To me, most of the mood and atmosphere in the part of the chapter that you mention comes from what is going on in Scout's mind and, to some extent, on her body. The mood and atmosphere is tense and nervous.
Physically, we see this in the fact that sweat is running down Scout's side.
Mentally and emotionally, it is created by the way Lee describes Scout's hopes and worries. Scout thinks she hears the fence squeak. She worries that Atticus is going to wake up when she hears him cough.
Between the sweat and Scut's worries, Lee creates a mood of tension.