When is visibility considered to be prevailing?

Visibility is considered to be prevailing when it is observed in at least 180 degrees of the horizon. This means that the visibility should be consistent across a significant portion of the observer's field of view, rather than limited to a narrow area. In practical terms, if pilots or meteorologists are assessing visibility conditions, they need to gauge whether visibility conditions are reasonable for operations, and observing it over a broad span ensures that the majority of the area is adequately observed.

In contrast, simply exceeding reportable values does not address the directionality of visibility, which is crucial for determining prevailing conditions. Likewise, describing visibility as stable and clear does not specify the extent of the visibility in terms of directional coverage. Finally, visibility being lower than two miles would typically be regarded as a limitation or restriction rather than a prevailing condition, especially since it might not reflect the overall visibility across a large area. Hence, the specification of a minimum of 180 degrees is critical for defining prevailing visibility.