# Why is there a desert behind the mountains? In Dwarf Fortress, nobody put it there.

- Series: [How It Spawns](https://miu.fm/howitspawns.md)
- Premieres: 2026-11-08
- Page: https://miu.fm/howitspawns/why-is-there-a-desert-behind-the-mountains-in-dwarf-fortress

## Story

Why is there a desert behind the mountains? In Dwarf Fortress, nobody put it there.

Before you play, Dwarf Fortress builds a whole world. Tarn Adams, one of its makers, lays out the order: an elevation map "generated based on a randomized fractal", then separate layers for temperature, rainfall, drainage, vegetation and salinity, and only then does each tract of land become a biome. It's tempting to let a fractal pick the biomes directly, he writes, but the game "achieved much better results by handling fields separately".

So where does the rain fall? Make it random noise too, and deserts land anywhere: in our rebuild, 19 % of the land in front of the range and 15 % behind it. Switch on the wind instead. Rain rides in from the sea; when the air hits the range it climbs and drops its rain on the near side, and it comes over the top dry. In our rebuild that leaves 4 % desert in front of the range and 71 % behind it.

Adams: "the world maps improved greatly when rain shadows were taken into consideration (mountains block rain-producing weather systems, casting a shadow of dryness on the other side)". By 2008 his dev log notes that the world generator "forces in" orographic rain, the rain mountains make, even in a world set to no rain. His rule: "We know the real world works".

The continent, the wind and the biome table in this film are our own simplified version, built in code. How Dwarf Fortress works out its rain in detail isn't in the sources we read, so our numbers are ours, not the game's.

Did your last fortress sit in a rain shadow?

Sources: Tarn Adams, "Simulation Principles from Dwarf Fortress", Game AI Pro 2 (CRC Press, 2015), chapter 41. Tarn Adams, Bay 12 Games development log (June 6, 2008).

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