What is the maximum validity period for a SIGMET?

The maximum validity period for a Significant Meteorological Information (SIGMET) is indeed four hours. SIGMETs are issued to provide timely warnings of significant weather that could impact the safety of flight operations, such as severe turbulence, icing, or volcanic ash. Given their purpose, it's crucial that the information remains current and relevant; thus, they are typically valid for a maximum duration of four hours from the time of issuance. This time frame ensures pilots and flight crews receive up-to-date information, enabling them to make informed decisions regarding flight safety.

Beyond this period, weather conditions can change significantly, and older SIGMETs may not accurately reflect the current situation, which is why they are not valid for longer durations.