What describes the Positive Stack Effect (Winter Stack Effect)?

The Positive Stack Effect, also known as the Winter Stack Effect, occurs when warmer air rises due to its lower density compared to cooler air. In colder outside temperatures, the temperature inside a structure tends to be higher, which results in a pressure difference that drives the warm air up and out of the building. This phenomenon leads to the upward movement of smoke and hot gases from a fire, as the warmer air generated by the fire rises through the building and exits at higher elevations.

When outside air is cooler, it is denser and creates a more pronounced upward push on the lighter, warmer air. This effect is particularly relevant in colder weather scenarios when buildings are heated, thereby enhancing the stack effect. The warm air inside effectively rises and exits, while cooler air is drawn in from lower levels to replace it. This explains why the correct choice is that air pushes smoke up when outside air is warmer, as it directly correlates with the principles of thermodynamics and buoyancy at play in this scenario.