From AMS Glossary
An expendable meteorological instrument package, often borne aloft by a free-flight balloon, that measures, from the surface to the stratosphere, the vertical profiles of atmospheric variables and transmits the data via radio to a ground receiving system.
Radiosondes typically measure temperature, humidity, and, in many cases, pressure. Radiosonde temperature sensors generally measure temperature-induced changes in the electrical resistance, capacitance, or voltage of a material. Radiosonde humidity sensors can be substances that respond in a known way to changes in ambient humidity or instruments that directly measure a characteristic of the air that is dependent on its water vapor content. Radiosonde pressure sensors are typically aneroid cells, a part of which flexes in proportion to pressure changes. Some radiosondes do not measure pressure, but pressure data are calculated from the hypsometric equation using temperature, humidity, and height data. Some radiosondes also measure wind speed and direction.