Weather advisories alert pilots to hazardous weather, such as high winds, turbulence, icing, instrument weather conditions, and thunderstorm activity. Airmets,
convective sigmets are all weather advisories.
Each type of advisory has criteria for its issuance, which determines which weather advisories are issued.
An airmet, which is of primary interest to pilots of smaller aircraft, is issued every six hours, or more often when necessary, when any of the following
conditions exist or are anticipated over a widespread area:
- sustained surface winds greater than 30 knots (STG SFC WND)
- ceilings below 1,000 feet (IFR, CIG BLW 010)
- visibilities less than three statues miles (IFR, VIS BLW 3 SM)
- widespread mountain obscuration (MTN OBSCN)
- moderate turbulence (MOD TURB)
- moderate icing (MOD ICE)
- potential for low level wind shear not associated with convective weather exists (LLWS POTENTIAL)
There are three types of airmets, Sierra, Tango, and Zulu.
- Airmet Sierra is issued for instrument weather conditions and mountain obscuration
- Airmet Tango is related to turbulence
- And Airmet Zulu is issued for icing
A sigmet is issued when hazardous weather conditions exist of interest to all aircraft, which are not associated with convective weather activity, when one of the
following conditions is met:
- severe or extreme turbulence (SEV TURB)
- severe icing (SEV ICE)
- widespread dust storm (WDSPR DS)
- widespread sand storm (WDSPR SS)
- volcanic ash (VA)
- tropical cyclone (TC)
A convective sigmet contains information about hazardous weather associated with convective activity, which is significant to all aircraft. It is issued hourly or as needed,
when any of the following weather hazards exist:
- severe thunderstorms with surface winds greater than 50 knots
- hail at the surface greater than or equal to 3/4 inches in diameter
- thunderstorms which are embedded inside layers of concealing clouds
- lines of thunderstorms
- widespread thunderstorms with heavy or greater precipitation, covering more than 40% of the area
Convective sigmets are not issued for Hawaii or Alaska.
Center Weather Advisories
If weather conditions are approaching the criteria for an airmet, sigmet, or convective sigmet to be issued, a weather warning called a center weather advisory will be
issued, to allow pilots to better anticipate and avoid these adverse weather conditions.