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A weather balloon in preparation for launch Picture taken at approximately 30 km above Oregon using a 1,500 gram weather balloon 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 and wind direction. This information is transmitted back to surface observers.
A weather balloon, also known as a sounding balloon, is a high-altitude balloon (HAB) that carries instruments into the stratosphere for measuring atmospheric pressure, temperature, humidity and wind speed by means of a small, expendable measuring device called a radiosonde. First invented in the late 19th century, weather balloons have overgone several remodels and are still used by researchers across Europe, North America, and Asia to this day. The balloon itself is made of latex and filled with either helium or hydrogen to produce lift. Balloons can be tracked by radar, RDF, or GPS. Balloons meant to stay at a constant altitude for long periods are known as transosondes. Weather balloons that do not carry an instrument pack are used to determine upper-level winds and the height of cloud layers. For such balloons, a theodolite or total station is used to track the balloon's azimuth and elevation, which are then converted to estimated wind speed and direction and/or cloud height, as applicable.
Excerpted from Wikipedia’s “weather balloon” article, available under the CC BY-SA 4.0 licence.