The Fire Triangle

As you have seen in the previous chapter, synoptic patterns are not enough to indicate the locations where forest fires are likely to occur and spread. In fact, aside from the weather, a fire's behaviour also strongly depends on fuel and topography, creating the three sides of what is usually known as the fire triangle (Figure 3.1).

Fig 3.1 - Fire triangle. Fire hazards are influenced by fuel, weather and topography.

In this third chapter the three sides of this triangle will be further explained. After answering the following multiple choice question the chapter will continue with "weather".

Question

Fire ignition depends most on which of the following meteorological factors:

The correct answer is D.

Lightning was the correct answer. However, most of the instances of ignition are not related to meteorological factors, but are mainly due to human activities such as barbecues, cigarettes, machines that burn due to overheating or arson.

Still, lightning is the primary source of ignition in wilderness and other remote natural areas. A fire ignited by lightning may propagate if no precipitation accompanies the event. For example, a study from 1931 for the Rocky Mountains states that 8% of the thunderstorms over that area are not accompanied by rain (Gisborne, 1931).

As mentioned above, the other parameters referred to are important, but for the fire's development rather than its ignition.

The Wildfire Alternatives project at the University of Arizona has shown through a logistic regression analysis that wildfire ignitions were spatially associated with the following factors:

  • Proximity to roads - people usually access areas by way of roads and spend more time close to them than far away
  • Location of campgrounds and picnic areas - people spend more time in these areas
  • Proximity of urban areas - there are more people in urban areas, and less time is spent the further they have to go from the urban area
  • Non-forested vegetation layers
  • Urban-wildland border complexity (see Fig. 3.2)
Fig 3.2 - Map of human factors of fire ignition in the Fort Huachuca area, with darker colors indicating an increased likelihood of a fire.

In general, human-caused fires are more quickly reported and more easily accessed and extinguished than lightning-caused fires.