Carlson on Linear Perspective for Clouds

This diagram is on page 78 of Carlson's Guide to Landscape Painting (1958), by John F. Carlson.

He explains:
"If a mountain or hill should rise in our landscape in such as way as to obstruct our view of the horizon, the arching and receding sky (or clouds) would ignore this temporary "horizon" and proceed back to the real horizon (out of our vision, of course) at its own ratio of convergence. It is this continuance of the sky back of the mountains, with the clouds coming up over it (and following their own perspective), that helps to give a sense of height to the sky and the mountain in our picture, as shown in Diagram No. 22. Of course, we speak of "converging" lines in the sky in only a figurative sense. We can only feel these unseen lines." (Ibid., page 77)