Visibility Group


Standards and Criteria for Observing and Reporting Visibility
Coding and Interpreting Coded Visibility Groups


A measure of the opacity of the atmosphere. 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-derived visibility value is obtained using the "prevailing visibility" concept. In this section, the term "prevailing visibility" shall refer to both manual and instrument derived values.

Units of Measure
Visibility shall be reported in statute miles at US observing sites. Note: The WMO standard is to report visibility in meters, where 2800 represents 2,800 meters and 9999 represents a visibility greater than 9500 meters (9.5 km).

Prevailing visibility
The greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous; the visibility that is continuous; the visibility that is considered representative of visibility at the station.

Sector visibility
The visibility in a specified direction that represents at least a 45 degree arc of the horizon circle.
When the manually-derived visibility is not uniform in all directions, the horizon circle shall be divided into arcs 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 when it differs from the prevailing visibility by one or more reportable values and either the prevailing or sector visibility is less than 3 statute miles.

Surface visibility
The prevailing visibility determined from the usual point of observation.

Tower visibility
The prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.
Tower visibility shall be reported in accordance with agency procedures. See remarks section.

Visibility Observing Standards. Visibility may be manually determined at either the surface, the tower level, or both. If visibility observations are made from just one level, e.g., the airport traffic control tower, that level shall be considered the "usual point of observation" and that visibility shall be reported as surface visibility. If visibility observations are made from both levels, the visibility at the tower level may be reported as tower visibility.

Visibility may be automatically determined by sensors operating in accordance with the Federal Standard Algorithms for Automated Weather Observing Systems Used for Aviation Purposes. This visibility algorithm calculates a mean visibility which is the sensor equivalent of prevailing visibility. The visibility data during the period of observation are examined to determine if variable visibility shall be reported.

Manual Observing Aids. Agencies shall establish procedures to ensure that insofar as possible, dark or nearly dark objects viewed against the horizon sky shall be used during the day, and unfocused lights of moderate intensity (about 25 candela) shall be used during the night as reference points for manually determining visibility. In addition, visibility sensors may be used to assist the observer in the evaluation.

Method of determining visibility. Manually-derived visibility shall be evaluated as frequently as practicable. All available visibility reference points shall be used. The greatest distances that can be seen in all directions around the horizon circle shall be determined. When the visibility is greater than the distance to the farthest reference point, the greatest distance seen in each direction shall be estimated. The estimate shall be based on the appearance of the most distant visible reference points. If they are visible with sharp outlines and little blurring of color, the visibility is much greater than the distance to them. If they can barely be seen and identified, the visibility is about the same as the distance to them. After visibilities have been determined around the entire horizon circle, they shall be resolved into a single value for reporting purposes. To do this, the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous shall be used; this is prevailing visibility. If the visibility is varying rapidly during the time of the observation, the average of all observed values across the horizon circle shall be used for reporting purposes. Prevailing visibility shall be reported in all weather observations. The reportable values for visibility are listed in Table A-3. If the actual visibility falls halfway between two reportable values, the lower value shall be reported.

Variable Prevailing Visibility. If the prevailing visibility rapidly increases and decreases by 1/2 statute mile or more, during the time of the observation, and the prevailing visibility is less than 3 miles, the visibility is considered to be variable. The minimum and maximum visibility values observed shall be reported in the remarks section.

Visibility at Second Location. When an automated station uses a meteorological discontinuity visibility sensor, remarks shall be added to identify visibility at the second location which differ from the visibility in the body of the report. See remarks section.

Table A-3 Reportable Visibility Values
Source of Visibility Report
Automated Manual
M1/4 2 9a 0 5/8 1 5/8 4 12
1/4 2 1/2 10 1/16 3/4 1 3/4 5 13
1/2 3 - 1/8 7/8 1 7/8 6 14
3/4 4 - 3/16 1 2 7 15
1 5 - 1/4 1 1/8 2 1/4 8 20
1 1/4 6a - 5/16 1 1/4 2 1/2 9 25
1 1/2 7 - 3/8 1 3/8 2 3/4 10 30
1 3/4 8a - 1/2 1 1/2 3 11 35b
a. These values may not be reported by some automated stations.
b. Further values in increments of 5 statute miles may be reported; i.e., 40, 45, 50, etc.

