buoyancy

Applied to a volume of air, this is the resultant force from the balance between the weight of the air contained in that volume and the force due to the pressure exerted by the air surrounding it. Although that balance can be positive, negative, or zero, the concept of buoyancy is usually identified only with the upward motion experienced by a bubble or volume of air in the atmosphere, as a consequence of having a lower density than that of the surrounding air.