Relto hut, showing illumination from the west
You'll already have noticed that one side of the sky is brighter than the other. If mountain-side defines "north", then the eastern sky (dock-side) is clear and blue; the western sky is clouded.
It's the western sky that's brighter, by the way. If that seems odd, look at the cloud textures. They're lighter -- whiter -- than the sky's basic blue color. And the pattern of shadow and light implies sunlight behind clouds -- even if you don't have the "sun" Relto page. (If you do, of course, the sun appears in the west.)
And the whole effect is subtly reinforced by Relto's illumination; everything is lit on the west side.
Relto hut, showing illumination from the west
Anyway, back to the sky. If you look carefully, you'll see there is a dome texture that runs all the way around the sky. In fact, it repeats itself -- there's only 180 degrees of texture there. Every feature in the sky is repeated on the opposite side.
Opposite sides of Relto's sky
The trick is, on the east side of the sky, the cloud textures are cranked way down. They're more transparent -- nearly invisible, in fact.
If you stand on the end of Relto and look south, you can see the dividing line. Not a sharp line; it's a band where the sky-dome's transparency fades almost to nothing.
Time-lapse photos, looking south from Relto
The cloud texture is always rotating around the dome. But as any given feature moves across this dividing line, it fades away. And on the north side of the sky, "new" features are always appearing.
And that creates the illusion of changing cloud patterns. Without this transparency trick, you'd just see a fixed pattern turning above your head.