The science behind the smell of fall

One of the first memories Egan Davis recalls about fall is an image of his father tending to the compost pile at the top of the garden with a bonfire roaring nearby. The family home in North Vancouver's Lynn Valley backed onto a forested ravine, where resplendent big-leaf maple trees dumped sweet-smelling leaves every October.

Davis remembers his mother carrying him up the hill to see his dad, a lifelong gardener, always at home outside.

"I remember kicking my feet through the leaves and that sugary smell," said Davis, now 45. "When I kick my feet through raked maple leaves now, I am back to that moment when I was four or five, and I'm wooshing my boots through maple leaves."

The memory is stamped into Davis' brain, a nostalgic flashback that pops up whenever he senses the smell of fall. It turns out there's a scientific reason Davis, and so many others, have a smell inextricably linked to the season in their head: it's a confluence of chemistry, biology and psychology that can trigger different emotions like anxiety, sadness or excitement depending on your personal collection of memories associated with the time of year.

It also means no two people smell and feel fall the same way.

How smell works

Smell happens when the receptors in your nose pick up aromatic molecules in the air. These molecules ooze out of objects and living organisms in the environment around you.

You smell more aromatic molecules during the summer because hot, muggy air holds more of the molecules and enables them to move through the atmosphere more quickly. The opposite happens when it's cooler and drier in the fall: air molecules contract together and leave less space for odour molecules to move through.

It means we're bombarded with smells in the summer, but are able to better pick out specific scents in the fall.

"In summer, we have just more of a mixture of scent and sort of a wide blend of scent ... we smell a lot more of everything around us," said Rachel Herz, a neuroscientist. "When it gets cooler and drier, specific scents tend to stand out more — we're able to kind of pull out the scent of leaves, the scent of bark, the scent of grass in more distinct ways."

The dominant earthy smells of fall are largely the product of plants hunkering down for the winter. Fallen leaves begin to decay and their sugars and organic compounds in the leaf break down, creating the classic musky-sweet smell of a leaf pile. 

Our emotional reaction to any smell comes from the personal meaning we've assigned to the scent — typically stamped on your brain when you're young, sensing smells and forming memories with them for the first time. (It's why so many people associate the smell of fall with a back-to-school emotion.)

The meeting of molecule and memory that happens when you smell something familiar triggers a visceral, split-second response in the amygdala and the hippocampus sections of the brain — the zones where emotional memory and associations are stored.

It's a reaction that happens before you can think about it.

"If you saw something or heard something, you'd think about what that meant first, and you'd analyze it and evaluate it: 'Oh, this makes me feel such-and-such,'" said Herz, who wrote a book about smell and emotion called The Scent of Desire.

"But with smell, it's the emotion first and then we try to figure out, 'Why am I feeling like that? Why am I feeling wistful or nostalgic or excited?'" Herz continued.

"Sometimes we can get back to it, but other times we just have the feeling and even we don't quite know why."

Davis' definition of the scent of fall grew to include the sugary smell of katsura trees after someone showed it to him for the first time when he was an 18-year-old freshman at the University of British Columbia. Davis also thinks of alder trees, blackberries and figs. It's his personal autumn lexicon.

Today, Davis teaches horticulture at UBC and takes his own students to sniff the katsuras when classes begin.

"They've never smelled this. I remember how exciting that was, and when I smell that now, I go right back," he said.