Taken from https://github.com/mmp/pbrt-v4-scenes/tree/master#disney-cloud

The original dataset is Copyright 2017 Disney Enterprises, Inc. and is licensed under the CCA-SA 3.0 Licence. The volumetric cloud model used for reference is a photograph by Kevin Udy provided on the Colorado Clouds Blog and licensed under the CCA-SA 3.0 Licence.