In the prologue of Natalie Babbitt's Tuck Everlasting, the narrator sets the story during the "first week of August," a time period the narrator describes as being "motionless, and hot." The motionlessness of early August is characterized in the absence of wind and rain; there is only lightning at night and no thunder. The narrator further relays that it's during these "dog days" of summer that people are likely to do things they later regret. This backdrop of motionless, oppressive heat not only serves to foreshadow the conflicts in the story to come but also serve to capture Winnie Foster's emotions that drive the story.
By chapter 3, the narrator describes Winnie as being as boiling angry as the weather was boiling hot:
She had come out to the fence, very cross, very near the boiling point on a day that was itself near to boiling.
She is so angry because, just like the heat is oppressive, she feels that her adult family members oppress her. As an only child, she is the only person whom her grandmother and mother look after, and they are constantly restricting her behaviors. For example, in Chapter 3, her grandmother yells from the window of the cottage to order her not to sit on the grass because the grass will stain her clothing. Similarly, her mother orders her to come inside the house so she doesn't get heat stroke. Winnie feels so oppressed that she is contemplating running away to become her own person. Hence, the oppressive heat of the weather reflects Winnie's feelings of being oppressed by her family and her feeling of anger as a result of that oppression.