Fire is a common and naturally occurring event in many areas of the United States. Low-intensity, small scale, carefully managed burning programs can be an important part of maintaining soil and ecosystem health. Burning programs and other fuel-reduction efforts can reduce the likelihood of a large and damaging high-intensity fire. When high-intensity fires occur, they can cause soil and seedling habitat destruction. Severe fires can kill off more than 75% of the overstory trees in an area and cause extreme danger for surrounding people and property.
In the 1930s, the Forest Service and other pioneering fire professionals in the United States developed a system for predicting the potential for large fire activity called the National Fire Danger Rating System (NFDRS). Many environmental variables such as fuels, weather, topography and risk can contribute to how easily a fire ignites and spreads.
The NFDRS helps local officials input data and receive information to determine the risk of fire danger in their area. This system has five levels of indicators:
Knowledge of the local NFDRS rating can help residents make decisions about their burning and camping activities. Delaying routine controlled burns, campfires, debris burning, and activities that may produce sparks until a lower-risk time can significantly reduce the risk of human induced burns.
Forests have vegetation at different levels that can fuel fire activity: the forest floor, understory, and canopy. The ways that fires burn and how we categorize them is closely connected to the level of the forest environment where they burn.
Also called underground or subsurface fires, ground fires occur when accumulations of peat, hummus, tree roots, and other dead vegetation burn underground. This type of fire moves very slowly and tends to smoulder rather than produce flame. Given these conditions, a ground fire can continuously spread for months.
Ground fires are tough to contain and suppress because of their below-ground environment. Also, compared to surface fires, they can cause significant harmful damage to soil environments and plant ecosystems. In times of drought, ground fires can smoulder underground throughout the winter and emerge again in spring as surface fires.
Surface fire occurs within the lower level of the forest and does not scorch the canopy to the extent that it will carry a fire. Surface fires are common but are usually low intensity. They partially consume the “fuel layer” while presenting little danger to mature trees and root systems.
A buildup of fuel and vegetation on the forest floor can cause surface fires of higher intensity. In drought conditions, surface fires can transition into damaging ground fires.
Crown fire is fast-moving fire in the upper canopy of vegetation. Crown fires involve the burning of vegetation both at the surface and crown fuel layer farther up the tree. The “ladder effect” causes hot surface or ground fires to climb up into the canopy. Crown fires are most common in coniferous forests and chaparral-type shrublands.
Because crown fires affect multiple levels of vegetation, they are often fast moving and extreme intensity fires. Wind conditions have a larger impact on the spread of canopy level fires; high wind conditions can cause flames to spread along interconnected and continuous foliage incredibly quickly.
Fire spotting occurs when the burning particles of an existing fire are carried by the wind to start new fires outside the zone of direct ignition of the original fire. Both wildland and structure fires generate particles which have the potential to be carried away and start additional spot fires. “Ember” describes any small, hot, carbon-based particle, but the term “firebrand” is used to describe small airborne particles carried by an airstream that are the cause of spotting. Spotting can occur in both short and long range forms. The distance of travel and speed of spotting depends on factors such as the forest environment (height, width, and type of tree species, canopy cover), type of terrain, and wind speed.
Short-Range Spotting: when firebrands land and ignite close to the path of the original fire. Not considered a significant factor in fire spread because the fire usually quickly overruns the developing spot fire
Long-Range Spotting: long range travel of firebrands beyond the main fire site. Often caused by the lofting of firebrands in a convection column
Among the types of fire, conditions that produce crown and spotting fires are the most dangerous to both natural and human environments. The transition period from a surface fire to a crown fire is especially important for human safety. Knowing when fire is at the point of expansion towards a more intense energy state can help residents evacuate in time to avoid injury and death. The danger of fast-moving and high-intensity fire is an important part of the risk assessment of the National Fire Danger Rating System.
Although there is no way to completely prevent danger from fire risk, mitigation options can decrease the likelihood of a severe fire damaging your home. Staying informed and connected to local information on fire risk and safety, such as fire danger map resources, is an important part of preparedness.