> When Nick says that a "sharp wild brace came suddenly into the air," he's describing two things which happen to be analogous to each other. Note that Nick feels that the culture and even the snow of the Midwest both have a genuine appeal. This is his subjective interpretation because the Midwest is more genuine to him than the superficiality of the East where people don't really know one another (recall that no one came to Gatsby's funeral; a very impersonal culture).
The air he feels as the train starts out in the Midwest is sharp (immediately noticeable), wild (natural), and braces (cla…