It's not difficult to see that these three cloud studies by John Constable are presented here upside down. While it is obvious that these paintings are not their right way up, this clashes with the idea of a cloud being a somewhat formless phenomenon. When we, when I, think of a cloud as a concept, it's this kind of fluffy ball, like a sheep without legs. This shape is of course without a distinct top or bottom.
As was the case with trees, the distinct orientation of clouds is a consequence of the mechanisms by which they form, as well as the inescapable truth that sunlight always hits the earth at an acute angle.