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 icing • Severe or extreme turbulence
• Duststorms lowering visibilities to less than three (3) sm
• Volcanic Ash

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.