Structure
The structure element the balance between natural fuel and structural fuel to determine how a fire will burn through an area. In intermix areas, there is enough fuel between structures that a surface fire can burn through. In these cases, structure density can help identify the interaction of fuel and structures.
In interface areas, the ratio of built environment to natural fuels tips much more toward the built environment. When structures are truly dense, as occurs in interface areas, parcel shape can have an influence on how the fire might burn. Wildfire Risk Extreme uses the distance to the nearest structure as the structure factor in interface areas.
Access/egress
One of the most important goals for firefighters is during wildland suppression operations is to get people out of a threatened area. While life safety does not necessarily have a direct bearing on the primary goal of assessing wildfire risk to structures, small roads that serve the dual purposes of evacuating residents, and transporting firefighters and equipment can increase the probability that a structure will burn before fire suppression resources arrive. For this reason, Wildfire Risk Extreme uses a combination of the distance to the nearest primary road and address density to calculate an estimate of how difficult access for suppression resources will be. Since there are few structures in wildland areas and access is generally good in interface areas, access difficulty is primarily a concern in intermix areas.
Wind-aligned roads
In areas with enough structure density that structures along roads form a wind-channeling corridor (interface areas), the orientation of roads relative to predominant wind direction becomes important. If roads are parallel to the direction from which wind is most common, embers and flame impingement can penetrate deeper into an interface area, increasing the loss of individual or multiple structures due to urban conflagration.
Wind information is provided by NOAA.
Vegetation cover
Used only in areas of moderate structural density (intermix areas), vegetation cover assess the fuel that occurs between structures. While other fuel-based factors are included in Wildfire Risk Extreme, (specifically, inputs for fire behavior in the Severity group), vegetation cover operates at a finder scale than other measures of fuel loading. Also, vegetation cover is not specific to a timber fuel, but includes grass and shrubs as well. Total vegetation cover is, therefore, used as a secondary factor in suppression difficulty and in the ability of a fire to burn through an intermix area.
Input data for vegetation cover comes from LANDFIRE.