A nonporous envelope of thin material filled with a lifting gas and capable of lifting any surrounding material and usually a suspended payload into the atmosphere. A balloon which is supported chiefly by buoyancy imparted by the surrounding air is often referred to as an aerostat. The balloon rises because of a differential displacement of air according to Archimedes' principle, which states that the total upward buoyant force is equal to the weight of the air displaced. The upper practical limit for useful ballooning is approximately 34 mi (55 km). Beyond this, the exponential nature of the atmosphere would require balloons of enormous size and delicately thin skin.