Told in short, quiet lines, "Sky" is about the feeling of lying back and watching the sky open up above you. It begins on the ground, with the "green of life" reaching and feeling, then lifts our eyes upward to a pale blue sky with no limits. From there the poem keeps climbing: clouds appear as "white ships" sailing and splitting apart, and above them are tiny gray specks, which seem to be birds, soaring even higher.
The speaker sounds calm and a little awed. Words like "relaxing" and "overtaking" suggest the sky gently takes over their attention, and nothing in the poem feels rushed or tense. The single-word lines work like slow breaths, and the repeated "-ing" verbs make everything seem to be moving without end. The ending, "Into vast expanse / Of sky," leaves the poem hanging open, which fits its subject. The mood is peaceful, free, and quietly uplifting.