Wednesday, January 27, 2016

Podcast: Physics and the Smell of Snow

Can we smell a snowstorm before it begins? Anecdotally, many people insist that they can detect an impending snowfall, but what does the science say? On today’s podcast, we’re joined by olfactory scientist Pamela Dalton, a researcher at the Monell Chemical Senses Center in Philadelphia, to explore the physics behind that crisp, snowy scent.

Smell is a very direct sense: odor molecules have to be physically transported into your nose to trigger olfactory receptors and send a signal to the brain. Just about everything around you gives off odor molecules all the time, but the smelliest stuff is very volatile — that is, easy to evaporate and move around. Just as the kinetic theory of gases would suggest, scents are susceptible to the whims of temperature, flying high in warmer climes and hunkering down when it gets cold. “When it snows it's usually around 32 to 35 degrees, maybe a little colder,” says Dalton, “and there are just not as many things available to smell at that temperature. The molecules tend to stay where they are. They're less volatile.” Thus, the “smell of snow” is, in part, the absence of any smell at all. That’s not all there is to it, however. In addition to temperature, humidity also plays a role in our sense of smell, since our noses work best when they’re nice and moist. Snow, like rain, falls when the atmosphere reaches its limit for holding moisture, and just before a snowstorm, the humidity rises. “That little added boost of humidity in the environment, either right prior to a rainstorm or prior to a snowstorm, allows our equipment to operate at a higher level and we're a little more sensitive to what is there to be smelled.”

The physics of weather conditions — temperature and humidity — undoubtedly affects how well our noses operate and what odors are available to be detected in the first place, but there’s another effect that feeds into our impressions of snow-scent: the physical sensation of breathing cold air. The trigeminal nerve, which allows us to feel things like hot, cold, tingly spice, and cool mint, is a completely different sensory system, yet it often feeds into what we think of as our sense of smell. “Snow is probably a combination of the chillness of the air and a little bit of moisture,” says Dalton. “Is it unique to snow? We don't really know, but it does certainly provide a sensation, that while the specific molecules may not be unique to snow, the combination of the cold, the humidity, and the molecules might actually make us think, ‘Wow, it's going to snow, it's gonna be a big snow,’ that sort of thing.”

Head anatomy with olfactory nerve
Image Credit: Medscape

Podcast image credit: branches with snow (Free photobank / CC BY 4.0

Podcast and post by Meg Rosenburg

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