Which of the following does not fall under the category of significant weather for PIREPS?

Mountains or terrain presence does not constitute significant weather for PIREPs (Pilot Reports). PIREPs are primarily focused on atmospheric conditions that can adversely affect aircraft operations, such as severe icing, low visibility, and widespread thunderstorms, all of which directly relate to the safety and performance of flight. These factors can create hazardous conditions during flight, necessitating their reporting to inform other pilots and air traffic control.

Mountains or terrain presence, while it can affect flight operations, is generally considered geographical data rather than a weather phenomenon. It does not pertain to immediate atmospheric conditions impacting visibility, precipitation, or turbulence. Therefore, it falls outside the definition of significant weather for reporting purposes in the context of PIREPs.