III.

Modern Scientific Ballooning

Three types of balloon are in common use for meteorological research.

The rubber or neoprene balloon is used for vertical soundings, either as a radiosonde-carrying balloon that transmits meteorological information, or as a pilot balloon, a small balloon sent aloft to show wind speed and direction. The balloon, inflated with a lifting gas (hydrogen, helium, ammonia, or methane), stretches as it ascends into thinner air. When the diameter of the balloon has stretched by three to six times (that is, when its volume has increased to 30 to 200 times its original amount), the skin ruptures, destroying the balloon.

The zero-pressure plastic (usually polyethylene) balloon is used to carry scientific instruments to a predetermined density level. The plastic balloon is filled only partly with gas while on the ground. As the balloon ascends, the expanding gas fills the envelope. This type of balloon has a valve that automatically discharges excess gas when the balloon has reached its equilibrium altitude, so that the balloon can maintain this altitude. When the Sun sets, the gas cools, the volume decreases, and the balloon descends to the ground, unless ballast is released.

The superpressure balloon is a nonextensible balloon that is sealed to prevent the release of gas. By the time the balloon reaches its equilibrium level, the free-lift gas has become pressurized. Variations in the temperature 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-density level.

The highest unmanned research balloon flight was made from Chico, California, in October 1972, reaching an altitude of 51,850 m (170,100 ft).

Around the world each day radiosonde balloons make over 1,000 soundings of the winds, temperature, pressure, and humidity in the upper atmosphere. These flights are made almost exclusively from land areas. As a result, adequate measurements of the atmosphere are made over less than 20 per cent of the globe. To obtain coverage over ocean areas, Global Horizontal Sounding Technique (GHOST) balloons have been flown experimentally from the southern hemisphere. (See Meteorology: Upper-Air Observations;Airship).