In chapter 13 what does Atticus mean when he says "the summer's going to be a hot one"?

He speaks literally and figuratively here.  In Alabama all summers are hot.  But, he mainly means that the summer is going to be full of protest, violence, and the "Roman Carnival" atmosphere because of the Tom Robinson trial.

This has been foreshadowed since the first page of the novel, when Scout/Harper Lee talks of how hot the town is in the summer.  Usually, summers are lazy in Maycomb.  No one ventures out in the heat.  But the Tom Robinson trial will bring in people from Old Sarum and other neighboring towns, causing tempers to flare.