What is the primary indicator of a possible flashover condition in a structure?

The primary indicator of a possible flashover condition in a structure is dark, aggressive smoke. This type of smoke typically signals that there is a high concentration of combustible gases accumulating in the upper layers of the space. As the fire progresses, these gases can reach a temperature where they ignite all at once, leading to a flashover.

Dark smoke is often the result of incomplete combustion, meaning that the materials involved in the fire are not burning efficiently. This inefficiency generates more smoke, indicating that there may be a significant amount of flammable gases building up. When these gases reach a critical temperature and concentration, the flashover can occur, which poses an extreme risk to both firefighters and any occupants still in the building.

In contrast, other indicators such as rapid temperature increases, clear visibility, and white smoke can provide different information about the fire's behavior and state, but they do not specifically signal the imminent risk of flashover as effectively as dark, aggressive smoke does.