apparently not even heat rises but hot air rise idk " one experience smell when some molecules interact with the nasal receptors. In order to smell the molecule of a some particular chemical species has to travel and reach the nasal receptor. Now since every chemical species has different boiling point or say vapour pressure it all depends now on to the heat energy. As the molecules gain heat energy it can travel through air. Further some times vapour density is too high compared to air density that these vapour always remains at the lowest possible position. So it is not always necessary that smell (or molecules) rise. Heat itself is nothing but energy that can not rise (in vaccum heat can not travel, but light another form of energy can travel). "