Mountains, Deserts and Biomes: Getting Your Fantasy Map's Climate Right

You fixed the rivers. Every one starts in high ground and runs downhill to the sea, nothing forks. Good.

Now look at the green bits.

If your continent has a desert on one coast, a rainforest on the other, and a strip of tundra in between for no reason, the map is making the same mistake the rivers used to make. It looks drawn rather than discovered. Water has one rule (down). Climate has about five, and they are almost as easy to learn.

This is the sequel to the river post. Same idea: principles that people notice being broken even when they can’t say why, and a quick fix for each.

Start by deciding where the equator is

Every rule below depends on latitude, so this is the first decision, and most fantasy maps never make it. Is your continent sitting on the equator, straddling the tropics, or up near the pole? You don’t have to draw the line on the finished map, but you have to know.

The sun does the work. The equator gets the most direct sunlight, so the air there is warm, wet and rising. The poles get the least, so they are cold and dry. Everything in between is a gradient, and the gradient is what puts a desert in one place and a forest in another.

If you are using the generator, be honest about what it does here. Its moisture layer is a noise field: wetter patches, drier patches, forests where the wet patches sit on low or middling land. It does not know where your equator is and it does not compute rain shadows, so it will occasionally put a forest where your climate logic says scrub. You decide the latitude and fix the outliers by hand with the tree and label tools. A procedural moisture map is a good first draft, not a climate model.

The five bands

Earth’s atmosphere sorts itself into bands by latitude, and the pattern is regular enough to use as a template. Roughly, from the equator outward:

Latitude	What the air is doing	What you get
0° to 10°	rising, wet, warm	rainforest, year-round rain
10° to 25°	seasonal, wet summer and dry winter	savanna, monsoon forest
25° to 35°	sinking, dry	the great deserts
35° to 55°	westerly winds, storms from the west	temperate forest, grassland
55° to 70°	cold, moderate moisture	boreal forest (taiga)
70° and up	cold, dry	tundra, then ice

Why deserts sit at 30°. Hot air rises at the equator, drifts toward the poles high up, cools, and comes back down at around thirty degrees north and south. Sinking air warms as it descends, and warming air holds its moisture rather than dropping it. The result is a belt of high pressure with almost no rain. The Sahara, the Arabian deserts, the Kalahari, the Australian interior and the Atacama all sit in this belt. This is the single most useful fact in this article: if you want a big desert, put it at about a third of the way from the equator to the pole.

Why the temperate zone is green. Between roughly 35° and 55°, the prevailing winds blow from the west and carry storm after storm off the ocean. That is where most of Europe, most of the United States and most of the places readers picture when they hear the word “kingdom” live. Oak forests, farmland, rain in autumn.

Why the far north is trees, then not trees. Boreal forest is a band of conifers running across Canada, Scandinavia and Siberia: cold, but wet enough for trees. Further north the growing season becomes too short for anything with a trunk, and you get tundra, then ice.

Your world does not have to copy these numbers. A smaller planet or a different axial tilt would shift them. But ignore the pattern entirely and you get the desert-next-to-rainforest problem, and readers feel it.

Wind decides which side of the mountain is wet

The bands tell you the overall picture. Mountains rewrite it locally, and this is where most mapmakers get the most mileage.

The rain shadow. When wind hits a mountain range it has to go up. Rising air cools, and cool air cannot hold as much water, so it rains on the slope facing the wind. By the time the air comes down the other side it is dry, and it warms as it descends, which makes it drier still. The side facing the wind is green. The side behind is brown. This is not a subtle effect. On the Olympic Peninsula in Washington State, the Hoh Rainforest on the windward side gets around 140 inches of rain a year, and the town of Sequim, about seventy miles away on the far side of the mountains, gets around 17. Same latitude, same ocean, one mountain range in between.

Which way the wind blows. So you need to know the wind direction, and the bands give it to you. In the tropics (roughly 0° to 30°) the trade winds blow from the east. In the temperate zone (roughly 30° to 60°) the westerlies blow from the west. Near the poles the wind is easterly again. So a north-south mountain range in the temperate zone is wet on its western face and dry on its eastern face. The same range in the tropics flips: wet on the east, dry on the west.

Two examples worth copying. The Atacama in Chile is the driest non-polar desert on Earth, and it earns that by stacking effects: the Andes block the trade winds from the east, and a cold ocean current runs along the coast to the west (more on currents in a moment). Deep in Asia, the Gobi sits in the rain shadow of the Himalaya and the Tibetan plateau, which wring out the monsoon before it can get anywhere near Mongolia, and it is also simply a very long way from any ocean.

To use this on your own map: draw the range, decide the wind, and shade the far side. If you have a desert you love that isn’t at 30°, the rain shadow is your alibi. Put a mountain range between it and the sea, on the upwind side, and the desert is now earned.

