This chapter presents procedures and practices for measuring and recording visibility. All visibilities referred to in this chapter are horizontal visibilities. An automated instrumentally-derived visibility value is a sensor value converted to an appropriate visibility value using standard algorithms and is considered to be representative of the visibility in the vicinity of the airport runway complex. A manually-observed visibility value is obtained using the "prevailing visibility" concept.

a.  Visibility. Visibility is a measure of the horizontal opacity of the atmosphere at the point of observation and is expressed in terms of the horizontal distance at which a person should be able to see and identify specific objects.

b.  Prevailing Visibility. Prevailing visibility is the greatest visibility equaled or exceeded throughout at least half the horizon circle, which does not necessarily have to be continuous. This is the visibility that is considered representative of visibility conditions at the station.

c.  Surface Visibility. The prevailing visibility determined from the usual point of observation is the surface visibility.

d.  Tower Visibility. Tower visibility is the prevailing visibility determined from the airport traffic control tower at locations that also report the surface visibility.

e.  Sector Visibility. Sector visibility is the visibility in a specified direction that represents at least a 45 degree arc (portion) of the horizon circle.

f.  Variable Prevailing Visibility. Variable prevailing visibility is a condition where the prevailing visibility is less than 3 miles and rapidly increases and decreases by 1/2 statute mile or more during the period of observation.

g.  Visibility Markers. Visibility markers are dark or nearly dark objects viewed against the horizon sky during the day, or unfocused lights of moderate intensity (about 25 candela) during the night.

Visibility may be determined at either the surface, the tower level, or both. If visibility observations are made from just one level (e.g., the air traffic control tower), that level shall be considered the "usual point of observation" and that visibility shall be reported as the surface/prevailing visibility. If visibility observations are made from both levels, the lower value (if less than 4 miles) shall be reported as prevailing visibility in the body of the METAR, and the other value shall be a remark.

Visibility shall be reported in statute miles or fractions thereof. See Figure 9-1, Reportable Visibility Values.

Charts, lists, or other positive means of identifying lights or objects used as visibility markers shall be posted near the observer's position. At local direction, separate lists or charts can be used for daytime and nighttime markers. In any case, the markers must be clearly identified as to whether they are daytime or nighttime markers.

If tower visibility is reported, separate charts or lists of markers using the tower as an observation site shall be posted in the tower.

Insofar as possible, markers of the type described in paragraph 9-2g should be used for determining visibility markers to construct visibility aids. The red or green course lights, television and radio tower obstruction lights etc., may be used as nighttime visibility markers. Because of their intensity, focused lights such as airport beacons shall not be used as markers.

9-8.   OBSERVATION SITES

Visibility observations shall be taken from several viewpoints at one location as necessary to view as much of the horizon as practical. In this respect, natural obstructions, such as trees, hills, etc., are not obstructions to the horizon. These natural obstructions define the horizon.

Before taking visibility observations at night, the observer should spend as much time as practical in the darkness to allow the eyes to become accustomed to the limited light.

Visibility shall be evaluated as frequently as practical. Using all available visibility markers, the observer shall determine the greatest distances that can be seen in all directions around the horizon circle. When the visibility is greater than the distance to the farthest markers, the observer shall estimate the greatest distance that can be seen in each direction. This estimate shall be based on the appearance of all visibility markers. If they are visible with sharp outlines and little blurring of color, the visibility is much greater than the distance to them. If a marker can barely be seen and identified, the visibility is about the same as the distance to the marker.

9-11.   EVALUATING PREVAILING VISIBILITY

After visibilities have been determined around the entire horizon circle (paragraph 9-10), the observer shall resolve them into a single value for reporting purposes. To do this, the observer shall use either the greatest distance that can be seen throughout at least half the horizon circle, or if the visibility is varying rapidly during the time of observation, use the average of all observed values. The prevailing visibility shall be reported in all observations.

When the visibility is not uniform in all directions, the horizon circle shall be divided into arcs (sectors) that have uniform visibility and represent at least one eighth of the horizon circle (45 degrees). The visibility that is evaluated in each sector is sector visibility. Sector visibility shall be reported in the remarks section of weather observations when it differs from the prevailing visibility by one or more reportable values and either the prevailing or sector visibility is less than 3 miles or considered to be operationally significant. The format for the remark is given in paragraph 15-27, Sector Visibility.

If the prevailing visibility rapidly increases and decreases by 1/2 mile or more during the time of the observation, and the average prevailing visibility is less than 3 miles, the visibility is considered to be variable. When variable visibility conditions are observed, the minimum and maximum visibility values observed shall be reported in the remarks section. Variable visibility shall not be reported in the body of the report. The format for the remark is given in paragraph 15-26, Variable Prevailing Visibility.

The reportable values for manual visibility observations are listed in Figure 9-1. If the visibility falls halfway between two reportable values, the lower value shall be reported.

Control tower personnel certified to take visibility observations shall:

a.  Notify the weather station or contract observer when they observe tower prevailing visibility to decrease to less than, or increase to equal or exceed, 4 miles.

b.  When the prevailing visibility at the tower or the surface is less than 4 miles, report all changes of one or more reportable values to the weather station or contract observer.

c.  As required by FAA directives, use the lower of either the tower or the weather station/contract observer visibility for aircraft operations.

Tower personnel shall record on graphic transcription equipment, MF1M-10C, or a separate tabulation sheet, the following information for each Control Tower visibility observation:

a.  Time of observation.

b.  Prevailing visibility at the tower level.

c.  Remarks (such as visibility in different sectors).

d.  Observer's initials.

(Note: This paragraph is not applicable at OID/OT-equipped sites.)

Procedures for weather station/contract observer personnel are as follows:

a.  Notify the tower as soon as possible, whenever the prevailing visibility at the weather station/contract observation point decreases to less than, or increases to equal or exceed, 4 miles.

b.  Re-evaluate weather station/contract observer prevailing visibility, as soon as practical, upon initial receipt of a differing control tower value, and upon receipt of subsequent reportable changes at the control tower level.

Figure 9-1. Reportable Visibility Values

9-18. - 9-19. RESERVED