Stack Plume Rise and Effective Stack Height (Briggs)

ScopeWhich is what governs ground-level concentration rather than the height of the stack. Briggs computes a buoyancy flux from the stack gas velocity, the stack diameter and the temperature difference, and the final rise in neutral conditions follows that flux to a fractional power divided by the wind speed. The rise is frequently LARGER than the stack itself: a hot plume from a modest stack can rise well over a hundred feet on a light wind, so the effective release height can be double the physical one. Ignoring plume rise makes a dispersion estimate wildly conservative; assuming too much makes it dangerously optimistic, and since ground-level concentration falls roughly with the square of effective height, the error compounds. Wind is the term that trades against itself, and that is the counterintuitive part. A strong wind dilutes the plume more, but it also bends it over and reduces the rise, lowering the effective height -- and the rise is exactly inverse in wind speed, so doubling the wind halves it. The two effects work in opposite directions and the worst-case wind for ground-level concentration is neither the calmest nor the strongest, which is why a real dispersion analysis runs a full year of hourly meteorology rather than a design condition. Two limits bound this hard. Stability is modelled only as a cap: in stable class E or F air the rise is the lower of Briggs' stable form and the neutral figure, and the extra rise unstable air gives is not credited. And DOWNWASH is not modelled at all -- a stack too short relative to nearby buildings has its plume pulled down into the building wake, which can eliminate the rise entirely and is why the good engineering practice stack height rules exist. A plume that downwashes has an effective height at or below the stack, and no buoyancy calculation will say so. This is a screening estimate of FINAL rise in neutral conditions on a buoyant plume; it does not compute momentum rise for a cool high-velocity plume, transitional rise close to the stack, stability effects, downwash, or any ground-level concentration. A regulatory dispersion model, the applicable modelling guideline, and a qualified meteorologist or air quality professional govern.

Formula or tablebuoyancy flux F = g v d^2 (Ts - Ta) / (4 Ts) in SI; neutral-condition final rise = 21.425 F^0.75 / u below a 55 m^4/s^3 flux and 38.71 F^0.6 / u at or above it; effective height = stack height plus that rise, and rise is exactly inverse in wind speed; for stable classes E and F the final rise is 2.6 (F/(u s))^(1/3) with s = (g/Ta) dtheta/dz, dtheta/dz = 0.020 K/m (E) or 0.035 K/m (F), as EPA's ISC3 applies it.

Source and editionThe Briggs plume rise relations by name, published in SI and converted at 0.3048 m per foot and 1,609.344 m per mile, both exact. A screening estimate for a BUOYANT plume: it does not compute momentum rise for a cool high-velocity plume, transitional rise close to the stack, unstable-air enhancement (stable classes E and F use the stable form, added 2026-10-09), building DOWNWASH, or any ground-level concentration.

Free accessOne published correlation and two exact length conversions.

What governsEstimate. AHJ and licensed professional govern.

API field namesstack_height_ft, stack_diameter_ft, exit_velocity_fps, exit_temp_f, ambient_temp_f, wind_mph, buoyancy_flux, plume_rise_ft, effective_height_ft, height_ratio, concentration_factor, double_wind_rise_ft

• Neutral conditions, final rise stability suppresses or enhances rise and is not modelledthe applicable modelling guideline
• Buoyant plumes only momentum rise for a cool high-velocity plume is a different relationa regulatory dispersion model
• Downwash is not modelled a short stack near buildings can lose its rise entirelygood engineering practice stack height rules