Buoyancy is an upwards force exerted on a mass that is less dense than its surroundings. Thermal buoyancy is the tendency of warm air or smoke to rise when located in cooler surrounding air, as the air and smoke become less dense with increasing temperature.

Thermal buoyancy causes smoke to move through openings and leakage paths in walls and ceilings above the fire, as shown in Figure 12. While the buoyant force is relatively small, it will drive smoke movement. In taller buildings, the effects of buoyancy can be much higher and possibly overcome the design of state-of-the-art smoke control systems. However, as smoke moves away from the fire, heat transfer and dilution reduce its temperature and the effect of buoyancy decreases.

Figure 12: Buoyancy of smoke

Rapidly roiling smoke indicates that the gases are at a high temperature. Even very dense fire products will be lighter than air when they are heated to high temperatures. In contrast, cooler smoke has a tendency to slowly drift upwards or can even settle downwards as it cools. Less buoyancy could indicate relatively low compartment temperatures, or it could indicate that cooling has occurred as the smoke has travelled some distance through colder air or uninvolved sections of the structure.