Just a word of warning to those of you planning to look at this demo, and then do some skies.
Have a good look at the photo. See the BASES of the clouds on the left.
The artist has separated the tones of the clouds, and then joined up the bases of the clouds into one large ultramarine SHAPE.
This destroys the sense of recession, and brings the further clouds, forward in the picture, instead of sending them away towards the horizon.
To get a real sense of recession in the sky, she needed to have kept the tones richer and darker for the clouds which were closest to the viewer, and then reduced the tone values as the clouds move "back" towards the horizon, since they are, strange tho it may seem, the furthest away from the viewer.
Recession could also have been implied even more, with more obvious overlap.
Do be awfully careful of this when you do your own cloud pics.