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.
Major manufacturers of balloons are Totex Corporation and Cosmopren in Japan, and Kaysam in the US.
See also: Rawinsonde.