What forms the connection between zones in a two zone fire model?

In a two-zone fire model, the connection between the zones—typically the hot upper layer (smoke layer) and the cooler lower layer—is formed by the interface where hot gases rise and cooler air descends. This dynamic creates a thermal stratification, with the upper zone accumulating heat, smoke, and combustion gases, while the lower zone remains relatively cooler and contains breathable air. The movement of gases and heat between these zones is influenced by factors such as convection, radiation, and pressure differences, allowing for the transfer of energy and smoke.