What is ventilation rate?
Ventilation rate is an operational indicator of the atmosphere’s potential to transport and dilute smoke, commonly calculated as the product of mixing height and transport-wind speed.
Engineering relationship
Ventilation Rate = Mixing Height × Transport Wind
Example: mixing height = 4,000 ft and transport wind = 15 kt, therefore Ventilation Rate = 60,000 knot-ft.
Why the product is useful
Smoke dispersion requires both vertical mixing and horizontal transport. A deep mixed layer with very weak winds can still ventilate poorly; strong wind beneath a shallow stable layer may also provide limited vertical dilution.
Ventilation rate is not air quality
Ventilation rate describes atmospheric dispersion potential. It does not directly measure PM2.5, AQI or human exposure.
One commonly used NWS category profile
- Poor: less than 40,000 knot-ft
- Fair: 40,000–59,999 knot-ft
- Good: 60,000–99,999 knot-ft
- Very Good: 100,000–149,999 knot-ft
- Excellent: 150,000 knot-ft or greater
These thresholds are not universal. Other jurisdictions and smoke-management programs use different operational categories. The physical ventilation-rate result and the category convention should therefore remain separate.
Daily evolution
Ventilation potential can change strongly through the day as the mixed layer deepens or collapses and winds change. A daily maximum can hide poor morning or overnight dispersion.
Interpretation checklist
- Use mixing height and transport wind for the same forecast period.
- Check units before multiplication.
- Preserve the issuing agency and category convention.
- Do not convert ventilation rate directly into PM2.5.
- Consider transport direction when assessing downwind exposure.
Interactive engineering tool
Open the Tecnobosque Smoke Ventilation Calculator →
Related concepts
Mixing Height · Transport Wind · Smoke Dispersion · Atmospheric Stability · Fire Weather
Technical references
National Weather Service — Smoke Dispersal and Ventilation Rates
Tecnobosque Reference Concept · reviewed September 2026. Ventilation-rate units and qualitative categories must remain attached to the conventions of the issuing agency.