A SIGMET (SIGnificant METeorlogical Information) advises of weather
potentially hazadous to all aircraft other than convective activity. In the conterminous
U.S., items covered are:
Severe or extreme turbulence
Duststorms lowering visibilities to less than three (3) sm
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.
For the lower 48 states and adjacent coastal waters,
Convective SIGMETs are issued hourly for
Thunderstorm-related aviation hazards.