From AMS Glossary
An evaluation of meteorological data received from a descending dropsonde.
The dropsonde is a small expendable instrument package that is released from an aircraft. As it descends, it radio transmits pressure, temperature, and relative humidity data back to the aircraft. By tracking the position of the dropsonde using radio-navigation techniques [e.g., Global Positioning System (GPS)], wind speed and direction data can be obtained. The processed data are usually presented in terms of height, temperature, dewpoint, and winds at mandatory and significant pressure levels. A dropsonde observation is comparable to a rawinsonde observation. Data from the dropsonde are usually received and processed in the aircraft that dropped the instrument.