Prevailing visibility in aviation is a measurement of the greatest distance visible throughout at least half of the horizon, not necessarily continuously. To take the prevailing visibility, controllers reference a number of visual reference points: usually buildings, hills, or other geographic features. The value obtained is usually reported on a METAR and recorded on the ATIS.
When the prevailing visibility is less than 4 miles,or goes from less than 4 miles to four miles or greater, tower personnel are required to take a prevailing visibility reading. If this reading differs from the ground reading (i.e., different readings for different heights), the lower value of the two is used for aircraft operations. 
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