How does the author use metaphor in the novel, Martyn Pig?

The author uses rain throughout the narrative as a metaphoric representation of trouble and/or difficulty for Martyn. The connection is established in "Part 1, Wednesday, Section 2" when Martyn's realization that he is in "a sweet mess" is juxtaposed with the beginnings of a rainstorm. Here it's interesting to note that within moments of this juxtaposition, Alex arrives, the narrative eventually revealing that she is an unexpected source of even more trouble for Martyn.