 Rawinsonde weather balloon just after launch. Notice a parachute in the center of the string and a small instrument box at the end. After release it measures many parameters. These include temperature, relative humidity, pressure, and wind speed. This information is transmitted back to surface observers. A weather balloon is an instrument that is carried aloft by a balloon to send back information on atmospheric temperature, atmospheric pressure and humidity by means of a small, expendable radio transmitter. 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 instrument consists of two basic units, connected by a thin string.
The balloon itself produces the lift, and is usually made of a highly flexible latex material.
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 layer.
Before launch, the balloon is filled with helium or hydrogen gas. Weather balloons may reach altitudes of 35 km or more, limited by diminishing pressures causing the balloon to expand to such a degree (typically by a 100:1 factor) that it disintegrates. Above that altitude sounding rockets may be used.
Major manufacturers of balloons are Totex Corporation and Cosmopren in Japan, and Kaysam in the US.
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