Simple; Vivid
Pat Frank was, among many other things, a journalist, so he writes with a simple, approachable style that's eminently easy to understand. For the most part, he serves meat and potatoes, focusing more on characters' actions than their motivations, and delivering info in a straightforward manner.
Despite this, Frank writes with vivid, impressionistic detail at certain points. The nuclear explosions are a good example. Check it out:
The gaudy mushroom enlarged with incredible speed, angry, poisonous, malignant. It grew until the mushroom's rim looked like the leading edge of an approaching weather front, black, purple, orange, green, a cancerous man-created line squall. (5.40)
With its blend of natural and artificial imagery, this passage is a lot more complex stylistically then, say, the novel's descriptions of its characters actions, which are delivered dryly, even during the more intense moments.
Why do you think that is? Is it because metaphor and imagery are the only ways to express the horror of a nuclear explosion? Or is it something else altogether?