Call it the 40-degree phenomenon.

Remember how Tuesday’s temperature sent us whooping in the streets? We were ready to invite pals over for a barbecue or cajole restaurants into opening their patios.

Yet that same temperature in October had us scrambling for gloves and scarves and arguing about turning on the furnace.

The difference is that by now we’ve acclimated to the cold, a transition that northerners have needed ever since cave dwellers woke up to fall’s first frost with their winter pelts still in storage.

“It’s our Neanderthal DNA,” said Dr. Kevin Fleming, who is in general internal medicine at the Mayo Clinic in Rochester. “We’re genetically equipped to handle this.”

Fleming says this like it’s a good thing — and it is — yet no one likes to think about how this shift actually works: The way to get better at withstanding the cold is by exposing ourselves to the cold.

To further an abominable image: Recent research says that our acclimating abilities are linked to the amount of brown fat we harbor.

Brown adipose tissue, or BAT, lies in deposits around our necks and clavicles, sort of like chinking in a log cabin. Once these fatty deposits are activated by exposure to cold, they generate heat, Fleming said.

Think of how our bodies react during the first cold snap. Say you’re sitting on the bleachers for your kid’s football game. Likely, you haven’t dressed warmly enough, because who wears mittens in October? By the third quarter, you’re shivering, which is your body’s attempt to generate heat. Physiologically, it helps, but psychologically, shivering makes us feel even colder.

“The only way you can generate heat without shivering is if you get more exposure to cold, which activates the brown fat,” Fleming said. Unfortunately, this heat doesn’t kick in like those chemical hand warmers, but is more gradual.

“It’s like how people who move here from the South hate it, but then get used to it after a couple of years,” he said. “Why does it take so long? Because they have to grow tissue.”

Despite this physical response, pay no mind to the folklore that blood thickens in the winter. It doesn’t, Fleming said, wondering if the old saying stems from what happens if you injure yourself outdoors. “You just bleed differently when it’s freezing.”

Summer is another challenge

Conversely, the human body also acclimates to heat.

A study last year in the Journal of Occupational and Environmental Hygiene said that older firefighters could tolerate a blaze where the heat is particularly intense more easily than younger or less experienced colleagues, who needed to stop and take a break.

This resilience translates to residents of hot climates, or to people in jobs where they’re exposed to heat stress. According to the research, those who can handle the heat are more likely to do their jobs without risking injury due to a lapse in alertness or coordination.

The human body is not endlessly adaptable, however. One climate factor especially resistant to acclimation is humidity.

“It’s a fatiguing thing,” said Fleming. “That’s why even people who live in tropical climates have adopted the siesta.”

All together now: It’s not the heat; it’s the humidity.

Common sense also figures in on how we deal with the cold.

In 2006, a researcher at the University of Oulu in Finland studied how humans adapt in northern climes, suggesting that success is not only physiological, but behavioral. In other words, we know how to dress for the cold.

‘Zip it!’

“In this study the geographical differences in the use of hats, gloves and scarves were associated with cold-related mortality,” Tiina Mäkinen wrote. “Overall, mortality has been shown to increase to a greater extent with a given fall in temperatures … among people wearing fewer clothes.”

In layman’s nagging: “Zip up your jacket!”

This being the coldest winter here in 30 years, it’s worth noting that some age groups really do struggle with the cold. The very young need help regulating their temperatures, which is why we bundle up our babies and youngsters. Older people also gradually become more vulnerable to temperature extremes.

“For the big middle — from teens to 70s — acclimation is pretty easy,” Fleming said. “But we need to monitor those at the extremes.”

He added that one thing that stymies everyone who lives in frigid climates is hand dexterity. It doesn’t adjust with the temperature because a cold body restricts the flow of blood to extremities, keeping it near the heart. Less blood means more fumble fingers.

So that set of keys you dropped in the snow where they disappeared from sight, and seemingly off the face of the Earth?

Could happen to anyone.