Weather advisories alert pilots to hazardous weather, such as high winds, turbulence, icing, instrument weather conditions, and thunderstorm activity. Airmets, sigmets, and convective sigmets are all weather advisories.
Each type of advisory has criteria for its issuance, which determines which weather advisories are issued.
Airmet
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:
There are three types of airmets, Sierra, Tango, and Zulu.
Sigmet
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:
Convective Sigmet
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:
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.