Why Snow Has That Crisp, Clean Smell

FLORA: Sci Fri producer Rasha Aridi is out in the cold doing some reporting for us. Rasha gave me the snow-down. Where are you right now?

RASHA ARIDI: Yeah. I am right outside my apartment building. We just had a big snowstorm come through. And I’m standing in, like, 7 inches of very fluffy snow, which is the most I’ve seen in years, so it’s a big day. And it also brings back a familiar scent, the smell of snow.

FLORA: I’m not sure I know the smell of snow. Can you describe the smell of snow?

[SNOW CRUNCHES]

RASHA ARIDI: OK. It smells clean and fresh and like a lot of nothingness, but part of it is also the feeling. Like, the air is so cold that my nose is tingling and the air in my lungs is really sharp. And so altogether, it just– it tells me there is snow, even if I would close my eyes.

So I got to thinking, why does snow have a smell? It really is just water, so what is this that I’m sniffing?

FLORA: Surely someone knows what snow smell is made of. And maybe you should go somewhere warmer for this part.

RASHA ARIDI: Yeah, don’t have to tell me twice.

JOHAN LUNDSTROM: I like snow, and I have experienced snow a lot in my life.

FLORA: Dr. Johan Lundstrom isn’t just a snow aficionado, but an olfactory expert. He’s trying to untangle how smell works at the Karolinska Institute in Stockholm.

JOHAN LUNDSTROM: Born and raised also in Northern Sweden, where we are quite familiar with snow, and we have it from basically September to April.

RASHA ARIDI: So, Flora, here’s the first thing you need to know. Snow itself is porous and spongy, so it’s really good at grabbing on to odors and trapping them. So when you smell snow, you’re mainly smelling what it kind of sponged up.

JOHAN LUNDSTROM: Let’s say you’re scooping up the snow from a bundle of roses. The chemical that you are smelling is going to be an alcohol. It’s called phenylethyl alcohol, which basically are in plants and roses.

FLORA: My snow never smells like roses.

RASHA ARIDI: Yeah, me, too. I’m in a city, so I’m picking up notes of pollution, fumes from cars, asphalt, stuff like that. So, unfortunately, not roses. OK. So the next thing you need to know is that when you sniff snow–

JOHAN LUNDSTROM: It’s actually physical objects that you are smelling. So those physical objects are very, very small chemicals that are traveling up to your nose.

RASHA ARIDI: And those chemicals bind to odor receptors, and that info gets sent to your brain. Then those molecules can get absorbed into your body. So, Flora, chew on that next time you’re in a stinky public bathroom.

FLORA: Absolutely not. I will not chew on that.

RASHA ARIDI: [LAUGHS]

FLORA: OK. Here’s a question for you, Rasha. Why weren’t you picking up car exhaust in your snow bouquet?

RASHA ARIDI: Yeah. So the snow holds on to those odors, but the air itself has been kind of cleaned up by the snow, kind of like a natural air purifier. In fact, the air will probably smell the cleanest a few hours into a snowfall, once it’s kind of scrubbed the air. And because snow holds on to odors, it’ll probably smell worse after it’s been piling up on the ground for a few days versus if it’s falling fresh.

FLORA: That adds up to me.

RASHA ARIDI: Yes. So the other thing that we need to take into consideration is that even though there are odors in the snow for us to sniff, whether or not we actually pick up those scents is another question. And that’s for a couple of reasons. One is the air itself. When the temperature drops, so does humidity, and the odors end up moving in slow mo, so they’re less likely to waft up into our noses.

FLORA: Oh my gosh. Is that why a pile of garbage is so much stinkier in summer than it is in the winter?

RASHA ARIDI: Yes, exactly. Things are stinkier in the heat, including us.

FLORA: Speak for yourself, Rasha.

RASHA ARIDI: Fair enough. And another reason we might not be picking up the subtleties of the snow bouquet is that our noses just don’t work as well in the cold. Like, we’re snottier, for example, which gets in the way of smelling.

FLORA: Rasha, is the smell of snow related in any way to petrichor, you know, that smell after it rains?

RASHA ARIDI: Yeah, so some scents might be similar. You might pick up that earthy, musty smell of a chemical called geosmin. It’s found in soil and water. So if it rained right before it snowed or if snow is sitting on a pile of fresh soil, it might pick up that scent. And our noses are actually really good at sensing geosmin. If you poured a teaspoon of that chemical into 200 Olympic-sized swimming pools, you’d still be able to smell it.

FLORA: Wow.

RASHA ARIDI: OK. So the last thing you need to know about the snowy smell doesn’t have as much to do with our noses as much as it has to do with our memory.

JOHAN LUNDSTROM: Only those odors that have some kind of meaning to us, particularly if they have an emotional meaning, tend to be remembered.

RASHA ARIDI: Smells that are most distinct to you tend to be associated with happy memories. Johan says it might have to do with how our brains are wired. So the part of our brain that processes smells sends that information straight to our limbic system, which includes parts of the brain like the hippocampus and amygdala, which are key in regulating emotions and memory. Other senses take, quite literally, a longer route to get to the limbic system, but smell has a shortcut.

JOHAN LUNDSTROM: This intimate, unfiltered connection between the sense of smell and the memory areas of the brain are helping us to really encode and remember odors for a very long time.

So every time I smell snow that has a very wet character, I’m taken back to our cabin up north, just around the Arctic Circle. If we go there in the spring sometimes and the lakes start to open up, then you have this very strong, wet, snow smell which is– for me, it’s a very pleasant experience.

FLORA: Hmm. Rasha, do you have memories that you think make your brain sensitive to the scent of snow?

RASHA ARIDI: Yeah, I think so. Every time I smell snow, it takes me right back to being a little kid, having snowball fights with my little brother, Neebal, and sitting in front of the TV, hoping and hoping that school would be cancelled the next day. So, yeah, really happy times.

FLORA: Maybe that’s why it’s so distinctive to you.

RASHA ARIDI: Yeah, I think so.

FLORA: Rasha, I wish you many more trips down smemory lane.

RASHA ARIDI: Oh, thank you, Flora. Same to you.

FLORA: Thanks, Rasha.

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