What is the most common and persistent weather hazard in aviation?

Fog is considered the most common and persistent weather hazard in aviation due to its ability to significantly reduce visibility, which is crucial for the safe operation of aircraft during takeoff, landing, and taxiing. Fog can develop in various weather conditions and can persist for extended periods, making it a frequent hazard that pilots and air traffic controllers must contend with. Its low visibility conditions often lead to delays and require diversion or holding patterns for aircraft, impacting overall air traffic management.

While thunderstorms, rain, and snow also pose significant challenges and can disrupt flight operations, fog's unique characteristics of unpredictability and sustained impact on visibility typically render it a more consistent hazard across different regions and seasons.