One morning, Scout looks out her window and discovers that "the world's endin'." Fortunately for Scout, what she believes is the end of the world is just snow. Maycomb County experiences some of the coldest weather on record, and the first snowfall since 1885. School, of course, is canceled.
Although Atticus doubts there will be enough snow, Jem and Scout decide to make a snowman. They gather snow from their own yard and "borrow" some from Miss Maudie. Jem uses dirt to form the snowman, then covers the dirt with snow. After adding a little more dirt to the midsection of the snowman, wood pieces for facial features and buttons, and finally a "stick of stovewood," their project is complete.
When Atticus views the snowman, he informs Jem that he has "perpetrated a near libel here in the front yard," as he recognizes a resemblance between the snowman and Mr. Avery. Atticus then advises the children to "disguise this fellow," so Jem adds Miss Maudie's sunhat and clippers.
Perhaps Scout and Jem make a snowman of Mr. Avery because they believe his shape resembles that of a snowman. Another possible reason could be because Mr. Avery believes Scout and Jem are the reason it snowed. He tells them, "It’s bad children like you makes the seasons change."