1. Rain Has a Named Smell, Petrichor, So Does Snow Have One Too?
The earthy smell of soil after rain was named "petrichor" on March 7, 1964, by Isabel Bear and Richard Thomas from the Australian research organization CSIRO, in the scientific journal Nature. [1] The theory is that yellowish oils accumulated in rocks and soil are released by moisture. [1]
Geosmin, a substance produced by soil bacteria such as actinomycetes, also plays a role in the smell of rain. In water, it can sometimes be detected at concentrations of about 4 nanograms per liter (four times one-billionth of a gram), which is roughly equivalent to one teaspoon in 200 Olympic swimming pools. [2] Another hypothesis suggests that tiny particles called aerosols, released when raindrops hit the ground, may carry these scents. [3]
In contrast, the smell before snow has no specific name or identified substance. In the research for this article, no peer-reviewed papers were found that directly investigated the chemical composition of pre-snow air. So, what is the nose picking up? The first clue is that cold air may make smells weaker, not stronger.
2. Why the Smell of Snow Might Be Weaker Than Other Weather Smells
On days when snow falls, the air is often cold from the upper atmosphere down to the ground. Whether snow forms is not determined by ground temperature alone. As temperatures drop, the movement of scent molecules in the air slows down, according to explanations provided by experts. [5]
When molecules move slowly, smells are perceived as weaker. Pamela Dalton, an olfactory scientist at the Monell Chemical Senses Center in the United States, states that people are less sensitive to smells in winter, and scent molecules are less likely to be in a "smellable state." [5] Dalton suggests that right before snow, the air might actually smell weaker than usual. This is an explanation, not a result confirmed by specific experiments on snow.
You might think of it like turning down the volume of music. However, this is about scent molecules, not sound. If the air smells weaker, why do we still sense the "smell of snow"? One clue may be a different sensor inside the nose.
3. How the Trigeminal Nerve in the Nose Reacts to Cold Air
When we breathe in cold air, the trigeminal nerve in the nasal mucosa reacts to the coldness. This nerve also handles sensations like the sting of chili peppers or the cooling feeling of mint. [4]
The trigeminal nerve is separate from the nerve responsible for smell. Yet, Dalton explains that we receive this information combined with ordinary smells, like coffee or pine, as a "scent experience." [4]
Dalton explains the "smell of snow" before it falls using three factors: coldness, humidity, and stimulated nerves. She describes it as a sensory experience rather than the detection of specific scent molecules. [4] Of these three factors, the role of humidity is where explanations diverge.
4. Humidity and Ozone: Two Different Explanations for the Sensation
Some news commentary articles explain that the air right before snow is more humid than usual, and that this humidity is related to the sensation. [4] On the other hand, other commentaries state that air cold enough to produce snow is actually dry. [6] The impact of humidity is inconsistent across different sources.
Some summary articles mention that snow has an "ozone smell." However, the three snow commentary articles reviewed for this piece did not mention ozone in their text. [4] [5] [6]
Ozone smells are explained in the context of thunderstorms. It is said that downdrafts in thunderstorms bring ozone from higher altitudes down to the ground. [7] This is a story about thunderstorms, not snow.
5. Why People Still Call It the "Smell of Snow"
Dalton, speaking in the context of rain smells, suggests that humans do not seem to have an innate reaction to weather scents. Instead, the idea is that we learn to associate these sensations with experience. [7]
The smell before snow might be explained by the same logic. However, this has not been verified specifically for snow. The explanation combining the cold-sensing nerve, weakened smell, and experience makes sense as a commentary. But since no papers have directly measured the smell of snow, there is significant room for experimental verification.
6. Simple Observations You Can Try Yourself
If you want to explore this further, there are simple observations you can make. Today, you can open the cap of a tube of mint-flavored toothpaste and breathe in near the opening. Check if you feel not just a smell, but a "cooling" sensation. This can help you think about how the trigeminal nerve works. Be careful not to get toothpaste in your eyes or mouth.
When you go outside on a chilly day, try to put into words what you feel in your nose and write it down. Distinguish between what is a smell and what is coldness. Writing them separately may help you see how your senses mix. For official information on temperature and snow forecasts, you can check data from the Japan Meteorological Agency.