During the storm, Roderick Usher insists on opening a casement (window) so that the narrator can experience the "whirlwind" outside as the wind changes directions, along with the dense clouds and "huge masses of agitated vapor" that light everything up outside with an eery, unnatural light.
To divert Usher from the storm, the narrator begins to read to him from a story called "Mad Trist." As he reads, the sounds in the story—"cracking and ripping," the shriek of a dragon, and the reverberation of something metallic—are echoed in real life in the house. The narrator is frightened. At this point, Usher reveals that he has known for "many, many days" that he buried his sister alive in a crypt beneath the house. Because of the heightened sensitivity of his senses, he has known for a long time that she was trying to break out of her vault.
Usher confesses this to the narrator, as well as that he did nothing to try to save her from being buried alive: he simply repeats that he "dare not. . . dare not. . . dare not. . . speak." He says he should be "pitied" for having been forced to listen to her all this time. Then, he confesses that he is terrified she will "upbraid" him for his "haste" in burying her. While he is clearly terrified and horrified, he is much more concerned for himself than for his sister.