A sky full of soft cumulus, drifting and casting shadows on each other.
Underneath is a field of noise, summed several times over at ever finer detail, which gives it the lumpy self-similar texture that clouds and mountains and coastlines all share. That grey field is then cut at a threshold: anything below the line becomes sky, anything above becomes cloud, and a narrow band in between gives the edge its softness rather than a hard cutout.
The shadows are what make it read as a sky rather than a pattern. The whole cloud field is worked out a second time, shifted slightly on the diagonal, and wherever that second copy sticks out past the first, the picture is darkened. That puts the shade beside each cloud instead of directly under it, which is what a low sun does. A second, tighter threshold varies the brightness inside each cloud, so thick middles come out white and thin edges stay grey.