It snows in Maycomb, a natural event that rarely happens there. This event provides some comic relief before the fire at Miss Maudie's that night. Jem and Scout are both excited and build a snowman that eventually becomes a snowwoman because Atticus makes them change the first one they build. It looks too much like Mr. Avery. There is such little snow, they have to borrow from Miss Maudie's yard. Even with that, they don't have enough snow, so they have to use dirt with the snow to build their snowperson. Mr. Avery shows his superstitioius nature when he says it snows because the children are bad, not doing what they're told by their parents.
The unusual snowfall foreshadows the racial trouble that will occur in the book, since the snowperson is both black and white.
The act of nature that appears in Maycomb, Alabama that convinced Scout that the world was ending was the rare occurrence of snow falling in Maycomb. It had not snowed in Maycomb since the late 1800's, so the children had never witnessed this event and were somewhat in shock of the snow fall.