Condition arising from the difference between the density of a given parcel of fluid and that of the surrounding fluid. It is this that makes a hydrogen- or helium-filled balloon or airship float or rise through the atmosphere. If an air parcel is warmer (therefore less dense) than its surroundings, the density difference implies an upward-directed force acting upon that parcel of air and it will rise, with positive buoyancy. When a parcel of air is colder (denser) than its surroundings the reverse condition causes it to sink, with downward (negative) buoyancy. If the fluid density is equal to that of the surrounding medium the fluid will experience neutral buoyancy and remain at the same level. Buoyancy per unit mass of a fluid is given by (ρ‎′ – ρ‎)/ρ‎, where ρ‎′ is the density of the surrounding medium and ρ‎ is the density of the parcel of fluid. See thermal.