The day and the look
Humidity, altitude and temperature, and how the vapor is lit and photographed.
The air
<WingVapor
air={{
altitudeM: 300,
temperatureOffsetK: 10, // a warm day
relativeHumidity: 0.9,
}}
/>Everything comes down to the humidity. The cooling a wing or a vortex makes is tens of kelvin at most, so at 30 % on a warm day nothing fogs, and at 95 % a gentle pull trails its tips. The altitude and the temperature offset set the International Standard Atmosphere's pressure, temperature and density, and so the speed of sound the Mach number is measured against.
Where to start, low over the sea on a warm day:
| to see | humidity | flight |
|---|---|---|
| tip trails | 0.85 to 0.95 | a hard pull, 5 to 7 g at Mach 0.5 |
| leading-edge vortices | 0.85 to 0.95 | a slow, high-alpha pull on a delta |
| the shock on the wing | 0.5 to 0.7 | a pull at Mach 0.9 to 0.95 |
| the vapor cone | 0.7 to 0.85 | Mach 0.95 to 1.05 |
The look
Vapor is white because it scatters, not because it is given a colour. What it looks like is the sun's and the sky's light, scattered by droplets of a given size:
<WingVapor
look={{
sunDirection: [5, 10, 5], // towards the sun, in world space
sunIntensity: 3,
skyColor: [0.55, 0.68, 0.9],
dropletRadiusM: 1e-6,
shutterS: 1 / 60,
}}
/>sunDirectionshould match your scene's sun, so the cloud is lit as the skin is.dropletRadiusM: vapor that forms in a millisecond makes many tiny droplets, around a micron. Smaller is denser for the same water.humiditySpreadandeddyM: how unevenly moist the air is, and how large its patches are. They break a sheet into patches.shutterS: the air goes past at the airspeed, so the patches streak along the flow by what it travels while the shutter is open. At 300 m/s and a sixtieth, five metres. The streaks are what make the vapor read as fast.anisotropy: a micron droplet throws most of the light forward, the silver of vapor seen against the sun.
Every field is in the reference.
Which phenomena
effects switches each one on or off:
<WingVapor effects={{ cone: false, selfShadow: false }} />It is compiled into the shader, so changing it rebuilds the material.