Please explain the following lines from "Ode to the West Wind."
Thou on whose stream, 'mid the steep sky's commotion, 15
Loose clouds like earth's decaying leaves are shed,
Like the bright hair uplifted from the head 20
Of some fierce Mænad, even from the dim verge
Will be the dome of a vast sepulchre, 25
Vaulted with all thy congregated might
You have quoted the second stanza of this poem. It can often be hard to follow the line of argument in such poems, but key to realise is that in this stanza, the wind's effect on the clouds is being described, and thus we are given a series of comparisons to explore this effect. Note the number of similes that are employed, using the word "like." For example, in lines 16-17, we are told that the clouds are shaken from heaven "like... decaying leaves... shed" from tree boughs. Likewise, the clouds are described as being "Like the bright hair uplifted from the head / Of some fierce Maenad." A Maenad was a woman in Greek mythology who performs frenzied dances in the worship of Dionysus, the god of wine. The effect of these comparisons are to convey a kind of hysteria or sense of impending doom.
In addition in this stanza, in line 28, the speaker of the poem addresses the wind directly, imploring the wind to "hear" his words. The impact of this is to create an invocation, as if the speaker were addressing a God, and therefore suggesting that humans are powerless in the face of such might and majesty.