Weather balloon

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Close up of a hydrogen filled balloon at Cambridge Bay Upper Air station, Nunavut, Canada
Rawinsonde weather balloon just after it was sent. 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 sent back to surface observers.

A weather or sounding balloon is a balloon (specifically a type of high altitude balloon) which carries instruments in the sky to send back information on atmospheric pressure, temperature, and humidity by means of a small measuring tool called a radiosonde. To get wind data, they can be tracked by radar, radio direction finding, or navigation systems (such as the satellite based Global Positioning System).