Pine needles. Wood smoke. Snow. These are the smells of winter, and for people who live with distinct seasons, wintry weather brings its own set of olfactory experiences.

But why does the cold of winter smell different from the heat of summer?

One reason is that odor molecules move much more slowly as the air temperature drops, said Pamela Dalton, an olfactory scientist at the Monell Chemical Senses Center in Philadelphia. That means that there are simply fewer smells to smell on a cold, crisp day than there are on a hot and humid one.

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It's the same reason why hot soup smells more than cold soup does and why the garbage truck leaves behind the strongest odors on steamy summer days.

What's more, our noses don't work quite as well when the ambient air is cold, Dalton said. In experiments that require biopsies of olfactory receptors that lie deep inside the nose, researchers at Monell have discovered that the receptors "bury themselves a little more deeply in the nose in winter," she said, possibly as a protective response against cold, dry air.

"We're not as sensitive to odors in winter," she added. "And odors aren't as available to be smelled."

Cold air also stimulates the irritant-sensitive trigeminal nerve, said Alan Hirsch, a neurologist and psychiatrist in Chicago. The trigeminal nerve is what makes you cry when you chop an onion and delivers a hit of spiciness when you inhale a whiff of strong mint.

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When odors stimulate both the trigeminal nerve and the olfactory nerve, the experience of smell becomes more intense.

There is a strong psychological component to our sense of smell, Hirsch added, and what we expect to smell has a big influence on what we actually smell.

In "The Invalid's Story" by Mark Twain, for example, a man is stuck on a train next to what he thinks is a rotting corpse but is actually a box of stinky cheese. So overwhelmed by the smell, he spends too long seeking fresh air on the freezing platform and develops a fever that ends up killing him.

"What you think a smell will be impacts whether you like it and what you perceive it to be," Hirsch said. "So, if you go outside in the winter and you are used to smelling snow or chestnuts in the fire or whatever you happen to smell outside, that's what you will interpret smells to be."

Of course, the smells that are available to be smelled differ as the seasons change. Summer brings flowers and dirt and barbeque smoke. In the most wintery of places, there isn't much outside on cold days except snow, blustery wind and cars warming up.

To cope with the smell deprivation of winter, many people compensate by burning more scented candles, cooking more aromatic stews and baking more cookies. That creates a greater contrast between the indoor and outdoor environments.

"You probably have an uptick of indoor scents in the winter," Dalton said. "Homes are closed up, windows are closed. We concentrate the smells of cooking and living."

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