Why does Scout think the world is ending?

Scout believes the world is ending because it was snowing outside. She had never seen snow before.

“The world’s endin‘, Atticus! Please do something—!” I dragged him to the

window and pointed.

“No it’s not,” he said. “It’s snowing.”

Jem asked Atticus would it keep up. Jem had never seen snow either, but he knew what it was. Atticus said he didn’t know any more about snow than Jem did. “I think, though, if it’s watery like that, it’ll turn to rain.”