Stack effect is described as which of the following?

• A. Hotter air inside, cooler air outside
• B. Hotter air outside, cooler air inside
• C. Equalized temperature between inside and outside
• D. No air movement

Answer: (A)

Stack effect is the buoyancy-driven movement of air caused by a temperature difference between indoors and outdoors. When the inside is heated, the air becomes lighter and rises, pushing its way out through the upper parts of the building. To replace it, cooler outside air is drawn in through lower openings. So the defining description is that the interior air is hotter than the exterior, driving an upward flow and a corresponding inward draft at lower levels. The other options describe no movement, equal temperatures, or the opposite condition, none of which capture this buoyancy-driven air exchange. In firefighting, this upward, buoyancy-driven flow can rapidly move smoke and heat to upper areas, influencing ventilation and tenability.