How is the nature described in the novel Frankenstein a metaphor?
Mary Shelley was a Romantic author and her novel Frankenstein is a renowned novel of the period. Given that the Romantics idealized nature, it is no surprise that Shelley used nature as a repetitive image in Frankenstein.
While nature can be recognized as an important image throughout the novel, one place where nature exists as a metaphor can be found in chapter 17 (the lines are Victor's own personal thoughts made "public" (to the reader) after agreeing to create a mate for his "son").
These were wild and miserable thoughts; but I cannot describe to you how the eternal twinkling of the stars weighed upon me, and how I listened to every blast of wind as if it were a dull ugly siroc on its way to consume me.
Prior to this feeling, Victor has heard the story of how the creature came to exist as he is (since Victor's abandonment) and Victor felt sympathy for his creation. It is this sympathy which allowed Victor to agree to creating the mate.
Essentially, when Victor feels the "eternal twinkling stars" weighing upon him and the "dull ugly siroc" nearing in order to consume him, this creates a very distinct comparison between nature and Victor's morals. It is not the stars which actually weigh down upon him. Instead, Victor's conscience is weighing down upon him.