Prevailing visibility is reported as variable when it rapidly increases and decreases by 1/2 or more miles and is less than:

Prevailing visibility is defined as the greatest distance in which objects can be seen and identified. When this visibility fluctuates significantly, particularly when it increases and decreases by half a mile or more, it indicates that conditions are changing rapidly. The criteria for reporting this phenomenon as "variable" focus on a threshold visibility of less than 3 miles. This standard is used to reflect that the variations could be impactful on operational decisions, especially in aviation operations, where such dynamics affect flight safety.

When visibility is under this threshold and shows considerable variation, it needs to be noted as variable to alert pilots and air traffic controllers about the potential challenges in navigation and safety. Thus, the determination of less than 3 miles as the reporting standard for variable visibility is crucial for effective weather observation and communication in aviation.