A balloon-borne instrument that measures meteorological parameters from the Earth's surface up to 20 miles in the atmosphere. The radiosonde measures temperature, pressure, and humidity, and transmits or "radios" these data back to Earth. Upper air winds also are determined through tracking of the balloon ascent.
http://www.casde.unl.edu/glossary/r.php

https://www.esrl.noaa.gov/gmd/obop/mlo/programs/esrl/ozonesondes/img/img_launching_sonde_hilo_1.JPG