An AIRMET (Airmen’s Meteorological Information) is a routine weather advisory issued to pilots concerning en-route weather phenomena that could be hazardous to aircraft operations. The primary audience includes pilots of smaller, single-engine aircraft and general aviation flying under Visual Flight Rules (VFR), as these aircraft are more susceptible to moderate weather conditions. Although the conditions are not usually severe enough to warrant a more urgent advisory, they still present a significant safety concern for flight planning.
The advisory informs pilots about the potential development or existence of widespread weather hazards across a designated area. For an AIRMET to be issued, the hazardous conditions must be affecting or forecast to affect an area of at least 3,000 square miles at any one time. This focus on moderate, yet extensive, weather allows pilots to make informed decisions to modify their route or altitude for a safer flight.
The Standard Validity Period
AIRMETs have a fixed, regulatory duration designed to standardize the weather forecast cycle for aviation users. A text-based AIRMET issued for the contiguous United States is valid for a maximum period of six hours. This six-hour period represents a scheduled forecast window, after which a new product is routinely issued.
The issuance of a new AIRMET occurs on a regular, predictable schedule, typically beginning shortly after the hour to cover the subsequent six-hour block. This routine allows pilots and flight planners to anticipate when the most current weather information will become available for their route of flight.
The six-hour validity is a maximum duration, meaning the forecast covers the weather phenomena expected during that entire time frame. The conditions described within the AIRMET may not exist for the full six hours, or they may shift their location within the defined area. The forecast is a time-bound prediction of a widespread hazard, not a guarantee of its continuous presence until expiration.
Types of AIRMETs and Associated Hazards
Although the duration is uniform, the specific atmospheric hazard determines which of the three types of AIRMET is issued. Each type is identified by a phonetic letter corresponding to the weather phenomenon being forecasted. This classification system provides pilots with an immediate understanding of the nature of the potential danger.
The first type is AIRMET Sierra, issued for Instrument Flight Rules (IFR) conditions and extensive mountain obscuration. IFR conditions are met when the ceiling is less than 1,000 feet or the visibility is less than three miles, affecting at least 50% of the area. The Sierra advisory alerts pilots to conditions where navigation by visual reference may be difficult, especially near high terrain.
AIRMET Tango is the second type, focusing on atmospheric movement hazards. This advisory is issued for areas experiencing moderate turbulence or where sustained surface winds are 30 knots or greater. It may also be issued for non-convective low-level wind shear, which is a sudden change in wind velocity hazardous during takeoff or landing.
The final type is AIRMET Zulu, which provides information related to temperature and moisture. This advisory is issued when moderate airframe icing is occurring or forecasted, and it includes information on the height of the freezing levels. Knowing the altitude where temperatures may drop to freezing is important for avoiding the accumulation of ice on the aircraft structure.
Issuance Updates and Cancellation
The six-hour validity period is a scheduled maximum, but an AIRMET’s actual lifespan can be shorter or its content modified if weather conditions change. The advisory is dynamic and subject to updates or amendments throughout the forecast cycle. Forecasters continuously monitor the weather; if the hazard intensifies, diminishes, or moves significantly, the AIRMET can be revised.
An AIRMET can be explicitly cancelled before its scheduled six-hour expiration if the hazardous weather conditions cease to exist or move completely out of the forecast area. This cancellation ensures pilots are not unnecessarily avoiding an area that is now safe, formally terminating the advisory’s validity.
For a more granular view of forecast changes, Graphical AIRMETs (G-AIRMETs) are available, presenting the same hazard information visually on a map. These graphical products offer an updated forecast every three hours, providing a more frequent snapshot of the expected conditions up to 12 hours ahead. While the traditional text AIRMET maintains the six-hour forecast period, the availability of these graphical products emphasizes checking the latest information before and during a flight.