Table A-4 Summary of Visibility Observing and Reporting Standards and Procedures
Visibility Type of Station
Automated Manual
Surface Represents 10-minutes of sensor outputs Visual Evaluation of Visibility around the horizon
Variable Reported when the prevailing visibility varies by 1/2 mile or more and the visibility is less than 3 miles
Tower Augmented Reported at stations with an ATCT
Sector Not Reported Reported at all stations

Coding the Visibility Group

For U.S. Stations:

  • Automated stations shall use an M to indicate "less than" when reporting visibility. For example, M1/4SM means a visibility of less than one-quarter statute mile.
  • Note: The term CAVOK is not used in the United States. See below.

International Standard for Reporting Visibility

  • WMO standards indicate that horizontal visibility, VVVV, is to be reported in whole meters. When the horizontal visibility is not the same in all directions, the minimum visibility is given for VVVV followed, without a space, by Dv, the direction of the visibility observed given by a one or two letter indicator of the eight points of the compass (N, NE, etc.). If the minimum visibility reported by VVVVDv is less than 1500 meters and the maximum visibility in another direction is greater than 5000 meters, then the visibility group VVVVDv is followed by the group VxVxVxVxDVv which is the maximum visibility and direction.

  • The term CAVOK is an acceptable contraction (meaning Ceiling and Visibility OK) for international use. It indicates that:
    1. No clouds exist below 5,000 feet or below the highest minimum sector altitude, whichever is greater, and no cumulonimbus are present.
    2. Visibility is 10 kilometers or more and,
    3. No precipitation, thunderstorms, sandstorm, duststorm, shallow fog, or low drifting dust, sand or snow is occurring.

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f. Tower or Surface Visibility (TWR_VIS_vvvvv or SFC_VIS_vvvvv). Tower visibility or surface visibility (see paragraphs 6.5.4 and 6.5.5) shall be coded in the formats, TWR_VIS_vvvvv or SFC_VIS_vvvvv, respectively, where vvvvv is the observed tower/surface visibility value. A space shall be coded between each of the remark elements. For example, the control tower visibility of 1 1/2 statute miles would be coded "TWR VIS 1 1/2". g. Variable Prevailing Visibility (VIS_vnvnvnvnvnVvxvxvxvxvx). Variable prevailing visibility shall be coded in the format VIS_vnvnvnvnvnVvxvxvxvxvx, where VIS is the remark identifier, vnvnvnvnvn is the lowest visibility evaluated, V denotes variability between two values, and vxvxvxvxvx is the highest visibility evaluated. There shall be one space following the remark identifier; no spaces between the letter V and the lowest/highest values. For example, a visibility that was varying between 1/2 and 2 statute miles would be coded "VIS 1/2V2" (see paragraphs 6.4.5 and 6.5.3). h. Sector Visibility (VIS_[DIR]_vvvvv) [Plain Language]. The sector visibility shall be coded in the format, VIS_[DIR]_vvvvv, where VIS is the remark identifier, [DIR] defines the sector to 8 points of the compass, and vvvvv is the sector visibility in statute miles, using the appropriate set of values in Table 12-1 (see paragraphs 6.4.6 and 6.5.7). For example, a visibility of 2 1/2 statute miles in the northeastern octant would be coded "VIS NE 2 1/2". i. Visibility At Second Location (VIS_vvvvv_[LOC]). At designated automated stations, the visibility at a second location shall be coded in the format VIS_vvvvv_[LOC], where VIS is the remark identifier, vvvvv is the measured visibility value, and [LOC] is the specific location of the visibility sensor(s) at the station (see paragraph 6.5.6). This remark shall only be generated when the condition is lower than that contained in the body of the report. For example, a visibility of 2 1/2 statute miles measured by a second sensor located at runway 11 would be coded "VIS 2 1/2 RWY11".