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Seasonal changes bring all sorts of new sights, sounds... and smells. But sometimes even the same things can smell totally different to us in the colder months than in the warmer ones.

It has to do with both what's there to smell and how our noses work. If you've ever been in a big city on a hot summer day, you may have noticed that it has a certain… bouquet. Part of this comes from the fact that bacteria become more active on hot days, so grosser things get grosser — like a dumpster in the hot sun.

Meanwhile, the cold might stop decomposition in its tracks. But there's another factor: it comes down to how smells actually reach our noses. Smells are a type of volatile organic compound, meaning they evaporate easily—and they need to evaporate before they can reach our noses.

The thing is, some of these compounds only do so at higher temperatures. As a result, there can simply be more scent molecules floating around as the temperature goes up. It's the same reason a bowl of cold soup doesn't smell much compared to a piping hot one.

But it's not just that there's more to smell in the summer. The air is also spreading smells around more efficiently. That's because gas molecules move around more quickly in hot air, so they pass odors along faster.

So, in the summer, there are more smells and they're traveling faster. Then there's humidity. It's probably not news to you that, in the Northern hemisphere, summer air is more humid than winter air.

And research suggests that molecules of water vapor likely act as buoys for odor particles and carry them around when, under dry conditions, they'd probably stay put. All of these effects add up to give garbage on a hot day that certain je-ne-sais-quoi. As temperatures drop, things start to stink less.

But you may also notice that crisp edge that gives winter air its distinct winter smell, like the smell of the first frost. Turns out, the harsh sensation of cold air triggers another sensory nerve called the trigeminal nerve. This nerve detects touch, temperature, and pain in your face.

It also interacts with your olfactory system, which is responsible for smell. The trigeminal nerve isn't actually part of the olfactory system, but the two are closely related in ways that we don't yet fully understand. Because it sits cozy with a lot of the other nerves, the trigeminal nerve can cause some crossed wires among our senses—which is why, for instance, some people get the urge to sneeze when they look at a light.

But the triggering of this nerve can also affect how we perceive smells. It effectively combines the sensation of temperature with our perception of a scent. And that combination—between our olfactory system and our trigeminal nerve —is why things like mint smell “cold” and pepper smells “hot”.

And it's part of the reason why winter smells like winter. There are many environmental differences between the seasons, and those can have a big impact on what smells are even around in the first place. But the way smells travel and interact with our noses can change our experience of smells for even cooler reasons.

Thanks for watching this episode of SciShow —and thanks to all our supporters on Patreon for making this episode possible! If you'd like to help us keep making videos like this, you can learn more at patreon.com/scishow. [♪ OUTRO].