buoyancy

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buoyancy Condition arising from the difference between the density of a given parcel of air or gas and that of the surrounding air. It is this that makes a hydrogen- or helium-filled balloon or airship float or rise through the atmosphere. Also, if an air parcel is warmer than its surroundings, the density difference implies an upwards directed force acting upon that parcel of air and it will rise, with positive buoyancy. When a parcel of air is colder than its surroundings the reverse condition causes it to sink, with downward (negative) buoyancy. See THERMAL.

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