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A weather or sounding balloon is a balloon which carries instruments aloft to send back information on atmospheric pressure, temperature, and humidity by means of a small, expendable measuring device called a radiosonde. To obtain wind data, they can be tracked by radar, radio direction finding, or navigation systems (such as the satellite based Global Positioning System). The balloon itself produces the lift, and is usually made of a highly flexible latex material (though Chloroprene may also be used). The unit that performs the actual measurements and radio transmissions hangs at the lower end of the string, and is called a radiosonde. Specialized radiosondes are used for measuring particular parameters, such as determining the ozone concentration. In North America prior to release the balloon is usually filled with hydrogen (though helium can be used as a substitute) gas. The ascent rate can be controlled by the amount of gas the balloon is filled with. Weather balloons may reach altitudes of 40 km (25 miles) or more, limited by diminishing pressures causing the balloon to expand to such a degree (typically by a 100:1 factor) that it disintegrates. The instrument package is usually lost. Above that altitude sounding rockets may be used. After sounding rockets, satellites are used for even higher altitudes. Major manufacturers of balloons are Totex Corporation and Cosmopren of Japan and Kaysam (now Kaymont) of the U.S.. Weather balloons are frequently used to explain UFO sightings.
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