n. The open wind speed at which some kind of crown fire is expected to initiate.
Torching index is defined in general terms so that it can be used in a variety of applications and geographic areas. Wind speed is used as an index because it strongly influences fireline intensity, and fireline intensity strongly influences the initiation of crown fire. As defined here, torching index can be estimated using a variety of fire models. Open wind speed is used instead of mid-flame wind speed to account for fire environments with different wind adjustment factors.
Open wind speed can be either 20-foot or 10-meter, depending on the prevailing standard measurement height above ground. Of the fire behavior calculation tools available, only the fire behavior model NEXUS directly computes torching index. In the BehavePlus fire modeling system, torching index represents the open wind speed at which the transition ratio is 1.