One theme is that of sweat. Sweat is symbolic of fear, anger, and labor. Her sweat and blood are used to wash and care for the white people's clothes. She spends long hours toiling in the sun. Her labor keeps her house which brings her a lot of joy, arguably the only joy in her life.
The heat of the summer can be compared to the simmering of her temperament when her husband taunts her. She has long suffered from his abuse and treachery, and as the summer continues, her anger simmers below the surface.Her fear of snakes causes her to sweat, as well. She is forced to reside in the house with it, and it is a source of constant upset for her.