Mushrooming is best described as which phenomenon?

• A. Thermal layering caused by lateral and downward migration of smoke due to a physical barrier (not temp/pressure equalization)
• B. Vertical plume rise due to buoyancy
• C. Uniform heating
• D. Smoke pooling at floor level

Answer: (A)

Mushrooming happens when hot smoke rises and then meets a barrier, such as a ceiling, causing the flow to spread laterally along the boundary and even sink slightly. This creates a cap‑like, mushroom-shaped layer of smoke due to thermal layering driven by the obstruction, not just by buoyant rise or even heating. If you think of a plume freely rising in open space, that’s buoyancy-driven vertical motion, not the mushroom shape. Uniform heating wouldn’t produce a cap with a distinct boundary, and smoke pooling at floor level describes the opposite lower-layer accumulation, not the ceiling‑ward cap.