Fire Behavior Triangle
By Louisa Evers, Golden Eagle Audubon Board Member
In my blog “Wildfire: A Primer on Behavior,” I explained the Fire Triangle. This blog is about another triangle - the Fire Behavior Triangle. While the Fire Triangle indicates what it takes to have a fire burn, the Fire Behavior Triangle indicates the primary factors that determine how fast the fire will spread, what the flame length might be, and what type of fire to expect. I will explain the different types of fire in a future blog.
Fuel Type and Arrangement
Fuel refers to the type of fuel, how it is arranged, and its moisture content. The three major types of fuels are grass, brush, and trees. Most fuelbeds are not exclusively one type or another, but usually a mix of two or all three fuel types. Firefighters typically refer to them by what type is dominant.
Arrangement refers to continuity both horizontally and vertically. Annual grassland fuels usually have horizontal continuity; perennial grassland
fuelbeds may or may not have horizontal continuity depending on the species mix and how productive the year is. Shrubland fuels may have only horizontal continuity or a limited amount of vertical continuity, depending on the shrub type and density. However, fire behavior models treat shrubland fuels as having only horizontal continuity.
Forest fuelbeds usually have both horizontal and vertical continuity. The horizontal continuity can occur in both the understory and the tree crowns. Vertical continuity refers to how connected the understory continuity is to the tree crown continuity. How connected they are varies with how tall the understory is, how low the tree crowns extend, and the flame length produced by the fire.
Fuel Moisture
Fuel moisture is a key factor in how a fire behaves. The drier the fuel, the easier it is for a fire to start, the faster it spreads, and the longer the flame length. Grasses don’t tend to support a fire until they are fully dried out or cured. That’s when grasses turn brown or yellow.
The extent to which fire spreads and the flame lengths produced in shrubs depends on how much dead material is in the shrub crown, whether the shrubs have oils and waxes in their foliage, and how moist the leaves are. Firefighters typically focus on how moist the leaves are, known as live fuel moisture. Fire behavior increases in shrubs when the live fuel moisture drops below 100%.
In forests, the focus is largely on the dead fuel on the ground, although live fuel moisture in tree canopies is important under certain conditions. Fine twigs, needles, sticks and logs on the ground are divided into four timelag classes. This is based on how long it would take for the moisture content of the dead fuel to reach equilibrium with the atmosphere (dry out). Fire behavior models are driven by the amount and moisture content of the fine dead fuels (1- and 10-hour classes). Firefighters pay attention to the needles, twigs and fine sticks because once moisture drops below 20%, fire behavior increases. The large fuels mostly affect smoldering fire spread, although a smoldering fire can spread persistently through logs, especially rotten logs.
Dead Fuel Classes
Topography
Topography refers to how steep the slope is and what aspect a given slope faces. Generally, the steeper the slope, the faster the fire can spread. A fire located at the bottom of a slope can spread faster than one at the top of a slope. Aspect governs how dry the fuels are given the slope steepness, the time of year, and the type of year, but is mostly a factor in mountainous terrain. Because we are in the northern hemisphere, south- and west-facing aspects receive more sun overall and more afternoon sun when conditions are hotter, so tend to be drier. North- and east-facing aspects tend to be wetter as they receive less sun and more sun in the morning when conditions are cooler.
Slope steepness
This difference in overall heating on different aspects can also create sharp differences in fuel type. For example, a south aspect may support grass and shrub fuel type while the opposite north aspect supports a tree fuel type. This type of difference is more noticeable at lower elevations where conditions are generally warmer and drier. At higher elevations, aspect may affect what tree species are dominant and how dense the forest is. Aspect along with fuel type affects how much snow accumulates, how quickly it melts, and how rapidly the fuels become dry enough to burn.
Slope Location
Fire behavior is also impacted by the location on the slope relative to the top and bottom. In mountains, the lower third of the slope is generally narrower and cooler and receives less sun overall, although the orientation of the stream drainage matters as well. Drainages that are oriented perpendicular to the track of the sun (north-south) tend to be moister than those that are oriented parallel to the track of the sun (east-west). The lower third of a north-south drainage may not dry out well enough to burn after a significant rain in late summer and early fall whereas the lower third of an east-west drainage might, especially as our climate becomes warmer and drier overall.
The middle third of the slope can play a unique role in some years. At night, the air cools and typically moves downslope to the lower third and down the drainage. The warm air readily moves off the upper third of the slope. However, in inversions, the warm air can become trapped between the cooling upper slope and the already cooler lower slope, creating what is called a thermal belt. Normally at night, firefighters expect the fire to slow down and not spread as rapidly due to cooling conditions, but that does not happen in the thermal belt.
