You might like to consider what happens at the end of this chapter and how the lightning bolt is used to foreshadow the very unpleasant discovery of Rochester's first wife. Let us remember that it is in this chapter that Rochester finally proposes and Jane accepts him. In fact, the chestnut tree under which Rochester and Jane sit while he proposes can actually be considered as a symbol of their relationship throughout the novel. Note the way that the elements and the chestnut tree belie what should be an incredibly happy time as Jane looks as if she is finally going to marry the man she loves:

But what had befallen the night? The moon was not yet set, and we were all in shadow: I could scarecely see my master's face, near as I was. And what ailed the chestnut tree? it writhed and groaned; while wind roared in the laurel walk, and came sweeping over us.

Clearly, this paragraph foreshadows the trouble that is to come. In a kind of pathetic fallacy, the elements themselves seem to predict bad times ahead for this relationship. As if to cement this, the next morning, Adele tells Jane that the tree under which Rochester proposed had been smitten into two, with half of it falling away. Of course, the symbolism is clear: Rochester and Jane cannot be together, at least yet. The lightning bolt is therefore used to clearly foreshadow the existence of Bertha Rochester who will, at least temporarily, prevent Jane achieving her happy ending.