How frequently are SIGMETs typically issued?

SIGMETs, or Significant Meteorological Information, are issued to provide critical information about severe weather that could impact the safety of flight operations. They are not bound by a fixed schedule like some other meteorological reports because their issuance is contingent upon the occurrence of significant weather phenomena. This could include severe thunderstorms, turbulence, volcanic ash, or other hazardous conditions that require immediate attention from pilots and dispatchers.

Consequently, SIGMETs are generated "as needed," meaning whenever a severe weather situation develops or is identified that may affect aviation safety. This flexibility allows pilots and dispatchers to receive timely alerts that are pertinent to current conditions rather than waiting for a regular reporting interval. This is essential for maintaining safety in aviation operations, as conditions can change rapidly.

The other options imply a routine schedule for SIGMET issuance, which does not reflect the nature of these alerts. Their primary function is to react to real-time weather situations rather than adhere to a predetermined issuance timeline.