maszyna/docs/weather.md

Weather: clouds, fog and precipitation

The weather of a scenery is a set of cloud layers, a fog and a precipitation. The simulation keeps it (environment/weather.h), the OpenGL 4.6 renderer draws it: volumetric clouds with their shadows on the ground, and a fog with the shafts of the sun and the light of lamps and headlights in it (see opengl46-renderer.md, Weather).

A scenery describes it in a weather section. A scenery without one is drawn from its atmo section as before: the overcast gives the clouds and the precipitation, the fog range the visibility. Whatever sets the old overcast or fog range while the simulation runs (the sliders of the user interface, the __simulation.weather memory cell, the setweather command of the network) does so too, and the clouds and the visibility follow it.

While the weather is described, the overcast and the fog range the rest of the simulation goes by (lamps turning on, the sound and the look of the rain, friction, the __simulation.weather memory cell) are worked out from it.

The weather section

weather
  metar "27015KT 3000 -RA BR BKN008 OVC015 08/06"
  visibility 12000
  haze 800
  groundfog 40 180
  fognoise 0.6
  clouds cumulus sct 1100 2600
  clouds cirrus 3 8500 9200 0.8
  precipitation rain 0.5
  cloudwind 250 12
  temperature 8
  elevation 310
  seed 7
endweather

Every entry is optional and they may come in any order; a later entry changes what an earlier one (or the report) set. The section wins over an atmo section, wherever the two are in the scenery. Heights are in metres above the ground reference (see elevation).

Entry	Meaning
metar "report"	a weather report (METAR) read first: the wind, the visibility, the present weather (rain, snow, drizzle, mist, fog, shallow and patchy fog, haze, showers, thunderstorms), the clouds with their heights in feet, the vertical visibility and the temperature. The station, time, pressure, runway visual range and anything after TEMPO, BECMG, NOSIG or RMK are skipped. Reports name only the convective genera (CB, TCU); the others are taken from the height, the amount and the precipitation (low and broken: stratocumulus; low, overcast and precipitating: nimbostratus; very low: stratus; middle: altocumulus or altostratus; high: cirrus or cirrostratus)
visibility m	meteorological visibility at the ground: how far a dark object is still seen against the horizon (5% contrast)
haze m	how fast the haze thins out with the height: it is e times less dense this many metres higher. Low for a mist, high for a dusty or smoky air
groundfog thickness visibility	fog lying on the ground, this many metres thick, with this visibility inside it. It fills the valleys below the ground reference too. 0 removes it
fognoise 0..1	how patchy the ground fog is
clouds genus amount base [top [density]]	a layer of clouds (at most 4, the ones covering the most are kept). The genus: cumulus (cu), towering_cumulus (tcu), cumulonimbus (cb), stratocumulus (sc), stratus (st), nimbostratus (ns), altocumulus (ac), altostratus (as), cirrus (ci), cirrostratus (cs), cirrocumulus (cc). The amount: few, sct, bkn, ovc, or oktas 0..8. Without the top the layer gets the typical depth of the genus; the density multiplies the typical one (0..4)
noclouds	removes the layers set before (by a report, say)
precipitation kind [intensity]	rain, snow, auto (by the air temperature, as the overcast above 1 always did) or none, intensity 0..1
cloudwind direction speed	the wind at the height of the clouds: where it blows from (degrees from the north, clockwise) and its speed in m/s. The higher the layer, the faster it goes
temperature degrees	the air temperature, applied with the weather
elevation m	world height (y) of the ground the heights of the weather are counted from. By default the ground under the camera where the simulation starts; give it for hilly sceneries, so the clouds and the valley fog stay at the same height whichever place the simulation starts at
seed n	another arrangement of the clouds

What each genus looks like:

Genus	Drawn as
cumulus, towering cumulus	heaps with flat bases and rounded tops of different heights; towering ones a few kilometres tall
cumulonimbus	storm towers up to the tropopause, whose tops spread into anvils
stratocumulus, altocumulus	lumpy sheets; altocumulus higher, thinner and of smaller lumps
stratus, nimbostratus	even, grey sheets; nimbostratus thick and dark, the rain cloud
altostratus	a thin, even veil the sun shines through dimly
cirrus, cirrostratus, cirrocumulus	thin sheets of ice high up: strands drawn out along the wind, an even veil, small ripples

Changing the weather while the simulation runs

The __simulation.metar memory cell holds a report of the weather the simulation goes to. Setting its text (with an updatevalues event, or from a script) changes the weather: the text is a report, or entries of the weather section (visibility 800 groundfog 60 300 clouds stratus ovc 150 400); its value 1 is the time of the change in seconds. A text of an event can't hold spaces: commas, semicolons or underscores separate the words as well (clouds,cumulonimbus,bkn,900, 24015G25KT_4000_TSRA_BKN030CB). The change goes smoothly: the layers of the same genera change their sizes and heights, different ones fade out and in. A change of the old overcast or fog range takes over the clouds or the fog at once, as it always did.

The weather can also be changed in the editor (Weather and time, Clouds, fog and weather report) and in the Weather debug window of the renderer. These change the weather of this computer only; they are not sent over the network.

What the weather made of the old overcast looks like

Overcast	Clouds
up to 0.5	fair weather cumulus, more and taller with the overcast, and a high cirrus
0.5 to 0.9	a broken stratocumulus sheet and altocumulus above it
0.9 to 1	an overcast stratocumulus sheet and altostratus
above 1	nimbostratus, lower and denser the heavier the precipitation, with cumulonimbus above 1.6

The visibility is 2.5 times the old fog range, divided by the overcast as the old fog was. The old fog thinned out a dark object to 5% at 1.7 fog ranges, the new one at the visibility: the two cross about where the old fog took a third.

The cloud model of the sky section (Global.asSky) and the model of the starry dome (skydome_stars.t3d) are no longer loaded: the clouds come from the weather, the stars from a catalogue built into the simulator (the Yale Bright Star Catalogue, 5th edition, about 9100 stars down to magnitude 6.5), turned with the sidereal time of the scenery.