Structures and roads are the defining elements of a landscape characteristic of interface areas. Unlike wildland and intermix areas, structures within an interface region are primarily threatened by flame impingement on exterior surfaces, ember cast, and events that may start as the result of a wildland fire but that are perpetuated through burning from structure to structure. Wildland fuel may no longer surround structures, and therefore fire behavior changes dramatically, as does risk to nearby structures.
Ember and flame impingement zones
Located within the interface, these zones are impacted by embers cast from adjacent wildland fuel. The size of these areas is dependent on the fuel type and prevailing weather conditions in the locales generating embers. For example, heavy timber and predominant hot, dry winds would cast embers farther than grass with no wind. The former conditions would produce larger interface zones that would extend deeper toward the center of a town/city.
Flame impingement is the direct impact of flames on a structure via conductive heat. This only occurs at the edge of interface areas where fuel meets the first structure in a community. Flames can only reach a short distance into the interface zone relative to the distance that embers can travel. Wildfire Risk Extreme will sometimes have a dedicated polygon that separates ember-only zones from flame impingement zones, but more often these zones are combined into a single polygon.
Urban-only conflagration zones
Urban conflagration refers to fires where structures themselves are the fuel. These fires may or may not start from a wildland fire, but once the fire burns exclusively via structures, it ceases to be a wildland fire and is referred to as an urban conflagration. Locales with higher structure density/lower spacing between structures, and subject to high winds preceded by warmer, drier conditions are more susceptible to urban conflagration events. It is important to note that, though urban conflagration-only zones start where ember cast diminishes, urban conflagration risk is also present inside ember/flame impingement zones.
