Negative buoyancy in a COTA cloud can exist even if the cloud is not strictly denser than air.

Negative buoyancy in a COTA cloud can exist even if the cloud is not strictly denser than air.

• A. Not true
• B. Only when the cloud is denser than air
• C. Not just when the cloud is denser than air
• D. Only when the cloud is lighter than air

Answer: (C)

Buoyancy is all about the density difference between the cloud and the surrounding air, not just a fixed label of “denser than air.” In real plumes, the cloud constantly entrains and mixes with ambient air, and its temperature, humidity, and pressure change as it moves. That mixing can raise or lower the cloud’s effective density relative to the local environment. Because buoyancy depends on how dense the cloud is compared with the air around it at that moment, the cloud can exhibit negative buoyancy even if, in a simple snapshot, it doesn’t look strictly denser than surrounding air. In short, negative buoyancy can occur due to entrainment and changing ambient conditions, not only because the cloud is inherently denser than air.

Buoyancy is all about the density difference between the cloud and the surrounding air, not just a fixed label of “denser than air.” In real plumes, the cloud constantly entrains and mixes with ambient air, and its temperature, humidity, and pressure change as it moves. That mixing can raise or lower the cloud’s effective density relative to the local environment. Because buoyancy depends on how dense the cloud is compared with the air around it at that moment, the cloud can exhibit negative buoyancy even if, in a simple snapshot, it doesn’t look strictly denser than surrounding air. In short, negative buoyancy can occur due to entrainment and changing ambient conditions, not only because the cloud is inherently denser than air.