ADDS - Aviation Digital Data Service

ADDS - AIRMETs Help Page (3 of 4)
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A SIGMET (SIGnificant METeorological Information) advises of weather potentially hazardous to all aircraft other than convective activity.
   SIGMETs are issued (for the lower 48 states and adjacent coastal waters) for the following weather-impacted reasons:
  • Severe Icing
  • Severe or Extreme Turbulence
  • Duststorms and sandstorms lowering visibilities to less than three (3) miles
  • Volcanic Ash
   These SIGMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time. However, if the total area to be affected during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time.    SIGMETs are issued for 6 hour periods for conditions associated with hurricanes and 4 hours for all other events. If conditions persist beyond the forecast period, the SIGMET is updated and reissued. Convective SIGMETS are issued hourly for thunderstorm-related aviation hazards.