What is a sign that a fire is approaching flashover?

Large volumes of darkening smoke are an indication that a fire is approaching flashover. This phenomenon occurs when the temperatures within the fire reach levels where the combustible materials in the environment can ignite almost simultaneously, creating a rapid spread of fire. The presence of thick, dark smoke signifies that the combustion process is intensifying, indicating that there is a significant buildup of heat and unburnt combustible materials in the atmosphere.

In a fire situation, dark smoke generally implies that the fire is burning more fuel-rich materials, which increases the potential for flashover. As the temperature continues to rise, the conditions for flashover become more favorable, with the dark smoke also indicating that the fire is consuming available fuel rapidly.

The other choices point to different fire behaviors: thick white smoke may indicate the presence of certain materials burning with less intensity, persistent low temperatures suggest a fire that is smoldering rather than fiercely burning, and audible crackling noises can occur in a variety of fire scenarios but do not specifically indicate a flashover condition. Thus, large volumes of darkening smoke are the most reliable sign of an impending flashover.