How the Making of ‘The Good Dinosaur’ Was Different From Other Pixar Movies
http://www.slashfilm.com/making-of-the-good-dinosaur/2/
Clouds in Pixar movies would usually be painted in by matte painters, using layers to create the illusion of animated drifts. But for this film, Pixar wanted to light the entire set and not have to go back and forth with a painter in the background. So they chose to populate the skies of the world with volume clouds.
The set team designed a library of different clouds with a set list of 11 different cloud types. Pixar’s artists and technicians also created a rain library, featuring different types of rain that could be mixed and matched and manipulated to create any desired look. The clouds would be combined to form unique looking formations, which were not just scenery in the background of the story.
The lighting team was able to have the volemetric clouds cast shadows on the ground, controlling the location of the sun to light each scene.