To report sector visibility, what must be true about its measurement compared to prevailing visibility?

Sector visibility is a specific measurement of visibility in a particular direction or sector within an observing area. It is reported when it provides important information for aviation operations, particularly when there are variations in visibility across different sectors.

For sector visibility to be accurately reported, it must differ from the prevailing visibility by one or more reportable values. Prevailing visibility refers to the greatest distance that can be seen throughout at least half of the horizon circle, and it serves as a baseline measurement for overall visibility conditions. When sector visibility is being reported, it signifies that there is a notable difference in visibility in a specific direction compared to what is observed in the prevailing conditions. This could be due to localized weather phenomena like fog, rain, or dust storms affecting visibility in one sector but not in others.

Reporting sector visibility that is the same or greater than the prevailing visibility does not provide useful information to pilots and air traffic controllers about potential hazards or variations, which is why it must show a clear difference to be relevant. Therefore, the correct choice emphasizes the necessity of having a distinguishable value to ensure that critical visibility information is conveyed accurately for flight safety and operations.