Fires and Plumes

In normal fire conditions, smoke rises and disperses without noticeable impacts on fire behavior. Rates of spread are governed by radiation and convection heat transfer, air movement in and above the fuel bed, and characteristics of the fuel bed. If fires begin to burn very intensely, air movement in and around the smoke column begins to impact the fire itself, resulting in plume-dominatedconvection-dominated fires.

Plume- or convection-dominated fires are characterized by high rates of heat release, and unpredictable and extreme fire behavior. These types of fires create dangerous conditions for fire fighters. The formation of plume-dominated fires is correlated with a high value on the Haines Index. How convection columns and plumes develop and how they can dominate fire behavior is discussed below.

Convection and plume development

A plume is characterized by rising buoyant gases, with the buoyant forces provided by intense localized sources of heat. Plume rise is a function of free convection in the atmosphere. As a fire heats and expands air near the ground, large density differences between the heated volume and the surrounding air mass are created, causing the heated parcel to rise. The potential height of the resulting plume depends on the heat energy of the source and rise velocity, which is affected by air stability, the exchange and conservation of mass, radiant heat loss, the buoyancy force, and turbulent mixing with the ambient air.

Atmospheric stability affects the size, shape, and growth of smoke columns. Stable air generally discourages height growth of smoke plumes because the rising heated air cools faster than the warm ambient air and temperatures tend to equalize at a relatively low “mixing height” in the atmosphere. Without a strong vertical movement of air, fire activity is generally lower with stable air masses unless there are strong surface winds. Unstable air masses create conditions for rapid vertical plume development and convectional lift, which increases surface air movement toward the fire. The convection column created by these conditions contains ascending columns of hot gases, smoke, and entrained debris.

Smoldering fires often create plumes that are neutrally buoyant, limiting widespread dispersion but allowing surface winds to dominate smoke trajectories. This can lead to accumulations of smoke in valleys and basins at night. Inversion layers (including marine or subsidence inversions) act to inhibit vertical motion in the atmosphere and affect smoke dispersal (Pyne et al. 1996; Potter 2002). Smoke will not dissipate in these conditions and can cause thick fog-like conditions that hamper visibility.

A fire is said to create its own wind when the convection column intensifies to a level such that air rushing into the column base to replace the air leaving the top of the column is stronger than the ambient wind field. Plume rise calculations are essential for determining smoke dispersion patterns and possible effects on fire behavior. Flame heights may increase in such convection columns and ascending gases from volatile materials may be completely consumed. Increases in combustion and convection act as feedback mechanisms on each other and can lead to convection columns developing to heights of 25,000 to 50,000 feet. The rush of air entering a convection column may transport firebrands up and out of the column causing spot fires to break out under severe conditions. Despite the volume and height of the smoke, smoke quantity near the base of the column may remain minimal particularly on level ground. In areas where topographic gradients exist, the slope may cause the convection column to tilt, creating low visibility upslope and increasing the rate of spread as firebrands fall out of the column.

High intensity fires can develop central convective columns with counter-rotating vortices that involve massive entrainment of the surrounding air mass (Clark et al. 1996; Haines and Smith 1987; Haines and Updike 1971). This stage of fire can produce fast-rising plumes and turbulent downdrafts that can exacerbate an already dangerous situation. The downdrafts and fallout from the column carry sparks and embers that ignite new fires. Cumulonimbus clouds often develop with accompanying lightning and rain. Dynamic plume rise brings gas and particles high into the atmosphere where strong winds can disperse the smoke hundreds to thousands of kilometers. As high intensity fires cool, however, the central column often collapses, creating numerous small convective cells that are less dynamic but equally active in carrying smoke into the atmosphere.