Fireline intensity
The first of the Severity group elements is fireline intensity. Surface fireline intensity is the heat energy release per unit of time from a three-foot (one-meter) wide section of the fuel bed, extending form the front to the rear of the flaming zone of a surface fire. Surface fireline intensity is a function of surface fire rate of spread and surface fire heat per unit area (BehavePlus Online Documentation; September 14, 2017). Fireline intensity is a good single measure of fire behavior that accounts for both heat release and the speed at which the fire is moving. It is also a measure of suppression difficulty.
Crown fire
The term crown fire refers to the movement of surface fire into the tree canopy via ladder fuels. Crown fire can involve a single tree (torching) or an entire stand of trees (active crowning). Fire behavior calculations evaluate whether expected flame length will exceed the height of the lowest branches of the dominant vegetation type (ladder fuels). If so, it is likely that the canopy will become involved. If tree crowns have enough fuel, are close enough to each other, and wind speed is sufficient, vertical fire movement becomes horizontal (tree-to-tree) and an active crown fire forms.
Crown fire potential is based on the percentage of pixels predicted to produce either torching or active crowning within an 0.5-mile radius. Higher crown fire potential clearly contributes to more wildfire risk to structures in an area.
Wind speed
Wind speed is such an important factor in wildfire that it is an independent element in Wildfire Risk Extreme, even though it is already used in fire behavior calculations. Wind can have an overwhelming impact on fire suppression that is not necessarily accounted for in fire behavior modeling outputs. Wildfire Risk Extreme does not, however, use raw wind values for this factor. Wind is typically only dangerous in conjunction with fuel and low humidity. Because of this, Wildfire Risk Extreme modifies raw wind speed values to make them more directly applicable to wildfire risk assessment.
Ember cast
Ember cast is the production of firebrands from burning vegetation that are carried aloft for some distance, then deposited on the surface ahead of the fire front. Such embers, if they have not been completely extinguished during their flight, can ignite any burnable material they contact, whether it be natural fuel or structures. Different fuel types produce different types of embers; some can carry for a significant distance and still be viable. Fire behavior modeling can predict maximum spotting distance, which can be used to evaluate the exposure of a given area from all directions. In Wildfire Risk Extreme, the predominant wind directions, and the speeds of those predominant winds, are incorporated so that more embers are cast downwind than upwind. While embers are still cast in all directions, more consideration is given to embers in alignment with predominant wind directions.
Not all embers coming from a fuel bed behave the same way. As shown in the illustration above, some embers may have a short lifespan (green and dark blue stars) while others remain active for a longer period (red and light blue stars). The EMBER_CAST score captures both situations.