In Harper Lee's To Kill a Mockingbird, what does Scout compare the courtroom atmosphere to?

In Chapter 21, as Scout and the rest of Maycomb are waiting for the verdict in the trial of Tom Robinson, she thinks:

"The atmosphere in the courtroom was exactly the same as a cold February morning, when the mockingbirds were still, and the carpenters had stopped hammering on Miss Maudie’s new house, and every wood door in the neighborhood was shut as tight as the doors of the Radley Place. A deserted, waiting, empty street, and the courtroom was packed with people."

Even though the courtroom is full of waiting people, Scout feels like it's similar to the emptiness of a winter morning when everything is still, even the mockingbirds. The courtroom has a feeling of anticipation, and activity has been suspended until the verdict is announced. 

Scout also feels that the courtroom has a somewhat surreal atmosphere. As the jury comes in to announce their verdict, she says, "What happened after that had a dreamlike quality: in a dream I saw the jury return, moving like underwater swimmers..." The action seems to occur in slow motion, as if they are swimming underwater, as the verdict they announce--Tom Robinson's guilt--seems so nonsensical to her. She has waited so long for the verdict that its announcement takes on a quality of being utterly unreal to her.