Wikis / MythicSeasons / Atmosphere / Physics

Physics

The physics section of atmosphere.yml tunes how air rises, how cloud and rain form and how the air settles back to its baseline. The shipped values work together, so change one at a time and watch the result with /ms atmosphere stats.

Every key except solver-threads can also be set per season, world or profile (see Where a block can sit). /ms reload applies a change within a step or two and keeps the running weather. Values outside the range move to the nearest end without a message.

Rates and weather time

Rates are per weather second. At the default timing one step is 8 weather seconds, so any rate of 0.125 (1 divided by 8) or more already acts in full every step, and higher values change nothing.

Convection and wind

Option	Default	Range	What it does
`buoyancy`	`0.035`	0 to 2	Higher gives stronger updrafts, taller clouds and more storms.
`pressure-iterations`	`40`	5 to 120	Passes of the wind calculation per step, a whole number. More make the wind follow terrain and updrafts more closely and cost more CPU.
`vorticity`	`0.0`	0 to 10	Extra swirl in storms. Above 3 divided by `grid.cell-size.xz` (0.1 for 32-block cells, 0.01 for 256) the winds grow out of control until they hit `max-wind`.
`max-wind`	`40.0`	1 to 200	Wind speed cap in m/s, for each direction (x, z and up-down) on its own.
`solver-threads`	`0`	0 to 16	CPU threads that share each step. 0 picks half the cores, at least 1 and at most 4. Only in `atmosphere.yml`, needs a restart.

A max-wind above 40 still changes the simulation, but /ms atmosphere stats, the wind placeholders and the atmospherewind mechanic and condition report each wind direction only between -40 and 40 m/s, and the ground wind speed only up to 25.5 m/s.

Temperature with height

Option	Default	Range	What it does
`lapse-rate`	`0.05`	0.0001 to 1	Degrees C the air cools per block of height, counted from the world's lowest block. Lower gives higher, rarer clouds.
`tropopause-height`	`336`	-2048 to 4096	Block Y where the air stops cooling with height. The weather stays mostly below it.
`ref-theta`	`12.0`	-60 to 60	Temperature in degrees C, at the world's lowest block, that the air settles back to over time. New weather starts from it.
`stability`	`0.017`	0 to 1	Higher damps tall storm towers and constant rain. The built-in default, used when no file sets the key, is 0.
`rad-tau`	`2400.0`	1 to 1,000,000	Weather seconds the air takes to settle back to the baseline of `ref-theta` and `stability`. Lower brings the upper air back faster.

The shipped lapse-rate is steeper than the real atmosphere on purpose: a Minecraft sky is too shallow for real cooling to form cloud.
tropopause-height keeps very tall worlds from filling their upper sky with permanent cloud, and does nothing in a world whose sky ends below it. A value below the world's lowest block is replaced by that block, with a warning.
The shipped seasons raise or lower ref-theta with the season, see Season Adjustments.

Air at the ground

Option	Default	Range	What it does
`surface-tau`	`120.0`	1 to 100,000	Weather seconds the ground air takes to follow the temperature and moisture of its biome, season and time of day. Lower is faster.
`surface-depth`	`16`	1 to 512	Blocks of air the ground warms, cools, moistens and slows down, in whole levels of `grid.cell-size.y` (at least one). Players read the ground wind from the level directly above.
`surface-max-humidity`	`1.0`	-1 to 2	Most moisture the ground pushes into that air, as a share of what it can hold (1.0 = saturated), so very damp or cold cells do not drizzle all night. The built-in default, used when no file sets the key, is -1 (no limit).
`drag-per-sec`	`0.10`	0 to 1	Friction on the wind in that air: the share of its speed removed per weather second.

If you make cells shorter (a smaller grid.cell-size.y), keep surface-depth at the height your other values were tuned for, so low cloud, fog and drizzle stay where they were.
Air masses raise and lower surface-max-humidity together with the moisture.

Cloud and rain

Option	Default	Range	What it does
`cond-rate`	`0.9`	0 to 10	How fast moisture the air cannot hold turns into cloud. At the default step length only values below 0.125 slow it down.
`auto-threshold`	`0.6`	0 to 20	Cloud water in g/kg above which cloud starts turning into rain. Lower gives drizzle, higher lets cloud build up and rain in bursts.
`auto-rate`	`0.12`	0 to 10	How fast the cloud water above `auto-threshold` turns into rain. 0 stops rain from ever forming.
`rain-evap-rate`	`0.08`	0 to 10	How strongly rain evaporates in drier air under the cloud, cooling that air.

Top of the sky

Option	Default	Range	What it does
`sponge-per-sec`	`0.08`	0 to 1	Share of the wind removed per weather second in the top two levels of the sky, so the wind that Forcing drives stays low up there.

Examples

physics:
  auto-threshold: 0.4

More drizzle, fewer bursts of rain.

physics:
  auto-rate: 0.0

Nothing ever rains and no storm forms. The shipped mars profile uses this.

worlds:
  resource_world:
    physics:
      max-wind: 15.0
      buoyancy: 0.02

A wind cap of 15 m/s and weaker updrafts, in resource_world only.

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