For a PIREP, which weather conditions are significant for a pilot to report?

The correct answer highlights crucial weather conditions that can markedly affect flight safety and operational conditions. Low visibility is significant because it can impair a pilot's ability to navigate and see other aircraft, which could lead to dangerous situations. Turbulence is also critical to report, as it can lead to uncomfortable or hazardous flying conditions, especially during landing and takeoff. Icing can pose a serious risk as it adversely affects aircraft performance, particularly during ascent or descent, and can lead to loss of control if not managed properly.

In contrast, the other options contain conditions that are less critical for immediate reporting due to their lesser impact on flight safety. For example, while fog and thunderstorms are indeed hazardous, excessive winds can vary in significance depending on the aircraft and current procedures. Clear skies, dry conditions, and high visibility generally indicate safe flying conditions, which do not necessarily need to be reported. Similarly, while cloud cover could be important in some contexts, warm temperatures and calm winds are typically not significant concerns in a pilot report. The focus should be on conditions that pose immediate threats or challenges to flight safety.