One of the eternal challenges of winter is figuring out whether it’s too cold outside to take your dog for a walk.

There are a lot of factors to consider, like the actual temperature, wind chill and even the surfaces your dog will be walking on. But what about dog-specific factors, like size?

There’s a stereotype about small dogs that they get cold more easily than big dogs, and that is partly true. Toy breeds do lose their body heat very easily because of their higher surface-area-to-volume ratio.

(MORE: Can dogs get frostbite?)

But it’s not, ultimately, the full story. Size isn’t everything when it comes to a dog’s susceptibility to cold weather.

I’ve witnessed this firsthand over many years of having dogs in my family, ranging from Walter, a cross between a string bean and a dog that just wants to snuggle, to my dog, Brody, who hits his head on furniture when he sneezes and has a shedding season that lasts year round.

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I bring up these dogs in particular for a reason. They are perfect representations of what does or doesn’t make a dog equipped to deal with cold weather.

Bigger is not always better (or warmer)

Most dogs are ok when temperatures are above 45 degrees (though a sweater and insulated booties are a good idea for sensitive dogs), but once you dip below freezing, some big dogs are a bit more like the little guys and need extra added insulation.

(MORE: How cold is too cold for a dog walk? A breed-by-breed temperature guide)

Chihuahuas and French bulldogs are sensitive to cold weather partly because of their size, yes, but also because of their thin coats (smooth-coat chihuahuas, to be more specific). 

Larger breeds like Great Danes have the same problem with their fur. It’s just not designed to act as insulation, and some breeds, like greyhounds, have very little body fat to further insulate themselves.

This is why Walter, despite his large size, turns into a heat-seeking missile in the winter, and it’s why walks for big dogs with thin coats need to be kept a little shorter in the colder months.

Once you hit 20 degrees, all dogs are at risk from the cold, like hypothermia and frostbite, and you need to make sure your dog is properly protected from the elements.

What makes a dog ready for the cold

Some dogs, however, are literally built for cold weather. 

Introducing: the double coat.

Breeds like Siberian huskies, Alaskan malamutes and akitas have a second layer of fur called an undercoat that consists of soft, wooly, crimped hair that is densely packed between the long, coarser hairs that make up the “guard coat,” which is the fur you see.

This thick layer of fur is designed to insulate your dog from extreme cold, and even keep your dog cool in the summer by trapping a layer of cooler air above their skin and protecting them from sunburns.

(WATCH: Dog gets the zoomies among the fallen leaves)

This is what makes Brody so much better at handling cold weather than Walter (and better than me, to be honest). While a human might need to put on an extra coat if it’s cold outside, he already has one tucked in close to his skin.

Bonus adaptations: A double coat isn’t the only thing that cold tolerant breeds use to keep them warm. Smaller, fluffy ears reduce the risk of frostbite and fluffy tails can be used to cover their nose, protecting them from the wind and warming the air they breathe in.

Breeds like Saint Bernards use their bulk and muscle mass to generate heat and slow heat loss.

It’s also up to the individual. Dogs that experience cold frequently can become acclimatized and need less extra insulation, although I’m of the opinion that a dog in a coat is always a good look (as long as it’s not so hot they get overheated, which is another thing to consider).

Sara Tonks is a content meteorologist with weather.com and has a bachelor’s and a master’s degree from Georgia Tech in Earth and Atmospheric Sciences, along with a master’s degree from Unity Environmental University in Marine Science.