balloon-borne instrument for making atmospheric measurements, such as temperature, pressure, and humidity, and radioing the information back to a ground station. Special helium-filled meteorological balloons made of high-quality neoprene rubber are employed for elevating the radiosonde to very high altitudes of around 30,000 m (100,000 feet); maximum altitude for balloon-borne radiosondes is about 50,000 m.

Two basic systems are used for collecting data and coding it for transmittal. In the mechanical type, the thermometer is usually a bimetallic strip or a stretched wire, the humidity element a hair or goldbeater's skin hygrometer, and the pressure system an aneroid type of capsule. In the electrical change type system, a mechanical bellows, responsive to changes in atmospheric pressure, operates a switching arrangement to alternately transmit measurements of temperature and humidity; the temperature is indicated by a temperature-sensitive resistor and the humidity by a humidity-sensitive arrangement consisting of a strip of polystyrene coated with lithium chloride. Wind velocity can be determined by tracking the radiosonde with a theodolite or with an automatic tracking antenna.