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prevailing visibility[pri′vāl·iŋ ‚viz·ə′bil·əd·ē]
In United States weather observing practice, the greatest horizontal visibility equaled or surpassed throughout half of the horizon circle; in the case of rapidly varying conditions, it is the average of the prevailing visibility while the observation is being taken.
The horizontal distance at which targets of known distance are visible over half the horizon. It is determined by viewing dark objects against the horizon sky by day and moderate-intensity unfocused lights by night. The half of the horizon need not be a continuous half.