The Burning Index (BI) value (10*feet) as calculated by the Oklahoma Fire Danger Model. This is probably the most useful index of the National Fire Danger Rating System (on which the Oklahoma Fire Danger Model is based) since BI directly relates to the intensity of the fire (and thus is related to the difficulty of containment) and is scaled such that BI/10 is equal to the flame length (FL) in feet at the head of the fire. It is an index which integrates both the spread component (SC) and energy release component (ERC).
The traditional U.S. Forest Service interpretation of burning index with respect to fire behavior and suppression is listed below:
BI < 40 (FL < 4 ft) Fires can generally be attacked at the head or flanks by persons using hand tools. Hand line should hold the fire.
BI = 40-80 (FL = 4-8 ft) Fires are too intense for direct attack on the head by persons using hand tools. Hand line cannot be relied on to hold fire. Equipment such as dozers, pumpers, and retardant aircraft can be effective.
BI = 80-110 (FL = 8-11 ft) Fires may present serious control problems torching out, crowning, and spotting. Control efforts at the fire head will probably be ineffective.
BI > 110 (FL > 11 ft) Crowning, spotting, and major fire runs are probable. Control efforts at the head of the fire are ineffective.
Burning index is a function of the fuel model being used, the live and dead fuel moistures, and the weather conditions. Accordingly, inaccuracies in the NAM weather forecast will lead to inaccuracies in the BI forecast. Of course, if the fuel types and loads are substantially different than those in the fuel model being used, there will be inaccuracies as well.
Finally, it is important to realize that these indices produced by the National Fire Danger Rating system are for the conditions modeled at 1-km resolution. In other words, the fuel model represents conditions over the entire 1-km square area, so indices such as BI are not meant to be used on a field-by-field basis. As an example, if the particular fuel in the area of concern (e.g., a particular field) differs from the assigned fuel model in that 1-km square, then the Oklahoma Fire Danger Model results for that square can be expected to be different than for the particular field in question (e.g., an open grassy area in a 1-km square that has been assigned a forest fuel model). Other limitations of the model can be found in the document entitled The Oklahoma Fire Danger Model.