Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Neutral (But with Dashes of Irony, Tragedy, Humor, and Hopefulness)
As a journalist, Hersey seems to try to let the events/actions themselves do the talking without adopting an attitude or slant toward his material. Even when he's intervening enough to draw attention to the scope or impact of some aspect of the tragedy, he maintains a neutral tone toward his subjects.
For a good example, look at how he finishes off the book's first chapter:
Everything fell, and Miss Sasaki lost consciousness. The ceiling dropped suddenly and the wooden floor above collapsed in splinters and the people up there came down and the roof above them gave way; but principally and first of all, the bookcases right behind her swooped forward and the contents threw her down, with her left leg horribly twisted and breaking underneath her. There, in the tin factory, in the first moment of the atomic age, a human being was crushed by books. (1.30)
As you can see from this passage, his tone is objective and removed, but he steps in just enough to get the reader to think about the incongruity (and perhaps irony?) of a human being crushed by an old form such as the book in this totally new world of atomic warfare.
That's a pretty impressive feat, if you ask us.