Procedural clouds were added to the Corona Sky in Corona 9.
To enable them, simply add a Corona Sky map as the scene environment and in its settings you will see the "Clouds parameters" rollout:
Here is what the Corona Sky map looks like with default Clouds parameters:
3ds Max viewport preview (left) and actual render (right). Note that the viewport preview is quite accurate.
Let's add Corona Sun now:
As we can see, the Clouds now get illuminated according to the sun.
If we lower the Sun closer to the horizon, we will see that the Clouds get illuminated with orange-red colors, like during a sunset:
Another Sun position:
If you set Corona Sun color to direct input - clouds will be colored based on the sun's color:
"Amount" controls the density of clouds:
Note that besides the main clouds there are also higher clouds - Cirrus. Their amount it controlled by the "Cirrus amount":
Cirrus amount: 1.0
Having both "Amount" and "Cirrus amount" set to 0.0 will result in completely cloudless sky:
Cirrus amount: 0.0
"Variety" changes the blend amount of different type of clouds. Think of it as "Seed" parameter for Clouds' shape:
Variety: 0.0 vs Variety: 0.5
Variety: 0.5 vs Variety: 1.0
Phase controls the minor cavities in the Clouds simulating their change over time. It is best used in combination with "Offset X" and "Offset Y" parameters to simulate clouds movement over time:
Phase: 0.0 vs Phase: 0.1
Phase: 0.1 vs Phase: 0.2
Seed randomly generates new clouds with the given settings.
"Altitude" controls how high above the ground the clouds are:
Altitude: 500m vs Altitude: 1300m
"Height" controls how vertically stretched the clouds are:
Height: 300m vs Height 1000m
"Offset X" and Offset Y" parameters move the clouds in world X and Y axes in meters. These in combination with "Phase" parameter can simulate the cloud movement:
In Corona Sun parameters an additional option is available for generating shadows from clouds:
This adds approximate shadows from clouds to the ground:
Cloud shadows OFF vs Cloud shadows ON
Plane count controls how many plane traces in total are in sky. You may need to increase it to see contrails in a specific camera view or offset them accordingly.
Plane count: 10 vs Plane Count: 40
for a specific view
Strength controls the opacity of contrails
Strength: 0.3 vs Strength: 1.0
Distortion controls the distortion amount in contrails
Distortion: 0.0 vs Distortion: 1.0
Time offsets the planes on contrail path
Time: 0.0 vs Time: 0.5
Offset parameters offsets the contrails relative to the clouds, just like in Main and Cirrus clouds.