VOLMET, or meteorological information for aircraft in flight, is the term applied to a worldwide network

Radio network

There are two types of radio networks currently in use around the world: the one-to-many broadcast type commonly used for public information and mass media entertainment; and the two-way type used more commonly for public safety and public services such as police, fire, taxicabs, and delivery...

 of radio stations that broadcast TAF

Terminal Aerodrome Forecast

In meteorology and aviation, TAF is a format for reporting weather forecast information, particularly as it relates to aviation. "TAF" is an acronym of Terminal Aerodrome Forecast or, in some countries, Terminal Area Forecast....

, SIGMET

SIGMET

SIGMET, or Significant Meteorological Information, is a weather advisory that contains meteorological information concerning the safety of all aircraft. There are two types of SIGMETs, convective and non-convective...

 and METAR

METAR

METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....

 reports on shortwave

Shortwave

Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...

 frequencies. In some countries, VOLMET stations broadcast on VHF frequencies too. Reports are sent using automated voice transmissions, in the upper sideband

Single-sideband modulation

Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....

 or J3E

Types of radio emissions

The International Telecommunication Union uses an internationally agreed system for classifying radio frequency signals. Each type of radio emission is classified according to its bandwidth, method of modulation, nature of the modulating signal, and type of information transmitted on the carrier...

 mode. As the name suggests, pilots on international air routes, such as North Atlantic Tracks

North Atlantic Tracks

North Atlantic Tracks are trans-Atlantic routes that stretch from the northeast of North America to western Europe across the Atlantic Ocean. They ensure aircraft are separated over the ocean, where there is little radar coverage...

, use these transmissions to determine what maneuvers to make to avoid severe storms or turbulence, as well as which procedures to use for descent, approach, and landing, such as a visual approach versus an instrument approach and the correct arrival procedure

Standard Terminal Arrival Route

In aviation, a standard terminal arrival route or standard terminal arrival is a published procedure followed by aircraft on an IFR flight plan just before reaching a destination airport.-Description:...

.

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.