A G-AIRMET is a graphical advisory of weather that may be hazardous to aircraft, but are less severe than SIGMETs. They are only valid at specific time "snapshots". Forecasters create graphical objects depicting the areas and attributes of AIRMET hazards, which are distributed in BUFR file format. G-AIRMETs are issued at discrete times 3 hours apart for a period of up to 12 hours into the future (00, 03, 06, 09, and 12 hours). They are issued at 03:00, 09:00, 15:00 and 21:00 UTC (with updates issued as necessary). AIRMET are issued by the AWC for the lower 48 states and adjacent coastal waters.
The aviation hazards depicted in the G-AIRMET are:
Areas of moderate turbulence, other than convectively induced, including the vertical extent.
LLWS is defined as wind shear below 2000 feet AGL, other than convectively induced, resulting in an air speed loss or gain of 20 knots or more. LLWS potential information is included after AIRMETs for moderate turbulence and/or sustained surface winds greater than 30 knots or statements indicating no significant turbulence is expected.
Areas of sustained surface winds greater than 30 knots. The direction and speed of the wind are not depicted; only the area where sustained surface winds greater than 30 knots will occur.
Areas of moderate airframe icing, other than convectively induced, including the areal extent.
Freezing level is defined as the lowest freezing level above the ground or at the SFC as appropriate. Freezing level information is included after AIRMETs for moderate icing or statements indicating that no significant icing is expected. Freezing levels are delineated using high altitude VOR locations describing the location of the lowest freezing level above the ground or SFC as appropriate. Freezing levels above the ground are delineated at 4000 feet intervals AMSL. Multiple freezing levels above the ground are delineated by high altitude and low altitude VOR locations. The range of freezing levels across the forecast area is also included.
Areas of cloud ceilings with bases less than 1000 feet AGL and/or areas of surface visibilities below 3 statute miles, including the weather causing the visibility restriction. The cause of the visibility restriction includes only widespread sand/dust storm, PCPN, FU HZ,BR,FG, and/or BLSN.
Areas of widespread mountain obscuration where VMC cannot be maintained, including the weather causing the obscuration. The weather causing the obscuration includes only CLDS, PCPN, FU, HZ, BR, and/or FG.