Which term describes the common boundary of interface between hot gases and cooler gases in a fire?

The correct term for the common boundary that separates hot gases from cooler gases in a fire is the neutral plane. In the context of fire behavior, the neutral plane represents the level in a structure where the pressures of hot gases above and cooler air below are equal. This phenomenon occurs during a fire when heat rises, creating a layer of hot gases that can be separated from the cooler, denser air below.

Understanding the neutral plane is crucial for fire fighters because it influences smoke and heat movement within a structure, as well as visibility and conditions for victim rescue. It also helps inform strategic decisions during firefighting operations, such as where to ventilate and the best entry points for rescue or suppression efforts.

The other terms mentioned, while they may relate to thermal dynamics in fires, do not accurately describe this specific interface. For instance, a thermal barrier typically refers to a material used to reduce heat transfer and is not a dynamic boundary like the neutral plane. Similarly, a cooling layer or heat boundary do not encapsulate the balance of pressures and temperatures characteristic of the neutral plane.