The VRaySky texture map is typically used as an environment map to help simulate outdoor lighting. The texture changes its appearance based on the position of the V-Ray Sun, which it is normally used in conjunction with. 

UI Path: Texture > V-Ray > VRaySky

Intensity Multiplier – Determines the intensity value when overwriting the Sun settings.

Turbidity – Determines the amount of dust in the air and affects the color of the Sun and Sky. Smaller values produce a clear and blue sky and sun as you get in the country, while larger values make them yellow and orange as, for example, in a big city.

Ozone – Affects the color of the sun light. Available in the range between 0.0 and 1.0. Smaller values make the sunlight more yellow, larger values make it blue.

Size Multiplier – Controls the visible size of the sun. This affects the appearance of the sun disc as seen by the camera and reflections, as well as the blurriness of the sun shadows.

Sky Model – Determines the procedural model that will be used to generate the VRaySky texture:

Hosek et al – When selected, the VRaySky procedural texture is generated based on the Hosek et al. method and uses a different ground and sky gradient.
Preetham et al. – When selected, the  VRaySky procedural texture is generated based on the Preetham et al. method.
CIE Clear – When selected, the VRaySky procedural texture is generated based on the CIE method for clear sky.
CIE Overcast – When selected, the VRaySky procedural texture is generated based on the CIE method for cloudy sky.
PRG Clear Sky – This procedural texture is generated using the PRG Clear Sky method. The updated PRG Clear Sky method allows for rendering the sky at various observer altitudes, up to several kilometers. It also supports rendering nautical twilight effects with sun positions up to 12 degrees below the horizon. Additionally, it features enhanced turbidity control, enabling the rendering of sky conditions with turbidity values ranging from 1.81 to 4.89. See the PRG Clear Sky New examples below.

Horiz Illum – Specifies the intensity (in lx) of the illumination on horizontal surfaces coming from the sky.
Note: Only available when CIE Overcast or CIE Clear are selected as the Sky model.

Altitude – Controls the observer's altitude in meters. As the altitude increases, sky clarity improves, and the horizon line becomes less defined.

Color Mode – Determines the way the color in the Filter color parameter affects the color of the sun.

Filter – Shifts the V-Ray sun's hue according to the Filter Color parameter. 
Direct – Sets the color of the V-Ray sun to the color in the Filter Color parameter. The intensity of the light no longer depends on the V-Ray Sun's position in the sky. Instead, intensity is controlled through the Intensity Multiplier. 
Override – Sets the color of the V-Ray sun to the color in the Filter Color parameter. The intensity of the light still depends on the V-Ray Sun's position in the sky.

Filter Color – Shifts the hue of the V-Ray Sun and Sky system towards the color specified in the field.

Ground Albedo – Sets the color of the V-Ray Sun and Sky system's ground.

Blend Angle – Specifies the angle in degrees where blending occurs between the horizon line and sky. Values close to 0.0 produce a sharper horizon line, while larger values produce a softer horizon line.

Horizon Offset – Allows the user to manually lower the horizon line.

Water Vapour – Controls the amount of water vapor which affect the sky control.

Invisible – When enabled, the sun becomes invisible to both the camera and the reflections. This is useful to prevent bright speckles on glossy surfaces, where a ray with low probability hits the extremely bright sun disk. This option is currently not compatible with the PRG Clear Sky model.

Sun – Specifies the Sun object, to which the Sky is attached.

Sun Direction Only – When enabled, the sky texture does not attach to the Sun. Instead, it only considers the Sun object's direction.

Example: PRG Clear Sky - Turbidity

In this example, you can see how the amount of dust in the air affects the color of the sun and sky. Smaller values produce a clear image, while higher values dim the sun and sky. 

Example: PRG Clear Sky - Nautical twilight effect

This example shows how the sun's position affects the sky, creating a twilight effect. Changing the sun's position could also require adjusting the Exposure of the V-Ray Physical Camera. In this example, Exposure Value is used for easier reading, but you can opt for Physical Exposure to be more physically accurate.