Have you ever left the house on a warm summer's day or a particularly cold dry winter afternoon and thought you can smell rain? Apparently, not everyone is able to nasally detect approaching bad weather – so is there any science to back it up, or is it just another myth?
The claim that it is possible to smell the rain before it arrives seems to divide people quite a lot. Just a quick glimpse at social media is enough to show that there are some who are adamant about their ability to predict the rain with their noses, while others seem far more skeptical. But whether you believe it or not, there is some evidence to support these claims. In fact, there are a few contributing factors to why rain can be sniffed out before it arrives, the most significant of which has a lot to do with petrichor.
The word petrichor comes from the Greek petros, meaning stone, and ichor, the fluid that flowed in the veins of the ancient gods. It refers to that familiar, oddly satisfying scent that is released by the ground after heavy rain, especially after a long dry spell. The term was first coined in 1964 by mineralogists Isabel Joy Bear and Richard Thomas.
For many people, petrichor is among their favorite smells in the world, but until relatively recently no one was quite sure why we find it so pleasant. Then, in 2020, a team of scientists found that the smell is also particularly attractive to other animals too.
The reason for this is that a common soil bacterium, Streptomyces, produces a compound called geosmin. It is the smell of geosmin that we are so peculiarly attracted to. In fact, the human nose is so good at detecting trace amounts of the compound that it outperforms sharks’ ability to smell blood in water. The bacteria produce geosmin to entice critters – mostly insects and other invertebrates, but larger animals as well – who get covered in their spores and then disperse them across wider distances.
So when it rains, as researchers showed in 2015, water droplets impact with a surface and then flatten out, which traps pockets of air in the pores of the ground. These pockets then burst out of the water and essentially become tiny aerosols. In the process, they take traces of whatever was on the ground with them, which includes geosmin, which can then be whipped up by the air and carried great distances – even miles ahead of the rainclouds themselves. It is likely it's these aerosol particles that people are detecting when they smell rain on the horizon. This phenomenon may also account for why bacteria have been found high in the atmosphere, as the tiny microorganisms are swept up by the winds.
Another contributing factor to the smell of approaching rain is the presence of ozone in the wind. This particular chemical has a sweeter scent to it than the slightly more earthy smell associated with petrichor.
Ozone is made up of three oxygen atoms, and derives its name from the Greek word ozein, which means “to smell”. It is a naturally occurring gas but can also be produced by man-made fertilizers or other pollutants. It can be made from an electrical charge – be it a lightning strike or an artificial source – which separates nitrogen in the atmosphere from oxygen molecules. Some of these molecules will recombine to form nitric oxide, which can sometimes turn into ozone, after reacting with other chemicals in the atmosphere.
The scent of ozone in the air can indicate that heavy rain is approaching, as a downdraft from the thunderstorm sends ozone to ground level where it can be detected by humans.
So next time you step out the door on a fresh day and your nose gets a whiff of something damp and intriguing on the wind, you’ll know it is probably time to grab an umbrella, just in case.
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