Smoke Management

Category Day

“Category day” is a system that was developed to determine the ability of the atmosphere to handle smoke through the depth of the mixing layer above the earth’s surface. It is based on “ventilation rate,” which is the mixing height in meters multiplied by the transport wind speed in meters per second. The greater the ventilation rate, the greater the ability of the atmosphere to ventilate the smoke and get it out of the area. The table below relates ventilation rate to category day, along with burning guidelines. Category day and ventilation rate information can be found at the fire weather forecast web sites of the National Weather Service discussed earlier. 

Category Ventilation Day Rate Burning Guidelines
I <2,000 No burning
II 2,000-4,000 No burning until 11 a.m. and not before surface inversion has lifted.
III 4,000-8,000 Fire out by 4 p.m. Daytime burning only after inversion has lifted.
IV 8,000-16,000 Burn anytime.
V >16,000 Unstable and windy. Excellent smoke dispersal. Burn with caution.



Smoke Dispersal

Good smoke dispersal occurs when burning on category III days or higher.

Oklahoma Dispersion Model in OK-FIRE

This model was developed to assess surface dispersion conditions several miles downwind. It breaks the atmosphere in six dispersion categories:

1-Very Poor (VP)
2-Poor (P)
3-Moderately Poor (MP)
4-Moderately Good (MG)
5-Good (G)
6-Excellent (EX)

The lower end of this scale typically occurs with inversion conditions, which inhibit mixing and causes poor dispersion. During such conditions, the smoke plume hangs together as it drifts downwind and anyone caught near the plume centerline could be smoked out. The upper end of this scale typically occurs with unstable atmospheric conditions, when the dispersion is good, both in the vertical and horizontal directions.

OK-Fire Website

Example of smoke dispersion conditions from OK- FIRE Web site. Red is poor and the greens range from moderately good to excellent, which are preferred for adequate smoke dispersal.

Current and forecasted smoke dispersion information can be found in the SMOKE section of the OK- FIRE Web site located at: http://okfire.mesonet.org/.

How to Minimize Smoke Problems

  • Burn smaller burn units.
  • Burn when weather conditions are forecast to produce the best dispersion. Consult both the category day forecast as well as the Oklahoma Dispersion Model forecast in OK-FIRE.
  • Burn when fuel conditions are likely to produce the least amount of smoke. This can be accomplished by selecting the correct combination of fuel moisture and fuel size class that needs to be removed or by the fuel type (i.e. leaf litter, grass fuels, amount of cedar in unit).
  • Use appropriate ignition techniques for smoke management. Consider using backfires to reduce the amount of smoke or mass ignition devices such as heli-torch or Delayed Aerial Ignition Device for larger burn units.
  • Conduct post-burn mop-up to reduce nuisance smoke. Outline actions to be taken after burn to reduce residual smoke. If post-burn smoke could be a problem, be sure to monitor unit to suppress any fuels that begin to smolder.
  • Reduce the amount of fuels in burn unit to reduce smoke emissions. This can be accomplished by burning frequently, grazing, or haying.
  • Reduce the impact of smoke on people. Be sure to notify all people downwind that could be affected by the smoke and use appropriate signage to in- form the public about areas were smoke will impact them.

Remember that even when the smoke leaves the burn unit, it is still your smoke and you should do everything possible to reduce the smoke impact outside of the burn unit. For more information see Smoke Management for Prescribed Burning E-1008 at: http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-5672/E-1008%20Smoke%20Management.pdf