Weather
The last factor in the Fire Behavior Triangle is weather – precipitation amount and timing, wind, relative humidity, and temperature. In Southwest Idaho, our precipitation regime is winter-spring dominant, meaning most of our precipitation comes between December and May. However, we are also in a semiarid climate, which means there is substantial interannual variation in both the amount of precipitation we get and when it comes. With climate change, that variability has been increasing. We get some precipitation in summer from thunderstorms, but that moisture reaches the ground mostly in the mountains due to a factor called orographic uplift. Orographic uplift occurs when an air mass encounters mountains and must rise to get over them. As the air mass rises, it can reach the level where moisture condenses into clouds and must fall as rain for the air mass to get over the mountains. In the rangelands, a different uplift mechanism comes into play called convective uplift. Intense surface heating from high summer temperatures causes an air mass to rise, but this mechanism is not as dominant as orographic uplift. Clouds may form, but no rain falls or else it evaporates before it reaches the ground.
Wetting Rain
Meteorologists define a wetting rain as 0.1 inches, however, that little amount is relevant only in grasslands. In shrublands and forests, a true wetting rain is closer to 0.25 to 0.5 inches, depending on canopy density. The denser the canopy, the more rain it takes to get through the canopy and onto the ground. In addition, summer heating and drying makes the surface fuels naturally resistant to wetting (hydrophobic). The drier the surface fuel, the more rain it takes to break down that hydrophobic layer and allow the water to penetrate. A wetting rain usually only slows the spread of a fire and lowers the flame lengths, at least temporarily. How long the effect of a wetting rain lasts depends on the amount and time of year with duration of effect increasing later in the year.
Temperature and Humidity
Air temperature and relative humidity are closely related in their effect on fire behavior. High temperature dries out fuels. Relative humidity is a measure of how much moisture the air mass holds as compared to how much it could hold at that temperature. For example, a relative humidity of 25% means the air mass has only about 25% of the moisture it could hold at a given temperature. As air temperature rises, relative humidity falls and as the air cools, relative humidity rises. The greatest fire behavior occurs when temperature is highest and relative humidity is lowest. Firefighters become greatly concerned when the temperature is 90 degrees or higher and relatively humidity is 20% or lower. Relative humidity in the single digits is especially concerning as fire behavior can become explosive with very rapid rates of spread and very long flame lengths for a given fuel type.
Wind
The last weather factor is wind, which often governs which way a fire will spread and the possible flame lengths. Several things affect wind, and it’s one of the hardest factors for meteorologists to forecast. For fire behavior, firefighters usually measure the winds at eye level, although forecasting focuses on the wind speed and direction 20 feet above the top of the vegetation.
Wind categories include slope and canyon winds, prevailing winds, frontal winds, and thunderstorm downdrafts. Winds are typically upslope and upcanyon in the day as surface heating causes air to warm and rise. At night, as the air cools, winds are typically downslope and downcanyon. These are also known as diurnal winds. The type of vegetation affects windspeeds with higher windspeeds in grasslands and shrublands and lower windspeeds in the forest. The denser the forest, the lower the windspeed near the ground.
Prevailing winds are the general winds for a given time of year. In summer, our prevailing winds are typically out of the southwest. Prevailing winds have a more significant effect in flatter terrain than mountainous terrain. The topographic complexity in the mountains, alters the direction of the prevailing winds and results in interactions between the prevailing wind and diurnal winds.
Frontal winds are important in fire behavior when they are associated with a dry cold front and the strong winds created. In winter, cold fronts deliver our moisture, but in summer, they often lack sufficient moisture to trigger rainfall, so its just wind. In flatter terrain, such as in the Treasure Valley, the wind direction is usually out of the southeast ahead of the front, switching around to the northwest behind the front. Fires can abruptly change direction as a front passes through. The situation is more complicated in mountains as the terrain interacts with the front to influence the wind direction making it hard for firefighters to predict fire behavior.
Thunderstorm Downdraft
The last type of wind is a dangerous thunderstorm downdraft. Rising air, also known as an updraft, can reach the condensation point at which a cloud starts to form. The updraft continues, causing the cloud to build up into the atmosphere. A thunderstorm occurs when the cloud reaches the tropopause, the boundary between the troposphere where we live and the stratosphere, and flattens out, creating an anvil shape. Once the cloud anvils, air starts to fall in the front of the cloud, known as a downdraft, while updrafts can continue at the rear of the cloud. Once it reaches the ground, the downdraft winds spread in all directions, making fire behavior very unpredictable. Occasionally these downdrafts are very strong, creating a microburst or a gust front. The winds in a microburst or gust front can be strong enough to topple trees, creating extremely dangerous conditions for firefighters.
Editor’s Note: Louisa has a professional background in fire ecology and many years experience assessing fire risk and predicting fire behavior. Watch for her next blog to learn more about Fire Science