This caught my eye simply because I can’t recall having seen such a sky, and can scarcely imagine it.  Clouded, but that doesn’t align with ‘stone-smooth’.  It is from the opening line of Edwin Muir’s ‘October in Hellbrunn’, written while in Salzburg.

The near-drawn, stone-smooth sky, closed in and grey,

Broods on the garden…

Source: Seamus Heaney, Finders Keepers: Selected prose 1971-2001 (London: Faber and Faber, 2003), p. 251