Aircraft Meteorological Data Relay
Aircraft Meteorological Data Relay (AMDAR) is a program initiated by the World Meteorological Organization. 
AMDAR is used to collect meteorological data worldwide by using commercial aircraft.

Data is collected by the aircraft navigation systems and the onboard standard temperature and static pressure probes.
The data is then preprocessed before linking them down to the ground either via VHF communication (ACARS)
or via satellite link ASDAR.

A detailed description is given in the AMDAR Reference Manual (WMO-No 958) available from the World Meteorological Organization, 
Geneva, Switzerland

Usage 
AMDAR transmissions are most commonly used in forecast models as a supplement to radiosonde data, to aid in the plotting of upper-air data between the standard radiosonde soundings at 00Z and 12Z.

References

External links 
 (Information no longer available)
WMO AMDAR Observing System site
(Dead link) NOAA AMDAR site

Meteorological data and networks
Meteorological instrumentation and equipment