Answer:
Allusion
Step-by-step explanation:
He is reflecting on a hot summer day, making you imagine this in your head. He is also trying to get you attached to your feelings of feeling miserable and over heated by saying that the "discontent will not pass". The real content would not be able to be comprehended. This makes it an allusion, as you are dreaming it up, but I understand how most youth would think that it's imagery as it's similar.