The cloudless, sunless sky is mentioned several times after the family has the car accident, and most notably by The Misfit shortly after we meet him (89). This peculiar sky is also mentioned by the narrator after the grandmother has been shot ("her face smiling up at the cloudless sky" ). If there's no sun and no clouds, we would expect it to be night. But the narrator never tells us it's night or dark, and the grandmother remarks, right after The Misfit mentions the sky, that it's a "beautiful day" (90). Is it just after sunset?
This sky is also an ambiguous image, in that it has two sides. On the one hand, there's something barren and austere about the cloudless, sunless sky because it's empty. You might see this emptiness as a reflection of the family's extreme situation at the end of the story: they're dying in the middle of nowhere, without anyone to help them. You might also think this sky complements the character of The Misfit. The Misfit is "empty" inside – he's lost all sense of good, but isn't passionate about evil either. He's also the character who, unlike the grandmother, isn't concerned with appearances, and wants to get straight to the heart of things. It may be significant that he's the first character to mention the sky.
On the other hand, a cloudless sky is often considered an ingredient of a beautiful day, and the grandmother says it's a beautiful day right after The Misfit mentions the sky. Looking at the image this way gives it two different consequences. The beautiful, cloudless day contrasts sharply with the events that are happening under it; murders in the woods in the middle of nowhere should happen during a dark and stormy night, not a beautiful cloudless day. There's something jarring about that which makes it all the more disturbing to the reader. But when it returns for the last time in the description of the grandmother's smiling face, the image of the cloudless sky seems to transform, suggesting the peace the grandmother found in her last moment.