Concentrating on the heavens again, but first, a verse from Swinburne’s “Dolores” that is very far from heaven.

Seven sorrows the priests give their Virgin; 
But thy sins, which are seventy times seven, 
Seven ages would fail thee to purge in, 
And then they would haunt thee in heaven: 
Fierce midnights and famishing morrows, 
And the loves that complete and control 
All the joys of the flesh, all the sorrows 
That wear out the soul.

Like the rant on astronomy, this concentrates on factual mistakes. If you think you see me make one, please tell me.

1) Don’t give your country unnatural weather unless a magical explanation is involved.

This may seem pretty obvious, but at times, amateur fantasy authors sacrifice reality for the sake of convenience, especially when their heroes are on a long journey. They need to go over mountains? No problem! Just take away the snow that lingers there all year long and make the traveling, even in winter, easy and pleasant. They need to cross a desert? No problem! They always find water just in time, and don’t seem to be in danger from the heat in the day or the cold at night, even if no one in the group has ever traveled a desert before. Other times, the consequences of such meddling are less dramatic, but they still insure that traveling heroes don’t ever run into rain or snow that could do more than create picturesque effects.

Keep an eye on how much time has elapsed in your story, as well as the statements you yourself make about it. If this is a country in the middle of a rainy season, it doesn’t make much sense for characters to travel for three months and never encounter rain. If you’re taking them across high mountains, it doesn’t make sense for the walk to be pleasant and easy all the way, even if it is in summer. If you’re not familiar with weather conditions for a particular type of country, research them. Trying to have the advantages of a kind of terrain without the inconveniences is just another way of trying to have your cake and eat it too.

2) Once you start weather going, don’t forget about it.

I can’t tell you how easy this is. I’ve started a rainstorm in one paragraph, and then nearly forgotten about it when I wanted the sun to glitter off something. I’ve read a few books where the author states in one part that it’s snowing, and then a few sentences later that the heroes were able to see perfectly ahead.

If it helps, write yourself notes in the middle of a scene, like, “Don’t forget it’s raining.” This may seem simple and obvious, but it’s the simple and obvious details, like having the heroes feel cool in the middle of broiling sunshine, that remind your readers this is make-believe.

3) Don’t forget the consequences of the weather, either.

Perhaps the rain is nice enough to fall at night, and the heroes start on their way after it has stopped. However, the ground should not have stopped being soggy simply because the rain is no longer falling. Think about times you’ve walked outside after a heavy storm. If you’re following a packed dirt road, it would now be mud. Grass is slick and untrustworthy, and prone to standing puddles in low places. Footing is probably difficult on hillsides, especially for horses. In a fantasy world without sidewalks or many paved roads, the going would be even rougher.

If a snowstorm has stopped, it’s highly unlikely the snow would simply melt away. It needs a strong sun and warmer air for that, and even then, the snow will be lying around in treacherous slushy piles. In cases where the air is cloudy, or sunny and cold, the snow sticks around, and can make walking very difficult. The deeper the snow is, the more effort is required to break through. Ground that takes a few hours to walk in dry conditions can take a day to cover. Snow will cost your heroes time—or should. If they are somehow able to reach a town faster than they could if the ground was uncovered, something is wrong. (Horses get slowed down by snow, too, and are likelier to plunge a hoof into a hidden hole and break a leg; they may actually be more of a liability than a help).

4) Weather will affect the march of an army.

Few fantasy armies should be attacking in winter unless they have tricks to counter the snow. And this should only happen if they’re from countries where the winters are heavy and they would have reason to develop such tricks. Traditionally, spring is the fighting season, not just out of the rulers’ need to preserve their own stores and soldiers in winter, but because it’s really hard to maneuver in snow for an army without technology.

Even the breaking of winter might not enable the army to move quickly. Study the way weather works with the landscape. Streams swollen with snowmelt in the spring won’t be easy to cross. Nor will soldiers be able to move quickly in the heavy rains that spring brings to many temperate climates, or in the mudslides and muck that roads of dirt or landscapes heavy in clay can become. In the summer, broiling heat presents its own problems. In a country like a desert or a jungle where the seasons show less change, these won’t be problems to the same extent, but the heat and (in the jungle) the thickness of the trees and the rain will present their own challenges.

