Radiosondes are packages of sensors suspended from a balloon filled with helium or hydrogen. As the balloon carries the sensors upwards, the instruments measure atmospheric properties such as pressure, temperature, and relative humidity and send the observations to a ground receiver. Tracking the position of a radiosonde with GPS also provides measurements of wind speed and direction. After a certain height is reached—sometimes more than 35 km—the balloon bursts and a small parachute is deployed so the instrument package is returned to the ground safely. With strong winds, a radiosonde might drift more than 200 km from its release point.

Radiosondes provide valuable in situ monitoring of atmospheric variables that can inform climate models, weather forecasts, and atmospheric research. They also provide an important source of ground truth for satellite observations.