AIM 7-1-15 Reporting Prevailing Visibility

a. Surface (horizontal) visibility is reported in METAR reports in terms of statute miles and increments thereof; e.g., 1 / 16, 1 / 8, 3/ 16, 1 / 4, 5/ 16, 3/ 8, 1/ 2, 5/ 8, 3 / 4, 7 / 8, 1, 1 1 / 8, etc. (Visibility reported by an unaugmented automated site is reported differently than in a manual report, i.e., ASOS/AWOS: 0, 1 / 16, 1/ 8, 1 / 4, 1/ 2, 3 / 4, 1, 1 1 / 4, 1 1/ 2, 1 3/ 4, 2, 2 1/ 2, 3, 4, 5, etc., AWOS: M 1 / 4, 1 / 4, 1/ 2, 3 / 4, 1, 1 1 / 4, 1 1/ 2, 1 3/ 4, 2, 2 1/ 2, 3, 4, 5, etc.) Visibility is determined through the ability to see and identify preselected and prominent objects at a known distance from the usual point of observation. Visibilities which are determined to be less than 7 miles, identify the obscuring atmospheric condition; e.g., fog, haze, smoke, etc., or combinations thereof.

b. Prevailing visibility is the greatest visibility equaled or exceeded throughout at least one half of the horizon circle, not necessarily contiguous. Segments of the horizon circle which may have a significantly different visibility may be reported in the remarks section of the weather report; i.e., the southeastern quadrant of the horizon circle may be determined to be 2 miles in mist while the remaining quadrants are determined to be 3 miles in mist.

c. When the prevailing visibility at the usual point of observation, or at the tower level, is less than 4 miles, certificated tower personnel will take visibility observations in addition to those taken at the usual point of observation. The lower of these two values will be used as the prevailing visibility for aircraft operations.