Why Is There So Much Fog in CDDA? The Weather System Explained
It’s not your map seed
Fog on five out of seven days isn’t a quirk of your world generation. Multiple players have documented the same pattern across different seeds, and there is an open bug report on the official GitHub tracker — issue #69605 — specifically about fog occurring far too often, including all day in summer when real fog would have burned off by mid-morning.
Short version: it’s a generation issue in recent builds, not a map seed problem. Your cameras aren’t malfunctioning. The sky is just wrong.
How the weather system works
CDDA models weather through three variables: temperature, humidity, and pressure. They evolve over time using simplex noise functions, seeded per-world but running continuously. The base calibration comes from historical New England data — Concord, New Hampshire specifically — so the seasonal shape is meant to resemble a real temperate climate.
Fog has a precise trigger condition defined in data/json/weather_type.json. It fires when humidity reaches 98 or higher, or when pressure drops below 993 hPa. There is no time-of-day filter and no minimum temperature-spread requirement. Hit the threshold, get the fog.
The noise functions driving humidity can spend long stretches in the high-90s range, particularly through seasonal transitions. Summer heat should theoretically push humidity down, but the interaction between the noise curve and the seasonal temperature model doesn’t always produce that cleanly.
Why recent builds feel foggier
The 0.H Herbert stable release (November 2024) built on a weather realism overhaul started years earlier — pull request #34515 — a significant rework of how the three weather variables interact. That PR improved seasonal variance overall. The fog threshold behavior wasn’t fully resolved alongside it.
Issue #69605, filed in late 2023, describes fog appearing all day in summer, in the evenings, and at night — times when it should have either never formed or burned off hours earlier. The issue remains open as of mid-2025.
Traveling to different map locations won’t change your fog situation because the weather generator runs world-wide. The seed is global, not tied to a specific tile or region.
The No Hope mod factor
If you’re running the No Hope mod, fog is intentionally amplified — it increases fog probability across the entire year as part of the mod’s difficulty design. A separate issue (#66995) notes that this has measurable downstream effects, including cutting solar panel output significantly. On a vanilla run with no mods and still drowning in fog? That’s the base generator behaving badly.
A separate problem: monster vision in fog
Issue #66586 is worth knowing about for city clearing specifically. In fog weather, monsters retain proportionally more vision than the player character, even though both should be equally impaired. Your deathmobile cameras lose range as expected — but enemies may be spotting you from further out than the fog visuals imply. Worth factoring in when you plan your approach.
What you can do about it
Debug menu weather override
The debug menu has a weather override option that forces a specific weather type temporarily. Useful right before a big city run. One caveat: there was a bug (issue #54133) where the debug weather override bled across all save files in some builds — not just the one you’re actively playing. Check your other characters if that matters to you.
Edit weather_type.json
The fog entry in data/json/weather_type.json defines the trigger conditions. Raising the humidity threshold — from 98 to 99, or removing the pressure-based trigger entirely — meaningfully cuts fog frequency without eliminating it. It’s a one-line change in the condition expression for the fog weather type. The change survives across sessions unless you update or verify game files.
Time your runs
CDDA fog doesn’t always dissipate when it should, but there are usually clearer windows around midday when temperatures peak. Checking the weather display before committing to a camera-heavy run costs nothing. Early morning and evening hours are consistently the worst — that’s when the temperature-humidity gap narrows and the fog conditions trigger most reliably.
Sources