Which of the following is NOT a condition reported by SIGMET?

SIGMETs, or Significant Meteorological Information, are designed to provide information about severe weather that can affect the safety of flight. They specifically report conditions that could pose serious hazards to aircraft in flight. The key conditions reported by SIGMETs typically include phenomena such as convective thunderstorms, severe turbulence, icing, and non-convective weather phenomena that may influence aircraft operations.

The reporting of low visibility due to fog is generally not included in SIGMETs. While fog can severely affect visibility and pose hazards for pilots, it is considered a more routine weather condition and is usually covered under other aviation weather advisories, such as TAFs (Terminal Aerodrome Forecasts) or METARs (Meteorological Aerodrome Reports).

In contrast, conditions like convective thunderstorms and severe non-convective weather phenomena (which include conditions such as severe turbulence) are critical information for pilots and are included in SIGMET reports. Hail is also a significant hazard associated with thunderstorms and would typically be reported in SIGMETs due to its potential to cause damage to aircraft.

Therefore, low visibility due to fog does not meet the threshold of severity required for a SIGMET, making it the exception among the conditions listed in the question.