The question is Shelley’s and finds its answer in what has gone before in the Ode to the West Wind:
Who chariotest to their dark wintry bed
The wingèd seeds, where they lie cold and low,
Each like a corpse within its grave, until
Thine azure sister of the Spring shall blow
Her clarion o’er the dreaming earth …
Spring is here already, waiting for the moment to blow her trumpet announcing new birth and rising.
Shelley cannot avoid Biblical reference: the seed must die to bear fruit (John 12.24) and while Shelley’s chariot may be borrowed from Donne, it refers to Elijah’s whirlwind departure from this earth:
And it came to pass, as [[Elijah and Elisha] still went on, and talked, that, behold, there appeared a chariot of fire, and horses of fire, and parted them both asunder; and Elijah went up by a whirlwind into heaven.
For me Winter arrived when I saw my first redwing of the year, come over from Scandinavia to spend the winter eating berries.
The watchful tree (a very early cherry) is flowering for Christmas in the park in Canterbury. (Jeremiah 1:11) Birds, trees, wind, whatever takes your eye; always look out for the signs of the times.