Airborne expendable bathythermograph

From Glossary of Meteorology

airborne expendable bathythermograph

(Abbreviated AXBT.) An expendable instrument that is dropped from an aircraft and used to measure the profile of temperature in the water column.

The probe consists of a thermistor in a weighted, streamlined case. It falls freely at a fixed, known rate so that the elapsed time can be converted to depth. It is connected by a thin, freely unwinding wire to a small buoy with a radio transmitter through which the data are transmitted to the aircraft, which continues its flight.

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