Thou on whose stream, 'mid the steep sky's commotion, Loose clouds like Earth's decaying leaves are shed, Shook from the tangled boughs of Heaven and Ocean,
Angels of rain and lightning: there are spread
The speaker continues to describe the West Wind.
This time, he describes the wind as having clouds spread through it the way dead leaves float in a stream. Leaves fall from the branches of trees, and these clouds fall from the "branches" of the sky and the sea, which work together like "angels of rain and lightning" to create clouds and weather systems.
Yep, there’s a storm coming!
Angels of rain and lightning: there are spread On the blue surface of thine airy surge, Like the bright hair uplifted from the head
Of some fierce Mænad, even from the dim verge Of the horizon to the zenith's height, The locks of the approaching storm.
The speaker creates a complex simile describing the storm that the West Wind is bringing. The "locks of the approaching storm" – the thunderclouds, that is – are spread through the airy "blue surface" of the West Wind in the same way that the wild locks of hair on a Mænad wave around in the air. Got that?
Let’s put it in SAT analogy form: thunderclouds are to the West Wind as a Mænad’s locks of hair are to the air.
A Mænad is one of the wild, savage women who hang out with the god Dionysus in Greek mythology. The point here about Mænads is that, being wild and crazy, they don’t brush their hair much.
Oh, and the poet reminds us that these Mænad-hair-like clouds go vertically all the way through the sky, from the horizon to the center.
Of the dying year, to which this closing night Will be the dome of a vast sepulchre, Vaulted with all thy congregated might
Of vapours, from whose solid atmosphere Black rain and fire and hail will burst: O hear!
The speaker develops a morbid metaphor to describe the power of the West Wind. The wind is described as a "dirge," or funeral song, to mark the death of the old year. The night that’s falling as the storm comes is going to be like a dark-domed tomb constructed of thunderclouds, lightning, and rain.
The poet ends by asking the West Wind once again to "hear" him, but we don’t know yet what exactly he wants it to listen to.