What causes Atticus to cry?

Atticus cries after the loss of Tom Robinson's court case. He knew that he had a slim chance of winning in the first place, but what is heart-wrenching to him is the fact that the people of Maycomb did not see fit to try Tom fairly, but rather, made their verdict based solely on skin color, despite Atticus's excellent defense and demonstration of Tom's innocence. The close-mindedness of Maycomb County is tragic in Atticus's mind, and this is what causes him to cry.