While I love reading about characters and their exploits, I can find plenty of authors who do character well. I know an author’s one to watch, though, when I can tell that he or she has devoted plenty of attention to climate and geology. However, most writers tend to prefer language and psychology over geology and climatology, so finding a world with a somewhat unrealistic climate and morphology is par for the course — and, to some extent, justified by SF/F’s engagement with the unreal.

This focus on culture doesn’t mean that writers don’t want to learn to build worlds with climates, and while I’m by no means an expert, I can offer a general overview. Below the cut, I’ve provided a really, really basic primer for how to start thinking about building an Earth-like world with a cohesive climate and different biomes. This isn’t meant to be a comprehensive guide, and I’m intentionally leaving out some of the gritty details on (for example) rock morphology, magnetic vs. geographic poles, and desertification because that’s advanced-level stuff. I am also choosing to avoid addressing the advanced-level stuff surrounding a) non-Earth-like worlds and star systems, b) major global climate shifts a la glacial ages, c) unusual weather events like hurricanes and rogue waves, and d) wizards and pilots with weather-changing powers. All of these subjects, however, are fascinating and deserve further research.


Here’s our planet; let’s call it Htrae. Note that it is a round world, although most round-ish worlds wind up being ellipsoids.


We’ll give it some continents and other landmasses, because landmasses are generally (but not exclusively!) where the action occurs.


The first thing we’ll have to figure out is where the equator is; the equator is the part of the planet that gathers the most sunlight. On Earth, we treat the Equator as a latitude of 0°, and the tropics (at latitudes 23° 26′ 16″ N and S) block off a region that tends to get a substantial quantity of sunlight. The equator also serves as a watershed for things like wind directions and ocean currents, so knowing where the equator falls is important!


Next, we’ll figure out which direction Htrae is rotating. I’ve chosen to make it rotate counter-clockwise (as observed from the north), like Earth; among other things, this means that the sun will rise in the east and set in the west on Htrae. For most Earth-sized, Earth-shaped planets where the sun rises in the east and sets in the west, the following advice will apply. (Feel free to modify the model if you’ve got technological climate control or weather wizards.) I’ve also marked off where the north and south poles are.


In general, heat distributes itself into cold places; thus, you would expect air to rise from the equator and descend on the poles. However, Earth-sized planets are big, and the heat generally dissipates before it gets all the way to the poles — so what winds up happening is that you get several different zones of rising and falling air. Moreover, because the planet is still moving, we can observe the Coriolis effect in action. (There’s a great little animation at the link.) I’ve drawn a really simplified model here of how winds tend to move in different zones, but there’s a much more complicated and accurate one here.


The same general rules that apply to moving wind also apply to moving water; in the northern hemisphere, ocean currents move in a clockwise direction, and in the southern hemisphere, they move counterclockwise. (Currents close to the equator are trickier to plot, and the Alaska/Oyashio current system is off in its own little world.) Note, as well, that currents pick up warm water at the equator, then take that warm water toward the poles; when the currents swing back up toward the equator, they’re cold currents. This effect is part of why the UK is so much warmer than Maine, even though it’s further north; Maine’s the direct beneficiary of the Labrador Current (cold), and Britain receives the warmth of the Gulf Stream and the North Atlantic Current that branches off of it. It’s like a hot water bottle for northern climates.


I’ve created a really basic model here for how warmth would get distributed on Htrae. Note that, while in general warmth is clustered at the equator, the currents make the southeast parts of Continent C unusually warm, and the northwest parts of Continent B unusually cool. The southwest part of Continent D is also surprisingly temperate, given its proximity to the tropics.


Of course, winds and waters aren’t the only important features of a planet. I’ve also drawn on some arbitrary tectonic plates, and it’s important to remember that those plates are always moving. They separate, press up against each other, or slide under each other. Where they press together, you’ll often find mountain ranges; at plate boundaries, you’ll often encounter volcanos, earthquakes, and tsunamis. On Htrae, we can see that the island ranges that link Continents A and C are probably volcanic, and thus the mountain range along the north shore of A is probably volcanic, as well. The range along the middle of Continent B is more like the Himalayas — a compression boundary, with mountains that grow slowly every year.


Mountains have a strong influence on how precipitation gets distributed. In general, the side of a mountain that receives the wind gets a lot of rain, and the side of the mountain in the lee of the wind winds up being drier. (Those of you who watched The Magic School Bus as kids or with your kids might remember the rain shadow effect.) Warm, moist air tends to carry water from the oceans and to distribute it as rain as the land rises; thus, mountains that are on the receiving end of winds rising from the warm currents tend to get drenched. You’ll often find rainforests there; that’s part of why the Northwest US is a rainforest, and the middle states are comparatively dry. The northwest is on the receiving end of the warm Alaska Current, and it’s got some high mountains to catch that rain — but that rain often doesn’t make it over the mountains. On Htrae, Continent A has a large central desert blocked off by mountains, with rainforests on either side; the northeast parts of Continent B are colder and probably only slightly less dry. These steppes (and possibly taiga to the west) catch the cool, dry air and miss out on that equatorial sun. The isthmus between B and D gets hammered with heat and rain, while the west coast of C probably experiences some severe weather patterns — a dry season and a monsoon season, nothing temperate about it.


For a good idea of how much rain it’s logical to expect in different regions, Earth is always a good reference. If you want to be able to classify the terrain and weather patterns you’re observing, it helps to break landscapes down into biomes.

And there you have it! The world of Htrae, and the climates that shape its people.