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SYNOP (surface synoptic observations) is a numerical code (called FM-12 by WMO) used for reporting weather observations made by manned and automated weather stations. SYNOP reports are typically sent every six hours by Deutscher Wetterdienst on shortwave and low frequency using RTTY. A report consists of groups of numbers (and slashes where data is not available) describing general weather information, such as the temperature, barometric pressure and visibility at a weather station. It can be decoded by open-source software such as seaTTY, metaf2xml or Fldigi.
This is the general structure of a SYNOP message. Numbers shown here are fixed (group indicators), numbers replacing the x's contain the weather data plus information about the station's position (and speed and direction where applicable).
IIiii or IIIII YYGGi 99LLL QLLLL iihVV Nddff 00fff 1sTTT 2sTTT 3PPPP 4PPPP 5appp 6RRRt 7wwWW 8NCCC 9GGgg 222Dv 0sTTT 1PPHH 2PPHH 3dddd 4PPHH 5PPHH 6IEER 70HHH 8aTTT 333 0.... 1sTTT 2sTTT 3Ejjj 4Esss 5jjjj jjjjj 6RRRt 7RRRR 8Nchh 9SSss
Although this coded data is still available from three American universities it has now been replaced by a universal digital coding system so data can be shared in the same format whatever the source of the observations. This enables Synop, Metar upperair and satellite data to be processed by a common computer system.
The short wave radio transmission of Synop data was common in the 1980s from Bracknell, Paris and Germany, but the introduction of a faster internet had apparently made this redundant.