What occurs during the Negative Stack Effect (Summer Stack Effect)?

During the Negative Stack Effect, also known as the Summer Stack Effect, cooler outside air leads to a situation where warmer air inside a building rises and creates a pressure difference. As the outside air is cooler, it becomes denser and consequently exerts more pressure at the lower levels. This denser outside air pushes the warmer, lighter air inside upward. In essence, this phenomenon enhances the upward movement of smoke and hot gases inside a structure, which can pose significant challenges in fire situations, as it may cause smoke to rise and accumulate near the roof, potentially spreading toxic gases and heat throughout the space.

In this context, the correct answer illustrates that when outside air is cooler, it drives the negative stack effect, causing air to push smoke downward in the building. This understanding is crucial for firefighters and emergency responders to manage smoke movement effectively during their operations, ensuring their safety and the safety of others.