In a multi-story building, why does smoke typically rise through vertical shafts like stairwells during the early stages of a fire, even without mechanical ventilation?

In a multi-story building, smoke typically rises through vertical shafts like stairwells during the early stages of a fire, even without mechanical ventilation, due to a natural physical process driven by convection, buoyancy, and the stack effect. A fire generates intense heat, producing hot combustion gases, which are visible as smoke. These hot smoke particles and gases are significantly less dense than the surrounding cooler, ambient air. Density refers to how much mass is packed into a given volume; hot gases expand, making their particles more spread out and therefore lighter per unit of volume. Because the hot smoke ....