The image is the fog itself and Sandburg uses the metaphor of the stealth, quickness, unpredictability and silence of a cat to describe the fog. I would say this technique is a derivative technique of zoomorphism. Zoomorphism is when you describe something (usually humans) in animal terms or with characteristics of an animal; “hungry as a wolf.”
In this case, the poet is describing a phenomenon in nature; the fog, which is moisture in the air that forms when water vapor condenses in high humidity and a significant difference between temperature and dew point. Fog seems to creep in slowly, like a cat and creeps slowly and silently away. The fog moves as if it were alive. It pauses (because of changes in fronts and air temperature and pressure), but Sandburg is simply describing this poetically; not scientifically. He is describing the aesthetic of it; what it looks like, not how it happens. When the fog pauses, it seems to stop and consider ‘something’ before moving on. Ascribing humanistic qualities or even animalistic qualities to non-living things has been a poetic technique for centuries.
What does the poem mean? I don’t know. Fog acts like a cat. Cats act like fog. Fog is often associated with foreboding and mystery because it moves slowly and silently like its hiding something. The same is said of black cats and people who move slow and are quite. Now we’re getting into anthropomorphism in the explication of the poem. Fog acts like cats and both act like humans, that is, whenever humans are behaving mysteriously. Then the analysis becomes about human social behavior and what verbal or here, what non-verbal clues can imply about a person’s personality, motivations and other assumptions. Note that the poet/narrator describes the fog like a cat who stops, and looks over the city. And the narrator gives us this perception that the fog, like a cat, is somehow up to no good or at least, up to something we just don’t know about. But, the poet is also sitting there, silently watching. If we forget that this is a poem, we have a speaker who is looking over the fog, looking over the city. The poet is also like a cat watching the fog. Now our analysis is about LOOKING. And that meaning or how we perceive the world depends on who is doing the looking. Observation of the world has artistic implications here, but in larger terms, observation is central to scientific investigation and ethical responsibility in social looking as in the gender bias of objectifying women or prejudging someone based on their race.
Look how far the tangents took us (me). I did this to illustrate a main point about reading and poetry in general. The meaning of a poem does have something to do with the author, his/her historical context, intent, and theoretical or social implications. But ultimately, the poem’s meaning is going to be determined by the reader (See Reader-response theory). In fact, this is a recent revelation (last 50 years) that the reader SHOULD be responsible for and free to interpret literature however he/she wants. There are a lot of reasons for this. One is that this liberation of the reader was to democratize meaning and perception and take meaning away from the ‘author’ity of the author; this authority limits the meaning of the text. Remember, we are talking about meaning being in the hands of the looker. The reader(you) is the final looker in this case.
Reader – Poet/Author – Poem – Fog.