As I sit here this moment and watch the tree branches outside settle and groan under the weight of falling snow, I started thinking back on how many fantasy books I've read, and how the weather behaved in such books. Surprisingly (or not), a lot of it was questionable. Why is that? Well, I 'spect most folks don't know the how/why behind how weather works and I figured – being a trained meteorologist and lacking another subject to talk about today – I'd offer some insight.
Muse: Aren't you a precious peach? And by 'precious,' I mean 'pretentious,' and by 'peach', I mean jackass.
Thank you, Muse, for that note of encouragement.
Just to preface: this is just guidance, which is based on the physics and observed phenomena of our real-life environment. It's not all-encompassing, nor is it absolute. Tolkein's Middle-Earth, for example, existed as a plane in space before becoming a planet revolving about the sun. As such, physics as we understand it would not apply. In the Mistborn trilogy, there are some imposed differences on the physics of that world, which altered weather patterns. Alien worlds can have any number of explanations and if the writer is in this situation and is having trouble coming up with plausible reasoning for why the world exists the way it does, I think they are better off employing some hand-waved version of, "A Wizard Did It," than some half-assed explanation that will be picked apart by the more discerning readers.
Okay. I'm going to split this subject into several pieces, starting with the big planetary picture, then focusing down to more finite areas. Hopefully, it will prove useful in some small way to any readers and I will answer questions as best I can.
All right, enough talk. Let's make the following assumptions:
– We're dealing with a spherical planet. If you have a flat earth, or a supernatural space, then this all may not be applicable.
– We're dealing with life-forms as we know them, based on carbon and water. If your aliens are silica, again, this may not apply.
– There are no world-altering magical effects per above. Straight, normal world.
If you make these assumptions, you are left with a ball of blue and green, hurling through space. Pretty, but thus far, no answers on weather. But back to the ball. Now before you worry about weather, you have to answer some questions about your world. This isn't too hard but sketch it down on a sheet of paper and keep it handy; the reference will help a lot.
1) Orbit. How far is the world from the sun(s), and how fast is it whipping around said sun(s)? This affects several things. Astronomers speak of something called the "sweet spot" (In terms of orbital distance) in which liquid water can exist. This is based on the solar output of the star(s) and the radial distance of the orbit. But it's worth noting that if you go for the aesthetics of multiple suns, it will affect the orbit, and thus, the seasons. Here's an an example of orbital tracks around a binary system. Some of this can be ignored, I think, easier than the weather, as most folks are not up to the effects of binary stars on the orbits and the cascading effects. As a note, orbital track also affects year length and seasonal cycles.
2) Tilt. The Earth is tilted on it's axis approximately 23 degrees relative to its orbit. This tilt is what gives us the seasons (simplified explanation here), and such effects as the varying lengths of the day over the seasons. If your planet is tilted zero degrees relative to the orbit, every day consists of 12 hours of day and 12 hours of night (or half each of however long your day is). A tilt of 90 degrees means the world's spin is such that it is flat relative to the orbit, meaning, among other things that over the course of the year, the sun would rise at every point on the compass, not just in the east, and that only the equator would get sunlight every day of the year.
3) Rotation. How fast does your world spin on its axis? Earth takes just a hair over 24 hours to complete one spin, hence the day length. A faster or slower spin has affects wind currents, the daily heating/cooling cycle, cloud formation, and any number of other factors. The Earth also spins counter-clockwise (as viewed from the north pole); if you flip that, your sun will rise in the west, not the east. Also, your prevailing winds would reverse direction (which means, in bulk of the United States, big weather features would move from east to west, not vice versa).
This is all very academic and fascinating … but in the long run, I recommend sticking with an Earth-like setup for your world. One, it's familiar. Two, it's a lot less stress on you. But I offer these items up for consideration because in the event you want things to be different, you need to consider these issues. You may catch some flack ("Why does this book have the north pole in darkness in the winter? Why does the sun always rise in the east?") but that will be minor. Besides, unless these are major plot points in the book, the world-building is the flavor, not the meat of the story.
Next: Part II, in which we discuss topography, prevailing winds and currents, which drive biome growth and sustainment, and why all those fancy terms are important when you want to discuss why Thrug the Barbarian is getting rained on in the desert.