According to Bowles' autobiography, Without Stopping, the novel's title came first: "Before the First World War there had been a popular song called 'Down Among the Sheltering Palms'. … [I was fascinated by] the strange word 'sheltering.' What did the palm trees shelter people from, and how sure could they be of such protection?"
Bowles' comment suggests that the adjective in "sheltering sky" is meant with some irony, and yet the only time the phrase occurs is in the very last paragraph of Chapter 23—Port's dying breakthrough—indeed, in the very last sentence: "Reach out, pierce the fine fabric of the sheltering sky, take repose." The irony here seems much less evident.