Final answer:
The passage uses a cloud bank metaphor to describe a temporary yet impactful dark mood of Colonel Grangerford, ensuring that operations around him run smoothly to avoid provoking his anger further.
Step-by-step explanation:
The passage describing Colonel Grangerford turning into a cloud-bank from "Huck Finn" uses the cloud bank as a metaphor for the colonel's mood transitioning to an especially dark and grim one. The phrase "it was awful dark for half a minute, and that was enough" suggests that it takes only a brief moment of the Colonel being in a bad mood to ensure that everything around him is affected, and runs smoothly for a while afterward to avoid provoking his ire further. This reflects the Colonel's imposing presence and the fear he instills in those around him to keep things in order, demonstrating a transient yet profound impact.