Jumping Fire: a few word about firebrands

 

Irregular Fire Spread and the Threat of Firebrands

During wildfires, the flames rarely spread evenly or predictably. Although they typically move with the wind, their spread can be highly dynamic and irregular, which affects the shape of burned areas. One of the more difficult phenomena to control is the presence of firebrands, burning fragments of plant material carried by the wind that can ignite new flames far ahead of the main fire front. These spot fires can cause the blaze to spread much faster than anticipated.

Mechanics and Behavior of Firebrands

This phenomenon, called fire spotting, has major operational implications. It affects not only the speed and direction of fire spread but also directly impacts the safety of firefighters. By understanding how it works, responders can anticipate risks and plan their tactics more effectively. Firebrands are partially burned fragments of leaves, twigs, bark, seeds, or cones lifted above the fire by convective (thermal) currents. The ability of these bits of vegetation to carry fire depends on their physical properties: mass, shape, and burn time. Particles that are too small, such as grass blades, may travel long distances, but typically burn up completely while still airborne. On the other hand, larger fuel fragments, like cones, may still be burning when they land, but are generally unable to travel far from the fire source. However, the glowing embers found far ahead of the fire front represent only a fraction of the original firebrands generated by the fire, as they lose mass and burn up during their lift, transport, and falling. Only fragments with sufficient burn time and an optimal ratio of mass to surface area, suitable aerodynamic properties can ignite new fire spots. Different types of forest fuels vary in this regard – for example, deciduous leaves can be lifted even by weak convective currents, while cones require strong upward columns of hot air. For a firebrand to be lifted, the upward air current must exceed the firebrand’s maximum falling speed in still air. Only those firebrands that are carried high into the atmosphere can be transported over long distances. Even small fires release burning plant fragments into the air. Tiny charred pieces often rise in the smoke; while too small and low-energy to ignite new fires, they work on the same principle as large firebrands in major wildfires. However, for these firebrands to ignite new spot fires, they must land on dry, flammable fuelbeds. The effectiveness of ignition depends on the size of the firebrand, its mass, the type of material, and its condition – burning fragments are much more dangerous than smoldering ones. Firebrands made of materials like eucalyptus bark or pine cone scales are particularly hazardous and often lead to successful ignition.

Challenges of Long-Range Spotting and Firefighting Strategy

Optimal conditions for long-range spotting occur in Australian eucalyptus forests. To reduce the risk of fire spotting during wildfires, low-intensity prescribed burning [LINK] of eucalyptus bark is conducted under strictly defined timing and conditions, when the fire is fully controlled. During wildfires, long streamers or ribbons of eucalyptus bark (known as candlebark) burn slowly and have favorable aerodynamic properties, enabling spot fires to ignite over very long distances—typically 10 to 30 km, though this is an extreme range. To start a fire 30 km away, a firebrand must remain burning for about half an hour. It’s no wonder Australians wryly joke: “The only effective firebreak is the one between us and New Zealand.” Long-range spotting makes fighting wildfires difficult because the fire jumps ahead of firefighters, spreads to new areas, and gains strength. The risk increases when firefighters find themselves between the main fire front and a new “spot” fire; evacuation routes can be cut off. Therefore, in conditions of frequent spotting, defensive planning must consider not only the fireline but also the possible fire spread caused by potential spot fires. Reconnaissance, observation, and patrols to detect new fire ignitions become crucial. Communication, mobility, and rapid response are the foundations of effective firefighting against this phenomenon. Estimating the maximum distance at which a new fire spot may appear is crucial for operational decisions and safety. The problem is that it is impossible to predict the exact location and timing of such ignition. When the spotting distance reaches several kilometers, the fire can ignite anywhere within that area. Fire intensity can increase where multiple fire spots merge. Therefore, spotting contributes to the development of large, challenging wildfires. The situation becomes even more dangerous when firebrands ignite not forests, but buildings. It only takes an ember falling into a gutter full of dry leaves or onto dry grass near a house. The fire can then spread to entire neighborhoods far from the main fire front, and the ignition of a single building often triggers a domino effect. Any spark can start a new tragedy – that’s why planning must look beyond what is visible and anticipate threats coming from seemingly safe directions.