5) Often, heroes don’t seem to be equipped to handle the weather.

It’s remarkable how many fantasy characters go into snowstorms in summer clothes, and yet emerge with nothing worse than frostbite. (This is another thing you might want to study if you don’t know much about it; frostbite can be very severe, and cause people to lose fingers and toes). If your heroes are going to be tramping through mountains, make at least a passing mention of warm clothes and climbing gear. If they’re going to be crossing forests, they’ll probably want gear in greens and browns, and machetes to cut the undergrowth. Marching under bright flags and in colorful robes is very fine, but unless the characters are already part of a conquering army and can afford to take their time, it’s not practical for most of the ground they cross.

Similarly, weather will affect the availability of supplies the heroes need. If it’s been raining all day, the wood they gather will be wet and not as good for a fire. Likewise, many animals will keep to their holes, and it won’t be easy to hunt. Gathering food like berries in such weather is no fun, either. If it’s snowed and the snow is in heavy drifts, humans will flounder while deer will bound away. If the snow is frozen, with a glaze on top, then the deer will break through and become stranded—but so will horses, and humans without snowshoes. (Cougars, on the other hand, can trot along the top). Your fantasy heroes should make plans to deal with this, not rely on secret caches of miraculously dry wood and hidden meat, or else be prepared to go without fires and fresh meat for a while.

6) The weather can be an ally, too.

If the heroes have a water crisis in a siege, rain can be a means of restoring it. Sunlight can dry out a wet area so that it becomes reliable to charge an army down again. Snow can prevent an enemy army from attacking and gain a vulnerable group a little more time to prepare. As long as it doesn’t always serve (or too obviously) the heroes’ best interests, weather is a good way to spare your heroes some suffering or even give them an advantage. As a natural phenomenon, it’s more believable than mages being able to pull magical firestorms out of their asses every single time the heroes need them. If it’s magical, then it’s a subtler way of fighting the enemy than those same magical firestorms, and adds some variety to your action scenes, as well.

7) If you have unusual places in your fantasy world, consider what sort of weather they have.

If you have a race that lives far underground, obviously they aren’t going to know much about seasons, and they would have no reason to build houses with gutters or pointed roofs to keep off rain or snow. Elves building in the middle of a forest, on the other hand, will have to build with an eye to rain from above, and possibly snow if they live far enough north for that. If their houses have flat roofs, why is that? Do they have gutters, or some other, magical method of getting rid of the excess moisture?

If you change the nature of homes completely, and have unique architecture for one race or group of people, then consider how it fits into the weather around it. Perhaps you have a race of eagle-descended people who like to be able to see for miles in every direction, so they build wide-open houses on hilltops. However, this leaves them vulnerable to wind, and to the weather that rides the wind. If the hills were high enough, the houses also wouldn’t be very warm. Do they not mind the wind, perhaps? Do they have some other, magical method of resisting the cold? Or do they live in a place where this kind of weather isn’t a problem, and why? All these things should be taken into consideration when you adapt architecture, so that you don’t wind up with a construction that’s absolutely beautiful in theory but useless in practice.

8) Geographical features can alter weather.

It’s possible to have a flourishing farm country beside a seacoast even if the rest of a country is cold, as long as a warm current runs up the seacoast and so the weather is balmier there. Likewise, it’s possible to have very good farming country on one side of a set of mountains and poor soil on the other. As rainclouds pass over mountains, the moisture gets squeezed out of them, and the part of a country in the “rainshadow” of the mountain range won’t receive as heavy a fall as the land on either side of it. This could work as a guide to where to place the desert Kingdom that you want.

Also, consider the direction of the wind. If you have an ocean nearby, storms could blow in from that. If you have a mountainous country, it will be quieter in the valleys than in the peaks, since the bulks of the mountains will help shield the valleys from the worst of the wind. On the other hand, an open plain with no obstacles to the wind is likely to be visited with incredible storms.

Just some more details to make the fantasy world more realistic, and to be able to know whether your heroes could hunt deer on a particular day or from which direction the wind is likely to come.