Fire Science Background

Three basic "ingredients"

Fire is a dynamic process that requires three elements: fuel, oxidizer, and heat. Plants, artificial materials, and even buildings can be considered fuels, and are often classified based on their fire hazard and flammability. A fuel burns by reacting chemically with an oxidizer (usually oxygen in the air). Because these reactions occur at high temperatures, the heat source is the third essential component of a fire. While the initial energy needed to start a fire can come from ignition sources, once flammable materials begin to burn, they generate enough heat to sustain the process until all fuel has been consumed. This can be seen as a feedback loop in which the heating source is both an input and an output (see diagram below). In addition to heat, fires emit particulate matter (PM), as well as gaseous emissions such as carbon dioxide (CO2).

Shematics of the fire process

The relation between fuel, oxidizer, and heat is often indicated with the fire triangle (see figure below). By removing any of these components, the fire will extinguish.

The fire triangle shows the components necessary for a fire.

How does a fire spread?

The fire triangle serves as a tool for understanding fire, but it does not explain how the fire will respond to its local environment. A fire, for instance, can spread quickly through grass and grow in size and intensity if it comes into contact with more flammable materials. Wildfires can also spread rapidly if embers are blown ahead of the main fire, causing new fires to start. Embers are small pieces of plants, trees, or buildings that can be carried for miles ahead of a fire by the wind. Three factors significantly influence fire behavior: fuel availability, weather conditions, and terrain topography. Similar to the fire triangle, we can define a triangle for fire behavior, as
illustrated below.

Triangle describing the fire behavior

The fuel effect varies largely according to its properties and flammability conditions (type of vegetation, moisture, etc.). Weather conditions, such as wind speed, relative air humidity, and temperature, can either aid or inhibit fire growth. The wind directly affects fire behavior, but temperature and humidity also have a large impact on fuel conditions. Furthermore, large fires can "create their own weather" due to the upward motion of the fire plume interacting with the external wind. Topography also affects weather conditions, like changing wind patterns in canyons or affecting the vegetation moisture content on either side of a hill. Additionally, topography can directly influence how the fire spreads; for example, on steep slopes an upward spreading fire preheats fuel on its path, as illustrated below, causing it to spread more rapidly (see figure below from National Wildfire Coordinating Group).

The Fire regime: comparing multiple fires

So far, we have looked at the early stages and growth of a single fire, but how can we compare multiple fires? In a particular region, multiple fires can occur over several decades, and their intensity can vary based on local factors. Climate, ignitions, and vegetation determine the fire regime, which describes the frequency and severity of fires.

Triangle for the fire regime

Climate and vegetation are linked; plants have adapted to regional characteristics (water, temperature, soil content, etc.), including fire, over millennia. This connection has, however, been profoundly affected by human activities (logging, farming, and introduction of non-native plants), as well as climate change. Fire-prone ecosystems that used to burn frequently with low-intensity fires did not experience large fires for decades, and their fire resiliency was significantly reduced. The Sierra Nevada region in California, for example, has witnessed the transformation of sparse forests with large trees into densely populated forests with smaller trees over the last century; with accelerated tree mortality from recent droughts and pests, and the quick suppression of small fires, the accumulation of dead vegetation and a dense forest has caused devastating fires in recent years.

As the third determinant of fire regimes, ignitions are defined as their distribution and their nature over time. Naturally occurring ignitions (such as lightning) can occur, but they are most often caused by human activities on a landscape (such as farming, controlled burning, or accidentally).

Same problem, different scales

The fire triangle described the initial stage of a fire, which requires fuel, oxygen, and heat to be present in the immediate environment. Due to their larger scale, the factors affecting fire behavior (fuel distribution, weather, and topography) become tougher to control when the fire grows larger. Further, the timeline of the problem changes. While it takes minutes and hours to ignite a fire, the fire's behavior is affected over days and weeks. What about the fire regime? In this case, we are talking years, if not decades.  Thus, the fire regime is determined by large-scale changes. Below is a graph showing how the different scales impact fire characteristics.

Comparison of spatial and time scales for the different aspects of fire

How homes burn

Home and building loss during wildfires occur as a result of some part of the building igniting from one or more of the three basic wildfire exposures: 1) direct flame contact, 2) embers, 3) radiant heat.


Embers cause the majority of wildfire home ignition by directly igniting your home or igniting vegetation or materials on or near your home that results in flames touching your house or a high heat (radiant heat) exposure that may break glass in a window. Should embers land on or near your house, they accumulate (like hail or snow) and can easily ignite the plants and mulch near your home, dry leaves, or lawn furniture. They also land on the roof, deck, or porch and, depending on the condition of each, they may find a gap to enter the house or catch accumulated dry leaves on fire. Embers can also enter the home or attic through a vent or open window. When embers enter the home or attic, they can easily ignite the contents of the house, and the home will burn seemingly from the inside out. When ember enter the house directly, there is often little damage to the surrounding vegetation, and many are left puzzled as to what caused the home to burn, as shown in the picture below.