Which of the following is not a type of SIGMET?

The choice labeled 'AT' is correct as it is not a type of SIGMET. SIGMETs, or Significant Meteorological Information, are primarily issued for specific types of weather that could impact aviation safety. The recognized types of SIGMETs include those related to hazardous weather such as thunderstorms, turbulence, volcanic ash, and more.

WS stands for SIGMET for severe thunderstorms, WC is for SIGMET concerning winter weather such as snow or ice, while WV denotes SIGMET for turbulence. Each of these codes is specifically designed to alert pilots and air traffic controllers about significant weather phenomena that may affect flight safety.

In contrast, 'AT' does not refer to any category of SIGMET. It does not align with the standards and classifications established by aviation meteorological authorities to ensure the safety of air travel. Understanding the specific types of SIGMETs helps pilots anticipate and react to dangerous weather conditions during flight, reinforcing the importance of this knowledge in aviation operations.