The woods crashing through darkness, the booming hills, Winds stampeding the fields under the window Floundering black astride and blinding wet (2-4)
The wind's power is compared to a stampeding herds' power because it isn't necessarily aware of what it's doing. It doesn't intend to destroy anything, it's just a side-effect of its nature.
then under an orange sky The hills had new places, and wind wielded Blade-light, luminous black and emerald, Flexing like the lens of a mad eye. (5-8)
The wind seems to be lording its power over the world at this point, using the "blade light" as a symbol of its lordship.
Once I looked up— Through the brunt wind that dented the balls of my eyes The tent of the hills drummed and strained its guyrope, The fields quivering, the skyline a grimace, At any second to bang and vanish with a flap; (10-14)
This exaggerates the wind's power for effect, yet again. It couldn't actually blow the world away, but the fact that it seems like it could is telling.
The wind flung a magpie away and a black- Back gull bent like an iron bar slowly. (15-16)
A gull is actually a lot flimsier than an iron bar. This simile makes the wind seem like it's accomplishing a feat of strength—but, in reality, it's just bashing some birds around.
We watch the fire blazing, And feel the roots of the house move, but sit on, Seeing the window tremble to come in, Hearing the stones cry out under the horizons. (21-24)
The wind's power is so great that it can disrupt people's ability to pay attention to one another. It totally forces itself on their consciousness.