Hot combustion gases are less dense than the surrounding air, so they accelerate upward as a plume. As the plume rises it drags in, or entrains, cool ambient air along its edges, which makes it widen roughly in proportion to height while its centreline temperature and its concentration of smoke both fall. Because the entrained air is what dilutes the plume, a fire of a given heat release rate produces far more smoke volume at ten metres above the fire than at one metre, even though the mass of fuel burned is unchanged. The rule to carry is that plume flow rate grows with height and with the fire's heat release rate, so a small fire under a high ceiling delivers a large volume of cool, weakly-smoky gas rather than a small volume of hot dense smoke. The confusion it resolves is expecting a fire's heat to arrive undiluted overhead. This is also the flow that lofts embers and that turns, under strong buoyancy, into the plume-dominated behaviour taught with fire weather. After this Concept you can predict how a fire's plume changes with height above it.
Fire plume theory characterizes the vertical variation of temperature rise, centerline velocity, and plume radius above a fire source using power-law scaling relationships in height above a virtual o…