Three short lines, and the whole poem turns on one small word: "my." The speaker doesn't complain that the heat is hell on them. It's the dog who is suffering, and the speaker is the one watching it happen. That shift tells us a lot. The heat is unpleasant enough in general, but what really bothers the speaker is seeing a pet they care about struggle.
The word "hell" is a casual, everyday exaggeration, the kind of thing people say without thinking. Here it also carries real weight. It makes the heat feel oppressive and inescapable, something that presses down on a creature who can't explain what's wrong or ask for relief. The line breaks slow us down, so each phrase lands separately: the heat, the hell, the dog.
The mood is quiet and worried, with a little helplessness in it. Nothing is resolved. The speaker simply notices, and that plainness makes the concern feel honest.