For most of the poem the air has the field to itself. Edward Thomas gives it "two voices / Of wind and rain", and the noise is huge and oddly pleased with itself. The air "triumphs", and it is "as loud as if in anger it rejoices". That phrase holds anger and joy in a single breath, so the weather is neither gloomy nor merely threatening. It bullies and revels at once. The earth, by contrast, is reduced to something like a choking animal: it "gulps and gulps in choked endeavour vain / To swallow the rain." The repeated "gulps" makes the line stumble, and the awkward inversion of "endeavour vain" seems to catch in the throat along with it.
The second stanza moves into the night, and at first nothing changes. "Only the wild air speaks", and the word "only" suggests the earth has been silenced altogether. Then comes the turn: "Till forth the dumb source of the river breaks." "Dumb" means voiceless, and "breaks" does two jobs, since the water breaks out of the ground and breaks the silence. What follows is a reversal of power carried by a single verb. In the first stanza the air was "Drowning the sound of earth". In the second the source "drowns the rain and wind". Thomas hands the same weapon to the other side.
The poem never explains this, but the title invites a reading. The earth that seemed to be failing to swallow the rain has simply filled up, and the overflow returns as the beginning of a river. Its apparent defeat was a stage in something larger, and "source" can mean both the spring itself and the origin of the earth's strength. That is an inference from the imagery rather than anything the poem states, but the shape of the two stanzas supports it.
The sound patterns do much of this work. The first stanza is drenched in the rhyme of "rain", "vain" and "rain" again. The word is repeated so insistently that the rhyme itself drums like weather. Meanwhile "earth", at the end of line four, has no rhyme partner anywhere in the stanza. It sits unanswered, as the earth does. In the second stanza "wind" is the unrhymed word, and it is being drowned. Only at the very end does "earth" find its match in "mirth", and the poem's final word at last gets an answering sound.
The line lengths follow the same drama. The short lines such as "Of wind and rain" and "To swallow the rain" are clipped and dutiful, like repeated blows. Then the second stanza builds to its longest line, "Bellows like a giant bathing in mighty mirth", where the heavy b and m sounds seem to roll around the mouth. The image is both comic and grand. A giant splashing in a bath is a huge force at ease, and it turns the water that was a burden into a plaything. The earth's answer to the air's "anger" is a great laugh. "Bellows" suggests a deep, animal roar, quite unlike the thin, high noise of rain.
The ending also closes a frame. The poem opens with the air that "triumphs" and ends on "The triumph of earth", so the same word is first claimed by one side and then taken by the other. The final line is short and level, with no exclamation and no flourish, which fits the way the poem treats nature throughout. Nothing is sentimentalised and nothing is moralised. We are not told that the rain is cruel or that the river is kind.
No person appears in the poem, yet someone must be listening, since everything arrives as sound. The phrase "half the night" hints at a listener lying awake while the weather changes its mind. The poem's quiet force comes from this. It makes the reader hear a contest between elements and then hear it reversed, and it suggests that patient endurance, the gulping and choking, may be how strength is gathered. In twelve short lines it turns a wet night into a small epic of silence finding its voice.