Wikipedia
VOLMET (French origin VOL (flight) and METEO (weather)), or meteorological information for aircraft in flight, is a worldwide network of radio stations that broadcast TAF, SIGMET and METAR reports on shortwave frequencies, and in some countries on VHF too. Reports are sent in upper sideband mode, using automated voice transmissions.
Pilots on international routes, such as North Atlantic Tracks, use these transmissions to avoid storms and turbulence, and to determine which procedures to use for descent, approach, and landing.
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.