Carl Sandburg is well known for his collection of poems about Chicago after the turn of the twentieth century. Fog describes a typical dense fog which often catches people unexpectedly and dissipates quickly; much like a stealthy cat which perhaps appears as if from nowhere and then leaves unseen. The fact that the poem is so short reinforces the image of the fog as compared to the nature of a cat and creates a situation where man has no control. The fog in its cat-like state does as it pleases. It arrives whenever it wants to and it leaves or "moves on" when it is ready and not at any predetermined time or rate.
The whole poem is an extended metaphor. A metaphor makes a direct comparison between two or more seemingly unrelated objects or creatures and assigns characteristics from the one to the other in order to create a visual picture. The reader of Fog can imagine the fog almost creeping around on its "little cat feet" as it begins to enter the space, in this case, the "harbor and city." The fog then simply sits on its "silent haunches," thereby extending the metaphor as the fog is still being compared to a cat.
Figures of speech allow poets in particular to create special effects without any real pictures. Alliteration is used in Fog to create a rhythm, giving it pace. It starts quickly, as the "fog comes / on little cat feet..." with the "f" sound being most prominent. It then slows down as it "sits looking / over harbor and city / on silent haunches." The "s" sound (including the c in city) enhances the somber mood and makes the reader almost want to whisper. The "harbor" and "haunches" also contribute to this.
Carl Sandburg uses personification in the poem Fog.