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The density of a cloud has a significant impact on it's appearance. The main control for density is the Density Scale setting on the volume, which scales the density from the input OpenVDB file. Low Density Scale values create an airier, less detailed appearance with more light bleed, while high Density Scale values produce a thicker, more detailed, or even solid appearance.
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The images below were rendered with Density Scales of 0.2, 1.0, and 5.0.
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Anisotropy
Adjusting the anisotropy settings in PxrVolume is also critical for getting a realistic cloud render. There are three anisotropy settings to pay attention to, Primary Anisotropy, Secondary Anisotropy, and Lobe Blend Factor. The Primary and Secondary Anisotropy settings control two anisotropic lobes that can be either forward scattering (set to +1) or backward scattering (set to -1). The Lobe Blend controls the relative contribution of the two lobes, where 0 uses only the Primary lobe and 1 uses only the Secondary lobe. Setting the Primary lobe to be forward scattering flattens out detail in some areas of the cloud, but it also adds a darkening effect to parts of the cloud facing the light source.
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