Page 59 - AFMAN 15-111, 12 MAR 2019

AFMAN15-111 12 MARCH 2019 59

11.2.5. Each layer is separated from other layers by a space. The sky cover for each layer
reported is encoded by using the appropriate reportable contraction. The report of clear skies
(CLR or SKC) is reported by itself. The abbreviations FEW, SCT, BKN and OVC will be
followed, without a space, by the height of the cloud layer. (T-0).

11.2.6. A partial obscuration will be encoded with the reportable layer construction
corresponding to the amount of the sky that is obscured followed by the layer height. A surface-
based obscuration will have a layer height of 000. A remark will also be appended for any
surface-based obscurations. For example: fog obscuring 2/8ths of the sky would be encoded
in the body of the report as FEW000 and clarified in the remarks as FG FEW000. (T-0).
11.3. Sky Condition Algorithms. An FBWOS derives sky condition by detecting the frequency
and height of clouds that move over the sensor over a continuously averaged period of 30 minutes.
The data collected from the sensor is processed into coverage amount, height and variability of
clouds.
11.3.1. FBWOS sensors are capable of reproducing a sky condition report comparative to that
of a human weather technician but may perform poorly in situations where clouds develop in
the vertical but have little horizontal movement (i.e. convective clouds either remain directly
over sensor and are over-reported or do not pass over the sensor and are under-reported). When
equipped with multiple ceilometers, the data from the discontinuity sensors are examined and
compared with the primary sensor to determine the requirement to report non-uniform ceiling
conditions IAW Attachment 3.

11.3.2. FBWOSs report sky condition from 100ft up to a maximum of 25,000ft. Note: Some
systems may report heights exceeding 25,000ft.

11.4. Standards and Reporting. Sky condition is an evaluation of all obscurations/clouds
making up layers at different heights and is reported in oktas (eighths) using a code form with
appended height above the surface. Sky condition is reported in all METAR and SPECI
observations.

11.4.1. Clouds and/or obscuring phenomena (not necessarily all of the same type) with bases
at approximately the same level constitute a layer. They may be either continuous or composed
of detached elements. A trace aloft is evaluated as 1/8th; however, a surface-based obscuring
phenomenon is only classified as a layer when it covers at least 1/8th of the sky. All layers are
treated as opaque.
11.4.2. Heights of layers are reported using a three-digit value representing hundreds of feet
above the surface and are rounded to a reportable increment. Layer heights up to 5,000ft are
reported to the nearest 100 feet, layer heights between 5,000ft and 10,000ft are reported to the
nearest 500ft and layers above 10,000ft are reported to the nearest 1,000ft. When a value falls
halfway between two reportable increments, the lower value is reported.
11.4.3. Sky cover is determined using the summation principle. This principle states that the
sky cover at any level is equal to the sky cover in all lower levels up to and including the layer
in question. In other words, no layer can be classified as less than the layers below it and no
total sky cover can be greater than 8/8ths. See Figure 11.1 for an example of the summation
principle.