Wikis / MythicSeasons / Atmosphere / Forcing

Forcing

The forcing section of atmosphere.yml drives the weather from outside: how much warmer the day is than the night, the wind in the upper sky, and slow wet and dry spells.

Most keys can also be set per season, world or profile (see Where a block can sit). /ms reload applies a change. Times are weather seconds. Values outside the range move to the nearest end without a message.

Day and night

The ground air moves toward this temperature:

biome temperature + season temperature + diurnal-offset + diurnal-amplitude x daylight

The biome temperature comes from biome-defaults, the season temperature from Season Adjustments. Daylight follows the world's time: 0 at tick 0, 1 at noon (6000) and 0 from 12000 to 24000.

Option Default Range What it does
diurnal-amplitude 9.0 0 to 60 Degrees C added at noon, fading to 0 at the start and end of the day.
diurnal-offset -4.0 -60 to 60 Degrees C added all the time, night included.

With the defaults the ground air is 4 degrees below the biome and season value at night and 5 above it at noon.

Wind

Only the top jet-layers levels of the sky (the jet levels) are pushed toward a target wind. Lower winds come from the simulation, and /ms atmosphere stats, <mythicseasons.windspeed> and <mythicseasons.winddirection> show the wind at the ground.

Directions are the way the wind blows toward, in degrees: 0 = +X (east), 90 = +Z (south), 180 = -X (west), 270 = -Z (north).

Option Default Range What it does
wind.direction 250.0 -720 to 720 Direction of the target wind. 250 blows toward north-northwest.
wind.speed-base 4.0 0 to 100 Target speed in m/s at the bottom of the jet levels.
wind.speed-top 28.0 0 to 200 Extra speed in m/s, from none at the bottom of the jet levels to almost all of it at the top.
wind.wander-a 35.0 0 to 180 Degrees a slow swing turns the direction to each side, the same at every level. 0 removes it.
wind.period-a 900.0 10 to 100,000 How slow that swing is. A full swing takes 6.28 times this (about 94 weather minutes).
wind.wander-b 20.0 0 to 180 Degrees of a second swing that differs by height, so winds at different heights point different ways.
wind.period-b 311.0 10 to 100,000 How slow the second swing is (a full swing takes about 33 weather minutes).
jet-layers 8 1 to 32 How many levels at the top of the sky follow the target wind, at most as many as the sky has. The built-in value is 5.
jet-tau 300.0 10 to 100,000 Weather seconds those levels take to follow the target. Lower reacts faster.

The top two levels are calmed by physics.sponge-per-sec, so the strongest wind sits directly below them.

Air masses

Air masses are large wet and dry areas that drift across the map and bring rainy and dry spells. They multiply the moisture the ground feeds into the air, and the physics.surface-max-humidity limit, by as little as 1 - strength in the driest spots and up to 1 + strength in the wettest.

Option Default Range What it does
air-masses.strength 0.0 0 to 1 How far wet areas are wetter and dry areas drier. 0 turns air masses off.
air-masses.size 8000 256 to 1,000,000 Size of the areas in blocks.
air-masses.speed 1.5 0 to 100 Blocks per weather second the areas travel.
air-masses.direction 250.0 -720 to 720 Direction they travel, like wind.direction.
  • The shipped seasons set strength to 0.3 (spring), 0.5 (summer), 0.35 (fall) and 0.6 (winter). atmosphere.yml itself leaves it at 0.
  • size, speed and direction only work at the top level of atmosphere.yml. In a season, world or profile block they are ignored with a console message.
  • The areas carry on after a restart and are the same on every server of a shared weather group. Changing size, speed or direction makes them jump once.

Examples

forcing:
  wind:
    direction: 0.0
    wander-a: 0.0
    wander-b: 0.0

A steady upper wind toward +X (east) that never turns.

forcing:
  diurnal-amplitude: 3.0
  diurnal-offset: 0.0

Noon is only 3 degrees warmer than the night.

atmosphere:
  forcing:
    air-masses:
      strength: 0.8

In a season file: long, clear wet and dry spells for that season.

Updated Nov 11, 2026