From AMS Glossary
A monthly broadcast of the mean values of the meteorological elements during the preceding month for stations of the national weather services belonging to the World Meteorological Organization (WMO).
The messages are disseminated worldwide through the Global Telecommunications System (GTS) as soon as possible after the close of a month, but not later than the fifth of the following month. This program was recommended to the predecessor of the WMO (the International Meteorological Organization) and widely implemented during 1936–39. The program, the elements broadcast, and the codes utilized have been revised several times since the program's implementation. The elements broadcast by a principal climatological station, according to the 1995 edition of the Manual on Codes (WMO Publication No. 306), include atmospheric pressure, temperature (means and extremes of daily mean, maximum, and minimum), wind speed, vapor pressure, precipitation [monthly total, frequency group (quintile) within which it falls, and extreme daily amount], sunshine duration, standard deviation of temperature, number of days with thunderstorms and hail, and number of days with precipitation, temperatures, snow depth, visibility, and wind speed beyond various thresholds. Provision is also made for the issuance of comparable normals, as far as possible, for the most recent climatological standard normal period (i.e., 1901– 30, 1931–60, 1961–90, etc.).
Secretariat of the World Meteorological Organization 1995. Manual on Codes, WMO Publication No. 06, Annex to Technical Regulations. Geneva, Switzerland, looseleaf.