The poem is told by an adult remembering childhood, and what they remember is a sudden blinding flash, like a giant camera catching everyone in their work clothes. The children run to a high field to look at the strange shape in the sky. They ask whether it is a new kind of cloud or a tree, and the poem never names it. The imagery strongly suggests an atomic blast, though the poem leaves that for us to infer.
The speaker's feeling is a mix of innocence and quiet dread. The children are curious, even a little awed, and they guess the thing is "brewing its own weather, or setting fruit." The last stanza changes the tone. Because they are farmers' kids, they read the white birds fleeing west the way they would read any warning from animals. The calm, matter-of-fact ending is chilling. The mood is eerie and restrained, and the understatement makes the danger feel even larger.