In what ways are clouds like leaves in "Ode to the West Wind" by Percy B. Shelley? Lines 15–23, stanza 2

The lines to which you refer read as follows:

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


This is the stanza which begins the second section of the poem, and, before we get to the comparison between the clouds and the leaves, it is helpful to understand the overall structure of the poem. In the first section of the poem, the speaker addresses the west wind, in an almost reverential tone, in awe of its power as it sweeps the autumn leaves "to their dark wintry bed." In the second section, the speaker, still addressing the west wind, describes how its power even extends to the heavens, as it is able to move clouds, or "Loose clouds like earth's decaying leaves." In the third section, the speaker describes how the wind is so powerful that it can even move whole oceans ("the blue Mediterranean"), meaning that it can create gigantic waves ("the Atlantic's level powers / Cleave themselves into chasms").

So, when Shelley describes the clouds as "like earth's decaying leaves" he is simply referring to how they, like the leaves, are at the mercy of the wind. Clouds are, of course, much bigger than leaves, but the wind is so powerful that it can move the leaves as easily as if they were as small and light as the leaves. The wind can just as easily shake the leaves from trees in autumn as it can shake the clouds loose from "the tangled boughs of Heaven."

This idea then follows in the third section. The power of the wind is described in reference to something even bigger than clouds—namely, the oceans. Shelley, with each successive stanza, is simply increasing his scale to emphasize the power of the wind.