It's cold out! Putcha coat 'awn!
Dogs with natural evolutionary advantages in coat quality have it easy, very easy. They simply stay outside, adjust to the climate changes of winter, spring, summer and fall; allowing their fur to shed dead hair and regrow healthy hair based on the arriving climate. Wolves, foxes, coyotes and dogs like huskies, chow-chows, akita's, malamutes and many other breeds have three 'layers' to their body-hair. The outer-most layer is known as whisker hair, this is for tactile use and aids in subtle detection of the environment through wind and touch. Next is the dogs primary hair, this is a portion of hollow hair which enables the dog to stay toasty in winter and breezy in summer.
This is done by trapping the hot or cold air within the follicle thus giving it the name 'guard-hair'. The last layer and most well known is the undercoat. This layer is very dense, very thick and very soft fluff which insulates the dogs body so well that outdoor dogs are known to stay comfortable in weather as low as minus 75-100°F (which scientists say would be extremely painful for a human to breath in). Canine undercoat is also known as chiengora and is warmer and more water repellent then sheep wool. It is indeed a myth that during the summer months, a dog will be cooler if he is shaved.
This is because as people, we feel cooler when we take off layers of clothes, only because our sweat glands exist all over our body and release moisture in order for us to cool off. This is one of humans greatest evolutionary advantages over other animals but we tend to anthropomorphise it over onto our dogs. Dogs do not sweat from their body, in fact they have very few sweat glands, most being on their feet and nose. When a dog needs to cool off he simply transfers the atmospheres air over his tongue, releases minuscule amounts of moisture from his pads and viola--mission accomplished. The only reason to shave a dog is for ease of maintenance and to dematt if hair growth has gotten out of control.
Dogs without an undercoat are still yet at a higher advantage to humans during the cold season due to higher running body temperatures in comparison to ours. Dogs run around 101-103°F compared to humans 97-98°F keeping them toastier in the snow than our hairless carcasses could cope. With that being said, unless your dog is specially equipped with an undercoat, during the coldest winter months it will not matter if his hair is 6 feet long or 6cm long, he will be unable to survive negative temperatures.
As for summer, dogs will stay cool under a mane of thick hair just as much as if they were shaved down like a new recruit. For double coated dogs in the summer, feel free to rake out the undercoat if they're not permanent outdoor residents; living indoors will not allow them to properly blow out (release the excess insulation) with the changing during spring and fall seasons. It is this blowing out that is essentially just like you taking your coat off-- and 90% of that gloriously warm wool will wind up on your floor and furniture.
Hardiness to temperature is different for individual dogs based on breed, environment, age, size, diet, hormones, activity and physical proficiencies. The Nordics (Husky, Akita, Malamute, Samoyeds etc.) can handle anything you can throw at them, the big boys without the double coats (Sheperds, Rotts, Danes, PBT's, Labs, Dobs etc.) can stand pretty strong in a moderate storm. The Chiweenie, maaaay....... need a sweater thrown on it, and some ear-muffs. Know you dogs coat and know how environments play in favor or against them, and enjoy being outside battling the elements with them wherever you venture! --Be free. Enjoy your dog!