What is mixing height?
Mixing height is the approximate depth of the atmospheric layer near the surface within which turbulence can actively mix air, smoke and other pollutants.
Typical units: feet or metres above ground level (AGL), depending on the issuing weather service.
In plain language
Mixing height is the vertical room available for turbulent dilution. A shallow mixed layer restricts vertical dilution; a deeper mixed layer gives smoke a larger atmospheric volume in which to mix.
Mixing height is not plume height
- Mixing height: property of the atmosphere.
- Plume height: altitude reached by smoke from a particular fire.
An energetic fire plume can interact with or penetrate stable layers, so mixing height should not be treated as a rigid ceiling for every plume.
Day–night evolution
Solar heating often deepens the mixed layer during the day. After sunset, turbulence can weaken and stable conditions or temperature inversions may produce a shallow layer, increasing the potential for near-surface smoke accumulation.
Relationship with ventilation rate
Ventilation Rate = Mixing Height × Transport Wind
Example: 4,000 ft × 15 kt = 60,000 knot-ft.
Interpretation checklist
- Check whether height is reported AGL or MSL.
- Preserve forecast or observation time.
- Identify the source, model or forecast office.
- Read mixing height together with transport wind.
- Consider terrain and temperature inversions.
- Do not equate mixing height with plume-top altitude.
- Do not infer PM2.5 concentration from mixing height alone.
Related concepts
Fire Weather · Atmospheric Stability · Transport Wind · Ventilation Rate · Smoke Dispersion
Technical references
National Weather Service — Fire Weather Definitions
National Weather Service — Fire Weather Terms
Tecnobosque Reference Concept · reviewed September 2026. Forecast definitions and vertical references should be interpreted according to the issuing weather service.