If you ask your students, you will find that most of them think of them as specific, rather than comparative, temperatures. By probing you can establish that, to most students, warm is a temperature higher than the observer s skin temperature, and cool is lower than the observer s skin temperature. And of course skin temperature changes. Those substances with levity naturally fall upward, away form the center of the universe, those with gravity fall downward, toward the center of the universe. Air is assumed to be nothing.
Hot air rises because it IS heat, and heat has levity.
To create a conceptual understanding of the cause and effect relationships involved in the process called convection requires understanding several component concepts. Generally text books and teachers simplify the explanation in not more than a paragraph, to save time and space. This is then reviewed in single sentence form, which is what the student memorizes as the complete explanation. The highly abbreviated version leaves the student with the impression that warm air somehow pulls itself away from Earth leaving a vacuum. The water vapor rises in the air.