What is the meaning of "on the blue surface of thine airy surge" in the second stanza of "Ode to the West Wind" by P. B. Shelley?

This poem of course amazingly captures the spirit and
majesty of the West Wind through a series of incredible descriptions, depicting the Wind
as both "Destroyer and Preserver" before imprecating the wind to inspire the speaker to
spread his thoughts and ideas through the world like the dead leaves that are blown by
the wind, that will in turn bring life.

The quote in the
second stanza that you refer to is part of a larger description that describes the
wind's effect on clouds. A series of comparisons are made that will help us understand
the description. Let us just recall what is said:

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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,

Angels of rain and lightning: there are
spread

On the blue surface of thine aery
surge,

Like the bright hair uplifted from the
head

Of some fierce Maenad, even from the dim
verge...

Thus we can see that
your quote is part of a much larger chunk of this poem that describes the clouds of the
"approaching storm" as "locks" on the head of "some fierce Maenad." The quote you pick
out refers to the way that the wind blows these clouds across the blue
sky.