The trumpet has a long history of being associated with military strength, and often it has been used as a symbol of power and victory. The speaker of this poem appeals to the west wind to share with him its power. He notes all the wind has accomplished, driving dead leaves like ghosts, blowing winged seeds to their "dark wintry bed," and stirring the atmosphere to burst forth in fire and hail. The speaker realizes that the wind can't affect him physically as it does with most of nature, but he hopes that perhaps he can harness its powers to spread his ideas. He begs the wind to drive his "dead thoughts over the universe" to instigate change.

Thus, the final two lines, including the metaphor of being a "trumpet of prophecy," depend on the stanza before. The speaker asks the wind to use his own lips as a "trumpet of prophecy" as a spark to awaken the earth. The speaker wants to be used in a powerful way to instigate changes as powerful as the wind's abilities to influence nature. The speaker hopes that he can use this same sense of power to create a new "spring" (a season of rebirth) following the "winter" (a period of death and decay) that he feels dominates his own spirit. He looks to the wind to be his own source of power and victory over his dead spirit, prophetically restoring him to new life and using him as an instrument to spread this transformation across the earth.

Shelley believed that he, as a poet, was a kind of prophet and that his art was not only a means of self-expression but also a vehicle through which he could influence mankind and change the world for the better. Like other progressives of his time, he believed in the ideals of social equality, freedom, and justice initiated in the previous era, the Enlightenment. In this Ode, he sees the West Wind as symbolizing a primal force that will empower him and empower his art:

Scatter, as from an unextinguished hearth
Ashes and sparks, my words among mankind!
Be through my lips to unawakened earth
The trumpet of a prophecy!

"Unawakened earth" represents the world as it is, which has not yet been able to implement the ideals in which Shelley and other progressives, like his friend Byron, believed. The "trumpet of a prophecy" is Shelley's poetry itself, in which he is predicting the transformation of mankind in accordance with those ideals.

The darker side of this prophecy is a personal one, to which Shelley has alluded just before this final statement:

Make me thy lyre, even as the forest is:
What if my leaves are falling like its own?

Like many of the Romantics, Shelley had a premonition of his own early death. The Ode is thus a kind of farewell statement—Shelley's wish that, just like the verdant earth, which "dies" and is reborn every year, his own physical death he foresees will be symbolic of this never-ending process and will be an inspiration to others.