Horses handle cold weather better than humans do; equines evolved in the cold climates of northern Europe and Asia. Their natural "comfort zone" (energy-neutral temperature zone, in which they don't need to expend extra energy to maintain normal body temperature if weather is not wet or windy) is from about 15 to 60 degrees Fahrenheit. The horse's body is better at creating and conserving heat in cold weather than dissipating it in hot weather.

BODY CHANGES
As fall changes to winter, the horse's body undergoes a series of physiological changes, some of which actually begin long before the first frosts. As soon as the days start to shorten in midsummer, the horse begins to grow a new hair coat, more dense than his summer hair. As you brush and groom him in late summer you'll notice he's shedding some of his short summer hair. His metabolism begins to change also, enabling him to store more fat for insulation and for energy reserves. A layer of fat under the skin makes it more difficult for heat to escape from the body, and protects against cold weather. The layer of fat needs little energy to maintain, and has few blood vessels. The surface vessels that radiate heat in summer draw back deeper in winter.

As soon as nights start getting cold, his body begins to change, even if the days are still quite warm. If you are working a horse hard, you will notice that he does not dissipate heat as well as he did earlier in the summer; he may sweat more, and his sweat may start to have some smell to it, more like the sweat of an unconditioned horse.

He grows thicker hair as part of his protection against winter cold, but if he is blanketed to prevent this extra hair growth, or clipped, he won't do well outside during winter storms. Clipping makes it easier to cool out and groom a horse that is being ridden or worked in winter, but it's not wise to clip a horse that will have to spend time outdoors. If he does have a good winter coat, don't blanket him or bring him into the barn just because of foul weather. Horses prefer being outdoors even in the coldest weather and do fine if they have some kind of windbreak, or a run-in shed to get out of driving snow or rain. A horse in good condition with a good hair coat is usually better off outdoors. Make sure horses go into winter with adequate body condition. A thin horse won't winter as well as a fat one. Horses should never be too fat, but they need enough for a good insulating layer under the skin.

Long winter hair traps a layer of warm body heat between the skin and the cold air. When it's cold, tiny muscles in the skin make the hair stand up fluffy, increasing the insulating effect, and blood vessels near the skin constrict, conserving body heat by keeping the blood closer to the warm interior of the body, not allowing heat to escape from blood vessels near the skin surface.

TO BLANKET OR NOT?
A normal winter hair coat is much more insulating than most horse blankets. Adding a heavy blanket or piling on several light blankets can actually make a horse colder because it flattens out his hair and destroys the insulating effect. Blanketing may be necessary, however, for a clipped horse, or for one moved north during winter without a chance to grow a heavy coat, or a horse forced to stand outside in a winter storm without a windbreak. If a horse becomes so wet and cold he has to shiver to maintain body temperature, he'll burn more calories and need extra feed, or he'll start losing weight. Under those conditions, he'd be better off indoors or blanketed.

Horses have a normal body temperature of about 100 degrees Farenheit (38 degrees Celcius). They maintain this temperature in cold weather with the help of several mechanisms which include shivering, changes in hormone levels, increased body metabolism, increased digestion of fiber (adding more fiber or more protein to the diet can help a horse keep warm, since digestion of these nutrients produce heat), growing longer and thicker hair which can stand up on the skin to make a layer of insulating air pockets, increased feed consumption, and increased activity. Cold horses on a frosty morning often run and buck to warm up.

VALUABLE FUR COAT
A well fed horse can manage at temperatures down to 30 or even 40 below zero Farenheit if there's no wind and he's not wet. Wind ruffles the hair and destroys its insulating quality. The downward direction in which the hair grows (along with the oil glands that waterproof the hair) help keep a horse dry in rain and snow. The density of the hair coat and the directions in which the hair grows make such a good overcoat that snow can form ice on the outer surface of this coat without the skin becoming chilled.

It takes a lot of moisture on the hair coat before the dampness soaks through to the skin, since most of the water runs off. Once a horse gets wet, however, he may chill. A wet horse loses body heat up to 20 times faster than a dry horse, because the moisture flattens out the hair and eliminates the air spaces between the hairs, greatly reducing the insulating effect. Even a warm winter storm (rain instead of snow, or snow that immediately melts) can be hard on a horse, if he gets soaked and then gets chilled by dropping temperatures before he has a chance to dry off.

His best defense against cold are a long coat and a layer of fat just beneath the skin; both of these help reduce loss of body heat. Most wild animals go into winter fatter than they are at other times of year; this is nature's way to protect them against cold and give them some reserves for energy and body heat. Long winter hair is the first line of defense, but its insulating quality is lost if the horse is wet or covered with mud.

It's important that a horse have shelter during wet weather. A horse will rarely take shelter from cold, but he will try to get away from rain or driving snow. Horses prefer the warmth of winter sun to a shady shed. If an outdoor horse's coat gets muddy, groom him to keep it from being matted down.

PROGRAMMED FOR COLD
Humans tend to get frostbitten toes and noses in severely cold weather, but horses rarely suffer frostbite. The horse's blunt muzzle is so richly supplied with blood that it can withstand extreme cold without freezing. His long nasal passages with their bone spirals and air pouch (which he also uses for snorting and whistling when he blows air through it) help warm the cold air before it reaches his lungs.

A horse's feet and legs are constructed in such a way that they can withstand extreme cold without discomfort or damage, even when standing in deep snow. His slender legs are just bone and tendons below the knees and hocks, requiring much less circulation than muscles, and are thus less susceptible to frostbite. This allows them to handle extended exposure to cold and snow with no ill effects. The cells in bones and tendons need less blood for maintenance and they also lose less heat. The horse is able to shunt most of the blood away from his feet and still have a very functional foot. When the feet start to get cold, the shunts open up so that the blood flows from the smallest arteries directly into the veins without having to pass through the smaller capillaries.

If the horse gets cold, the blood vessels in his skin constrict to minimize heat loss, and the hair shafts stand on end for better insulating. If he continues to be cold, he starts to shiver, with his muscles rapidly contracting and relaxing--which quickly raises his metabolism rate and amount of fuel burned in the muscles. With his large blocks of muscle, the horse can shiver much more readily and more comfortably than a human. Since most of this muscle action is being converted to heat, this is a very effective way to warm himself. It takes a great deal of energy, however, to shiver for a prolonged period; this can use up his energy stores.

The horse has several other unique features that enable him to cope with winter. He is less vulnerable to snowblindness than a human, since his horizontal pupils can close more tightly than our round ones, filtering out more of the damaging ultraviolet light. His thick eyelashes protect his eyes from winter wind and extremely cold temperatures. If the wind blows, he instinctively turns his back to it. This protects his thin-skinned face and neck, which have more surface blood vessels. His rump and back have thicker skin and hair, and less surface blood vessels, and can withstand the wind better. He uses his tail to protect his more delicate underparts. His mane and forelock give waterproof protection for head and neck. Horses in groups stand close together to block the wind, and thus benefit from each others' body warmth. With a chance to prepare himself for cold weather, the horse can be quite comfortable and happy outdoors in winter.