Training and Safety Tip: Maintain weather awareness
Airmets, sigmets deserve attention before, during flight
Most pilots can tell at least one story about a flight that did not go as planned because of unpredicted weather along the route. We all know that weather forecasts aren’t perfect, but that can lead to concerns regarding the safety of the flight.
Having access to updated weather information aloft raises your awareness of potentially hazardous weather en route enabling you to make a better-informed decision about whether to continue the flight, divert, or land. Two products in particular offer warnings about hazardous weather conditions: airmen's meteorological information (airmets) and significant meteorological information (sigmets).
There are three types:
Airmet Sierra, for instrument flight rules conditions, is issued when ceilings are less than 1,000 feet and/or visibility is less than 3 miles for at least half of the area contained in the airmet. It also indicates when there is potential for mountain obscuration.
Airmet Tango is issued to warn of moderate turbulence or when the winds at the surface are at least 30 mph (sustained, not gusts).
Airmet Zulu is for moderate icing as well as freezing levels.
Graphical airmets, or G-airmets, show areas Sierra, Tango, and Zulu hazards. With their visual depiction, including color symbol keys, they are a great flight planning resource.
The conditions identified in airmets are generally most relevant to light aircraft. Sigmets, on the other hand, are issued when there is a possibility of significant weather that could be a hazard to all aircraft, including large commercial jets. All pilots should take note.
Sigmets are not scheduled; rather, they’re issued as needed and remain valid for four to six hours, depending on the type of weather being forecast. The geographical region is smaller and more localized than that of airmets.
Sigmets warn pilots about severe icing, severe or extreme turbulence, clear air turbulence, reduction of in-flight visibility to less than 3 miles, and volcanic ash. While sigmets deal with nonconvective weather, there are also convective sigmets that are issued specifically for hazardous weather related to thunderstorms. This includes severe thunderstorms, hail, and embedded thunderstorms.
General aviation pilots flying light aircraft should take all weather warnings, especially sigmets, very seriously. Flying into conditions that prompted a sigmet will likely be catastrophic. Air traffic control will broadcast hazardous weather alerts to all pilots on their frequency, and pilots can access additional information through flight service stations.
More information about airmets and sigmets can be found in Chapter 13 of the Pilot’s Handbook of Aeronautical Knowledge and Chapter 7 of the Aeronautical Information Manual. Also, the National Oceanic and Atmospheric Administration's aviation weather website has great resources including a detailed explanation of airmets.
