An AIRMET, or Airmen's Meteorological Information, is a concise description of weather phenomena that are occurring or may occur along an air route that may affect aircraft safety. Compared to SIGMETs, AIRMETs cover less severe weather: moderate turbulence and icing, sustained surface winds of 30 knots or more, or widespread restricted visibility.
AIRMETs are broadcast on the ATIS at ATC facilities, and are referred to as Weather Advisories. AIRMETs are valid for six hours. NOTE: The definition has changed and no longer says "light aircraft"; AIRMETs are intended for all aircraft.
- AIRMET SIERRA (Mountain obscuration or IFR) ceilings less than 1000 feet and/or visibility less than 3 miles affecting over 50% of the area at one time; extensive mountain obscuration
- AIRMET TANGO (Turbulence) moderate turbulence, sustained surface winds of 30 knots or more
- AIRMET ZULU (Icing) moderate icing, freezing levels
For an authority to issue an AIRMET, applicable conditions must be widespread. "Widespread" means that the applicable area covers at least 3000 square miles. Because conditions across the forecast period can move across the area, it is possible that only a small portion of the area is affected at any time.
AIRMET’s are routinely issued for six hour periods beginning at 0145Z during Central Daylight Time and at 0245Z during Central Standard Time. AIRMETS are also amended as necessary due to changing weather conditions or issuance/cancellation of a SIGMET.
- http://www.faa.gov/air_traffic/publications/ATpubs/AIM/aim0701.html - FAA Aeronautical Information Manual (ch 7 "Safety of Flight" section "Meteorology")
- http://www.crh.noaa.gov/lot/aviation/airmet_info.php - NOAA (National Weather Service Weather Forecast Office)
- http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/6A07EFAA9A68C1A8862576C70052E9E5?OpenDocument - FAA Advisory Circular Website (link to AC 00-45G "Aviation Weather Services" where the definition is found)
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