Coasts, currents and the inside of continents

Distance from the sea. Most of the moisture in the air came out of an ocean. The further inland the air travels, the more of it has already fallen as rain. So continental interiors are dry, and the bigger the continent, the drier the middle. Central Asia is the extreme case: steppe fading into desert thousands of miles from any coast. If you have drawn a huge landmass, the centre should be grassland or worse unless a big inland sea is feeding it.

Cold currents dry the coast; warm ones wet it. Oceans move in great loops. On the west coast of a continent in the subtropics, the current tends to be cold, flowing from the pole toward the equator. Cold water means less evaporation, and it chills the air above it so it cannot rise and form rain clouds. This is why so many coastal deserts (Atacama, Namib, Baja California) sit right beside an ocean. On the east coast at the same latitude, a warm current flows the other way and feeds humid, rainy weather. Florida and the Namib are at similar distances from the equator. One is swamp, the other is sand.

The Mediterranean pattern. West coasts between about 30° and 45° get a specific and very useful climate: dry summers, wet mild winters, olives, vines, scrub on the hills. California, central Chile, the Cape of South Africa and the actual Mediterranean all have it. If you want a region that feels like classical Greece or Andalusia, it goes on the west side of a continent, a bit poleward of the desert belt. The east coast at the same latitude is humid all year (the American South, eastern China): forest, rice, storms coming in off a warm sea.

Altitude is latitude in miniature

Going up cools the air by roughly the same amount as going toward the pole. Climbing a tall enough mountain at the equator takes you through rainforest, then temperate forest, then conifers, then alpine meadow, then bare rock and snow, all in a day’s walk.

Plateaus are cold and often dry. A high plateau in the tropics can be cool and pleasant (the highlands of Ethiopia and Mexico both host old cities for this reason). A high plateau in the temperate zone can be brutal steppe. Either way, a big raised area on your map should not share the biome of the lowlands around it. And permanent snow sits low on mountains near the pole and very high on mountains near the equator, so if every peak has the same white cap regardless of where it is, that is a small tell.

Where each biome belongs

Pull the rules together and you get a placement guide:

Biome	Where it goes
Rainforest	on the equator, or a tropical coast facing the trade winds
Savanna, monsoon forest	just outside the equatorial belt, with a wet season and a dry one
Desert	around 30°, in a rain shadow, beside a cold-current coast, or deep in a continent’s interior (ideally two of those)
Grassland, steppe	between desert and forest; temperate continental interiors
Temperate forest	the westerly belt, wherever the rain reaches; your default kingdom landscape
Boreal forest	poleward of the temperate zone, across the whole width of a continent
Tundra	beyond the tree line, at the top and bottom of the map
Swamp, marsh	any flat lowland where water arrives faster than it drains, especially river deltas

Swamps are the exception. Wetlands are not a latitude thing. They form wherever the ground is flat and low and the water table is high: deltas, the inside of a river bend, the shore of a shallow lake. A swamp on a hill is wrong. A swamp where a big river meets the sea is almost mandatory.

None of this needs to be labelled. Readers do not want a climate key. They want a forest that is where a forest would be, and they will notice the difference without knowing they noticed.

Scale changes which rules matter. On a world map the latitude bands are everything, because one continent might run from the equator to the pole. On a regional map covering a few hundred miles, latitude barely changes across the page and the local effects take over: which side of the hills is green, how far the flood plain reaches. As with map scale generally, decide what the page covers before you decide what grows on it.

A checklist before you call it done

Run down this list once and most climate problems disappear.

1. You know where the equator is, even if it isn’t drawn.
2. Big deserts sit around 30° from it, in a rain shadow, beside a cold-current coast, or deep in a continent’s interior. Preferably more than one of those.
3. Every mountain range has a wet side and a dry side, and the wet side faces the prevailing wind for that latitude.
4. The middle of any large landmass is drier than its coasts.
5. Rainforest touches the equator or a windward tropical coast. It does not sit next to a desert without something (mountains, a lot of distance) in between.
6. Tundra and ice are at the top and bottom of the map, or on the highest ground, not in the middle of a temperate kingdom.
7. Swamps are in low flat places with too much water, usually near the mouth of a river.
8. Forests thin out as you move toward deserts and toward the poles, rather than stopping at a hard line.

Break the rules on purpose. Once you know them, break them deliberately and make the break the story. A rainforest in the middle of a desert means a spring, a god, or an aqueduct somebody built. A frozen kingdom at the equator means a curse or a very old war. The point of getting the climate right everywhere else is that when it goes wrong, the reader knows it means something.

Generate a continent, decide where the sun is, and go looking for the places where the moisture layer disagrees with you. Those are either mistakes to fix or mysteries to write.