Wherefrom sunbeams descend without warning? (3,2,3,4)

I believe the answer is:

out of the blue

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'without warning?' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for out of the blue that I've seen before include "Suddenly" , "Entirely unexpectedly" , "Without warning" , "Quite unexpectedly" , "From nowhere" .)

I've seen this clue in The Telegraph.

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