Page 39 - AFMAN 15-111, 12 MAR 2019

AFMAN15-111 12 MARCH 2019 39

Chapter 8

VISIBILITY

8.1. Introduction. This chapter describes the observing and reporting standards for visibility.
Visibility is a measure of the opacity of the atmosphere and is expressed in terms of the horizontal
distance at which a person is able to see and identify specified objects. Visibility values are
reported in SM at U.S. locations (including Hawaii, Alaska and Guam); at overseas locations, the
unit of measure will be the same as that published for the installation in DoD FLIP airfield
approach plate minimums (i.e. usually meters). (T-2).

8.2. Visibility Group (VVVVVSM). The surface visibility, VVVVVSM (VVVV in meters for
overseas locations), is encoded using the values listed in Table 8.1 A space is encoded between
whole numbers and fractions of reportable visibility values. For example, a visibility of 1 1/ 2 SM
is encoded 1 1/2SM. The visibility group at US locations always ends in SM to indicate that
visibilities are in statute miles. Only FBWOS locations may use an M to indicate "less than" when
reporting visibility (e.g., M1/4SM (M0400) means a visibility less than 1/4 SM as reported by
AN/FMQ-19).

8.3. Visibility Algorithms. The visibility algorithm calculates the average visibility by
evaluating the sensor data gathered from a single point during a 10-minute period ending at the
actual time of the observation. Additionally, the data gathered during the averaging period is used
to determine if variable visibility should be reported. Where the FBWOS has meteorological
discontinuity sensors, the data from the additional sensors is examined to determine if their values
meet criteria for generating a visibility remark.

8.4. Standards and Reporting. The FBWOS sensors determine the average visibility over time
and record the corresponding reportable value as the prevailing visibility. If the visibility falls
halfway between two reportable values, the lower value is reported.

8.4.1. Prevailing visibility is determined by weather personnel as the greatest distance that can
be seen in at least half of the horizon circle and may not necessarily be continuous (i.e. may be
composed of sectors). Prevailing visibility represents a value determined from a height of
approximately 6 feet above the ground level and is reported using Table 8.1
8.4.2. Tower visibility is reported in column 13 when either the tower or prevailing visibility
is less than 4SM and they differ by at least one reportable value; see Attachment 3 for format.
Note: Tower visibility does not replace the prevailing visibility reported in the body of the
observation.
8.4.3. Variable prevailing visibility is reported in column 13 when the prevailing (or sensor
derived average) visibility is less than 3SM and rapidly fluctuating by at least 1/2SM during
the period of observation; see Attachment 3 for format.

8.4.4. Sector visibility is reported in column 13 when the visibility in at least 1/8th of the
horizon differs from the prevailing visibility by at least one reportable value and either the
prevailing or sector visibility is less than 3SM; see Attachment 3 for format.

8.4.5. Visibility at second location is reported in column 13 at locations equipped with two or
more visibility sensors when the visibility at the designated discontinuity sensor is lower than
the visibility in the body of the observation by a reportable value; see Attachment 3 for format.