Yep, stole the song title.
Random Fact: Dante Gabriel Rossetti was so devoted to his wife Elizabeth Siddal that when she died, he buried a manuscript of his poems with her. However, several years later he wanted the poems back. He had to sneak into the graveyard at night, dig her up, and recover them.
1) Decide how much of a problem the cold is.
What is your northern environment like? Pine woods that regularly get heavy snowfalls? Mountains with snow that stays all year-round? Tundra? Ice floes? (Those last two are very rare in fantasy; currently I’m trying to remember the last time I saw a tundra setting, and not recalling it).
Whatever you decide on will (or should) affect the way your characters think and feel about the cold. If it goes away when summer comes, or if they could escape it by moving downhill, then they must choose to live where they do. What makes the heavy winters or the height and the dangers of the mountains worth it? Is there game to be hunted or ore to be mined that’s only available in those particular places?
If, on the other hand, you have a tundra, ice floe, or similar setting where the cold never goes away, the characters will have to adapt. You can take this to a “normal” extreme, by having them wear heavy furs and live on and with the ice, or by having them adapt so well that they don’t have to wear clothes at all. There are a few human tribes like this, though not many. In such cases, the characters would probably suffer if forced to move into a desert environment.
The point: Acknowledge the cold. Don’t take the usual fantasy course of only mentioning it when it’s convenient and forgetting it the rest of the time.
2) Know how your people would eat and survive the winter.
People in northern forests and mountains could have gardens, but only during the summer or on the lower slopes. The soil is likely to be much poorer than in the south, meaning that huge yields will be uncommon. Food would also have to be stored against the winter months, so fruit and vegetables that cannot be dried or otherwise preserved would be luxuries only. For most of their diet and survival, the people would rely on animals: goats for cheese and clothing, sheep (if they have them) for wool, the animals they hunted for meat, and, during harsh winters, whatever domestic animals they had for body warmth and close contact.
Tundra dwellers will live differently again. More of their diet will consist of meat from hunting. If they live near water, fish and seals are the most likely candidates. They may hunt whales, but such hunts take a long time, and if they live far away from everyone else there won’t be a huge market in scrimshaw or ambergris. In summer, life will be easier as everything madly spawns and has young. The soil is even thinner and rockier than in a pine forest, and is unlikely to grow large crops that humans can eat. Clothing would come mostly from fur.
3) Transportation will not be so simple as in most fantasy environments.
Thanks to the Amazing Mechanical Horses of most European-based fantasy, heroes don’t have to worry about their mode of travel. Even if they break a leg, there always seem to be materials around to build a travois, and clear roads to carry it along. Rain and snow sometimes get mentioned, but don’t slow them up a lot.
It’s very, very different in a northern environment.
Horses have trouble in deep snow. Given most fantasy authors’ tendencies to be dramatic and pile the snow up and up, or use blizzards, the problems will multiply. A horse floundering through snow is an exhausted horse, and if the hero runs into enemies, it won’t be as able to help him fight or run. Some kinds of snow, such as that frozen with a glaze over the top, will be almost impossible to move in at all. The horse’s hoof goes through with a crack, but then the leg stays there. The horse has to lift another leg and move it forward, then another, and then another. It’s very slow progress, and chancy if this is the only horse your characters have to lose. Snowshoes are better transportation, though of course slower than horses.
Dog sledges will help in an environment that isn’t mountainous, but again, the hero is limited to how fast dogs can run. He’ll also have to keep food on hand for them, which is something heroes usually forget to do for the Amazing Mechanical Horses. (They can just graze, runs the idea). That means hunting or bringing along a large cache of dried fish and meat, which will take up room on the sledge. It will be different if there are waystations or inns along the way, but even those should be rarer than in the south unless the hero is running a well-known trail.
The point on this one: Horses can’t go everywhere, not even the Amazing Mechanical Horses.
4) Predators will be more of a problem.
For one thing, there’s likely to be more of them. It’s perfectly reasonable not to expect a wolf in the middle of a settled southern estate. It would be strange not to find them in forests with plenty of game, even if some humans live there. Since fantasy humans don’t have guns, they lack the modern world’s usual way of getting rid of wolves. The packs are unlikely to approach unless they’re starving, but they will be competition for deer, moose, and elk in winter, and possibly take livestock that’s not well-guarded enough.
For another, lean winters can drive animals to do things they might not otherwise. Taking livestock is probably going to be much more common than killing humans (unless you have evil or possessed beasts), but they will be braver and more desperate, less likely to run at the mere sight of a human.
And finally, many northern predators have advantages in that environment that humans don’t. There are large ones to cope with. In a tundra or ice floe environment, there will be polar bears and killer whales. In a woods or mountains environment, there will be wolves, cougars, and possibly lynxes. They can all be dangerous if provoked, especially if they happen to be hunting the same prey as a human. And all of them can move more easily in conditions that would trap humans. The glazed snow I mentioned above, which horses have a hell of a time getting through, provides ground that a puma can just skate right over.
5) Expectations and beliefs are unlikely to be the same.
For one thing, the characters will probably try to come to terms with the cycle of harsh winter followed by brief, furious spring, summer, and autumn, followed by harsh winter again (or six months of darkness if they dwell far enough north). Their religion might concentrate on this. Perhaps they believe the winter is a punishment for their sins. Perhaps they think of the winter as a powerful god. Perhaps they see the earth as ultimately inhospitable to them and not very loving.
They’re unlikely to follow a carbon copy psuedo-Wiccan religion that celebrates all the Celtic holidays, though. Imbolc, which was traditionally the time the lambing began, would either have to mean something different in this type of environment or not matter at all.
The characters are probably also going to be better at survival, tougher, and more stoic. I’m always surprised when I encounter whiny teenage characters in a fantasy book that concentrates on the north. How do they have time to whine, given all the things they have to do simply to survive, and where did they get the beliefs (such as being entitled to silk sheets) that no one else around them seems to have? Northern communities will usually be isolated for a good part of the year, so it’s a lot harder to propose that the character is simply influenced by a fairy tale from the south.
6) Avoid making the northerners the superior culture.
As fun as I find fantasies set in northern environments, a lot of them have this distressing tendency. Southerners are depicted as soft, fat, weak, unable to do anything worthwhile (they have art and music, but those are not somehow worth the northerners’ time), and often as slaveholders. And it’s the narrative making the judgment most of the time, not just those characters raised on tundra or ice floe or mountain.
Try to show the flaws in both systems. While northerners may know much better how to survive in the wilderness, how to skin animals, and how to deal with the cold, they would probably be helpless in a jungle environment, and wouldn’t know things that most southerners would take for granted, such as how to adjust to the violent changing of the seasons.
Study and research is necessary here, I think, if only because so many fantasy authors either don’t live in cold environments or do so only in heated homes with modern facilities and no need to rely exclusively on hunting.