The superpressure balloon is a rigid, nonstretchable balloon that is sealed with a plastic film to prevent the release of gas. It is also used to take horizontal measurements. By the time the balloon reaches its constant-altitude level, the gas in its envelope has become pressurized. Variations in the air pressure caused by the heat of the Sun produce changes in the internal gas pressure, but the volume of the balloon remains fixed. So long as the balloon remains under pressure, therefore, it continues to float at its predetermined constant-altitude level. Plastic films strong enough to withstand the high pressures within the balloon became available for use in about 1960. Flights of several months duration are possible with superpressure balloons.
