Wildland fire displays some remarkable characteristics and behavior. Flames are one of the visible manifestations of the combustion process and are the main source of heat from a fire. Measuring or estimating flame height, length, depth, angle, and char height can help to understand fire intensity and predict fire effects (Johnson and Miyanishi 2001).

Two theoretical definitions of flame height are: the height where flames reach the maximum temperature, which is difficult to measure in wildland fires because of varying flame heights; and the vertical distance from the flame base or ground to the time-averaged yellow flame tip as shown in the figure (Johnson and Miyanishi 2001).

Flame characteristics

Measurements of the visible flame tip require height markers positioned in the fire and either ocular observation or videotaping a moving fire (Adkins 1995, Hamins et al. 1996). A motor-driven 35mm still camera or a video camera can be used to record the fire for measuring characteristics such as flame angle and height (Johnson and Miyanishi 2001). For a smoke-covered flame, an infrared camera can be used to penetrate the smoke and measure the maximum flame temperature and visible flame tip (Johnson and Miyanishi 2001).

Flame length is another estimator of fire intensity. Flame length estimates have been made for many experimental fires, wildfires and prescribed burns. In the South, flame lengths may vary from less than 1 ft in slow moving backfires in leaves and short grass to over 100 ft in extreme wildfire conditions in dense shrubs and pines. Flame length is measured from ground level from the mid-point of the base of the flame to the tip of the flame, whether the flame is vertical or slanted.

Flame characteristics

In theory, flame length has been defined as the distance from the fuel surface to the point on the flame axis where the CO concentration is 5000 ppm. In wildland fires, it is difficult to determine these theoretical parameters and measurements are generally based on video or time-lapse photography. Flame length also provides an estimate of the difficulty in fighting fires. For instance, flame lengths less than four feet can be attacked at the head of the fire, while fires with flame lengths over eleven feet should not be controlled from the head and often lead to crowning and long range spotting.

Flame depth

Flame depth is the distance from the front to the rear of the flame zone at the fuel surface. Flame depth is dependent on the spread rate of fire, the length of time flames remain over any particular point in the fuel bed (residence time), and the type and quantity of fuel being burned.

Tilt or flame angle

Tilt or flame angle is the angle between the unburned terrain whether flat or sloped and the leading surface of the flame. Flame angle is highly influenced by wind. For a wind-driven flame the angle is less than 90^o and for a flame backing into the wind it is greater than 90^o.

Bark char height is the height from ground level to the highest point on the bole where bark char is evident (outer bark blackened by fire). Char height on individual trees is measured halfway between the upper and lower char heights on opposite sides of the tree. Bark char height is used to approximate flame height or flame length, particularly when measuring these variables during the course of a fire is impractical or impossible. Flame length estimated from bark char height can then be used to reconstruct fireline intensity using Byrams equation (e.g., Waldrop and Van Lear 1984). However, evidence suggests bark char height is not a reliable estimate of flame height, flame length, or indirectly, fireline intensity. For example, Cain (1984) found that for both back fires and head fires in a 9-year-old loblolly-shortleaf pine stand, bark char height underestimated flame length by 50 percent and fireline intensity by 75 percent during prescribed burns. Cain noted, however, that bark char height may provide adequate post-burn measures of relative, but not absolute, fire intensity where better measurements are not possible. In fact, Cain found that crown scorch height estimated fire intensity no better than bark char height.

Bark char height has also been use to predict fire damage. For example, Waldrop and Van Lear (1984) found a direct relationship between bark char height and crown scorch above a threshold value of 3 feet in a loblolly pine plantation. However, since bark char is not a very reliable indicator, it should only be used in cases where attempts are made to reconstruct fire behavior months after the scorched foliage has fallen (Wade and Johansen 1986). The accuracy of bark char height is particularly questionable in previously burned stands because sloughing of charred bark sometimes requires several years (Cain 1984).