From The RadioReference Wiki
VOLMET, or meteorological information for aircraft in flight, is the term applied to a worldwide network of radio stations that broadcast TAF, SIGMET and METAR reports on shortwave frequencies. Reports are sent using automated voice transmissions, in the upper sideband or J3E mode. As the name suggests, pilots on international air routes use these transmissions to determine what procedures to use for descent, approach, and landing, such as a visual approach or an instrument approach and the correct STAR. The actual term comes from the contraction of 2 French words, loosely translated as 'Flying Weather'.
The VOLMET network divides the world into specific regions, and individual VOLMET stations in each region broadcast weather reports for specific groups of air terminals in their region at specific times, coordinating their transmission schedules so as not to interfere with one another. Schedules are determined in intervals of five minutes, with one VOLMET station in each region broadcasting reports for a fixed list of cities in each interval. These schedules repeat every hour.
VOLMET Broadcasts are great for propagation measuring, since they have set schedules and frequencies all over the world and can give an clear indication of when propagation is strong or weak in areas. See the VOLMET station list by William Hepburn for a comprehensive list of these stations.
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