Which of the following is an indicator of a backdraft situation?

Opaque smoke signals serve as a significant indicator of a backdraft situation because they indicate the presence of a heated environment with a rich accumulation of combustible gases. In a backdraft scenario, the fire may be smoldering rather than actively burning, leading to the production of dark, thick, and dense smoke. This opaque smoke is a byproduct of incomplete combustion, often filling spaces with flammable gases that can ignite explosively upon reintroduction to oxygen.

When dealing with a potential backdraft, fire investigators and responders look for signs indicative of a buildup of these combustible gases, which can be lethally hazardous if the situation is not properly assessed before ventilation or entry. Therefore, the appearance of this opaque smoke is a crucial cue in identifying the distinct risk of a backdraft event.

Other options indicate phenomena that can be associated with various stages or types of fire situations but do not specifically characterize backdraft conditions. High visibility of flames and consumption of fuel without flames suggest active combustion, while spontaneous combustion refers to a condition where materials ignite without an external ignition source, which is different from the dynamics of backdraft phenomena.