Final answer:
When heated gases cool to the temperature of the surrounding air, they cease rising and start spreading horizontally, a process called stratification. In physics, it relates to the principles of heat and mass transfer.
Step-by-step explanation:
When heated smoke and fire gases rise and then cool down to the temperature of the surrounding air, they cease to rise and begin spreading out horizontally. This behavior is called stratification. It occurs because the smoke and fire gases become denser as they cool, losing the buoyancy that was causing them to rise. Instead, they spread out in a layer, which can have implications for firefighting tactics as the stratified gases can influence the behavior of the fire.
In physics, this is related to heat and mass transfer principles where hot gases rise due to lower density, and when cooled, their density increases and they spread out horizontally. An example in nature of a phase change similar to this phenomenon is sublimation, where a solid turns into a gas without becoming a liquid, and its reverse, deposition.