why does the Wild West Wind iS call "Thou breath of Autumn's being" in Shelley's "Ode To Thw West Wind?

The West Wind is actually a wind that Shelley experienced in Italy at the Autumn time of the year. His "Ode to the West Wind" is actually his apostrophe to the wind as a sign of autumn which is now coming. His use of personification in saying autumn has "breath" suggests, as we see in other references later in the poem, that the wind is as much a living, life-giving being as the season itself. The west wind makes autumn what it is.