What does the rain mean to Gatsby when he dreams about Daisy?

Rain is an important symbol in The Great Gatsby, especially with regards to Jay and Daisy's relationship. At the time of their illicit rendezvous, it's pouring. The inclement weather aptly symbolizes the awkwardness that both Daisy and Gatsby are feeling at that particular moment. It also represents the doomed nature of their relationship; there's no real depth to their connection, and all the genuine emotion is on Gatsby's side.
The sun does eventually come out, and things seem to have improved between them, yet it can only be a temporary reprieve. The sudden change in weather reflects the ephemeral nature of this relationship; the bright, sunny moments will come and go, but they will never last. The lovers' feelings—especially Daisy's—change with the rapidity of the weather on a day of sunshine and showers. And we're left in little doubt that it's the showers that will ultimately prevail.