Lingered upon the pools that stand in drains,
Let fall upon its back the soot that falls from chimneys,
This street of houses is obviously not a particularly pleasant one; the city drains are clogged up and there is soot from the chimneys everywhere. The yellow fog and smoke are reflected in the pools of standing water in the drains.
That Prufrock is noticing the fog (perhaps from inside the house, perhaps not there yet, but knowing what it will be like, because it’s always the same), and has time to be poetical about it (making it pretty, but noting its noxiousness), suggests his separateness from the life around him, and his ambivalence.
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