Technically, this poem is written in free verse, but before you kick up your heels and throw caution to the wind (see what we did there?), you should know that this is free verse that feels a lot like iambic pentameter. Sometimes Hughes uses nine or eleven or twelve syllables instead of ten, but pentameter (and the feeling of pentameter) is generally where this poem's at. Oh right, so what is iambic pentameter? Although defining it is one of the classic stand-bys of all English classes ever, we'll give a short refresher.
An iamb is a two-syllable measurement, a unit of rhythm known as a foot. The first syllable is unaccented and the second is accented, so it has a soft/hard beat. The beat goes like this: daDUM. Also, "penta-" is a prefix that means five—so iambic pentameter is a meter comprising five iambs, for a total of ten syllables. The beat goes on: daDUM daDUM daDUM daDUM daDUM.
Here's an example of two lines from the first stanza that fit this pattern pretty closely. Note that the second line has eleven syllables instead of ten. Don't get too hung up, though. Hughes is more interested in the general effect of pentameter than in using a really technical and precise version of it. You should still hear a general rhythm of daDUM daDUM daDUM daDUM daDUM.
This house has been far out at sea all night,
The woods crashing through darkness, the booming hills, (1-2)
As for the full form of the poem, Hughes orders it into six stanzas, each with four lines. Plenty of the lines are enjambed in interesting ways, too:
[…]. The house
Rang like some fine green goblet in the note
That any second would shatter it. (16-18)
So why all this iambic pentameter and enjambment? Well, on a few levels the poem's form pretty nicely reenacts a wind storm. It's generally rhythmic along the lines of iambic pentameter, but it's not mechanical. At the same time, the wind may have a certain consistency in its blowing, but it certainly can't be said to follow any pre-set, regular pattern. And you often get lulls in a wind storm, only to have the wind kick back up again. The enjambment between stanzas here does that same thing. As a reader, we comes across a lull as each stanza ends, but the last lines of those stanzas are often picked right back up again, pulling us back into the blustering storm. Hang on to your hats.