During the drying period, these molecules chemically combine with other elements on the surface of the rock. Then, when it rains, an aromatic cocktail of fatty acids, alcohols and hydrocarbons is released.
After a storm passes, what’s often left behind is a musty, earthy whiff of dampness. This is the smell of geosmin, which is a byproduct of the metabolism of bacteria or blue-green algae. Geosmin can be a comforting invitation to gardeners eager to dig in the dirt, but it’s less beloved when it contaminates wine or drinking water, leaving a soggy, unpleasant odor.
All these weather-driven chemicals could carry messages. Some biologists suspect that petrichorians running through streams act as a signal to freshwater fish, indicating when it is time to spawn.
Microbiologist Keith Chater at the John Innes Center in England has suggested that geosmin’s fragrance may be a beacon that helps camels find their way to desert oases. In turn, the bacteria that produce geosmin use camels as a carrier for their spores.
But do these scents send meaningful messages to people? Anthropologist Diana Young of the University of Queensland has studied Aboriginal traditions in Australia’s Western Sahara.
There, the first rains before the humid winter and summer months are an important event, as the fragrant scents of wet gum leaf oils, eucalyptus, animal droppings and dust mingle.
The rains bring game like kangaroos and emus, quenching thirst and turning the red desert into a green landscape. For these people, the smell of rain is associated with the color green, Young says, a relationship she calls “cultural synesthesia.”
In fact, many of these indigenous people make their own perfumes using plant and animal fats and rub them on their bodies, a symbolic connection between the body and the landscape. The scent is believed to be protective and purifying, and connects current generations to their ancestors.
Although humans don’t seem to have innate responses to these odors, we learn to associate them with our experiences, notes psychologist Pamela Dalton, a member of the Monell Chemical Senses Center. Floods may forever leave us with damp and musty memories, but for many the smell of rain is cleansing, refreshing, and a relief from the harsh summer heat.