Symbolism, Imagery, Allegory
Hope you brought your umbrellas. During Creation, Bob linked the weather to his feelings so that it would rain when he was sad and storm when he was angry, and so on. Obviously, not such a great idea, as anyone who's lived through a hurricane, blizzard, drought, or, really anything but a perfect 78 degree summer day could tell you.
See, Bob has a thing for human women. And when he falls in love, the weather (just like his emotions) goes crazy. Told you he was a typical 19-year-old boy. Check out what happens when he meets Lucy for the first time:
The temperature rose ten degrees throughout the zoo and tulip buds burst open with little muted pops, spreading instantly into full flower … Behind her, twenty-eight rainbows spread silently across the sky like oil in a puddle. (13.34, 49)
Not so bad, right?
Well, it's only just beginning. As their relationship progresses, everything gets worse. Only a few weeks later, the weather forecast isn't looking too great: "Newspapers reported the worst spring weather in the history of spring weather. The rain seemed to have developed a personality of its own—sharp and vindictive one minute, heavy and morose the next. So peevish was the mood that it might have been programmed by some gigantic, love-struck, miserable, sulking teenager. Which, of course, it was" ( 21.1). Not so great anymore, huh?
When the weather starts clearing up after Bob and Lucy have sex, it's basically a sign for us that he has completely forgotten about her—because otherwise the weather would still be terrible. Since we can gauge Bob's emotions (which he otherwise doesn't really talk about) via the weather, it's pretty helpful. It is also a constant reminder of how selfish he is, since his emotional roller coasters result in people dying and massive natural disasters that take over whole cities.
You know how they say that Mother Nature is a fickle mistress? Yeah. Try "Bob is a fickle master." The weather is a symbol of how weird, arbitrary, and incomprehensible the world is—because it has a weird, arbitrary, and incomprehensible God.