Symbolism, Imagery, Allegory
In case you can't tell by the title, this book is packed with sky imagery. And most of Lennie's sky-related metaphors are describing one thing: l'amore, a.k.a.her passionate, "demented" (20.24) love for French boy Joe Fontaine. In all fairness, though, it sounds like just touching Joe feels almost supernatural:
For a moment, in his arms, with my mind so close to his heart, I listen to the wind pick up and think it just might lift us off our feet and take us with it. (12.73)
The lightness in the passages about Joe is a real switch from the all-consuming heaviness of Lennie's grief. And a key part of how this lightness is communicated is through sky imagery—in the passage above, this takes the form of the wind. So while Lennie struggles to make peace with the fact that her sister is six feet underground, Joe elevates her heart.
That said, Lennie doesn't reserve sky references exclusively for her dreamy French make-out buddy. While Lennie doesn't seem to belong to a particular religion, she does, in her more optimistic moments, imagine Bailey in the sky, which conjures up heavenly ideas. For instance:
Bailey grabs my hand
and pulls me out of the window
into the sky,
pulls music out of my pockets.
"It's time you learned to fly," she says,
and vanishes. (32.1)
Here we have Bailey coming from the sky and encouraging Lennie to learn to fly, a metaphor for Lennie coming into her own and letting herself blossom. Bailey seems almost angelic here, doesn't she? But more importantly, again we see sky imagery being used to evoke lightness in Lennie. Bailey's in the sky encouraging her sister to be her fullest self, and loving Joe makes Lennie's spirits soar, too. So while Lennie attributes the sky to other people, really, when we get down to it, the sky imagery is all about Lennie and learning to live her life.