Extreme Fire Behavior 1

Weather - Fire Behavior
Page Last Modified / Reviewed: 
May 2020


Mild, moderate, and EXTREME fire behavior has always been present in cases of shelter deployment. It is critical for firefighters to maintain situational awareness of live and dead fuel moisture conditions, as well as predicted and current weather conditions. As fire behavior changes, it is critical to act decisively in regard to the changing situation.

Discuss the following indicators of extreme fire behavior:

  • Relative humidity less than 15%.
  • 1-hour fuel moisture less than 5%.
  • Transition from surface to crown fire.
  • Smoldering fires that exist in the early part of the day are beginning to burn very actively as the day progresses.
  • Approaching thunderheads with dark clouds beneath.
  • Presence of dust devils indicating an unstable atmosphere.
  • Increased spotting.
  • Sudden calm.
  • High clouds moving fast in direction different from surface wind.
  • Predicted Haines Index of 5 and 6 in those areas where this level is a good indicator of atmospheric instability.
  • Fire whirls.

Image of a fire whirl twisting up from a forest fire.


Additional Resources

Incident Management Situation Report (IMSR)
Incident Response Pocket Guide (IRPG), PMS 461
NWCG Standards for Helicopter Operations, PMS 510
RT-130, Wildland Fire Safety Training Annual Refresher (WFSTAR)
Interagency Standards for Fire and Fire Aviation Operations (Red Book)
Wildland Fire Lessons Learned Center

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