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AVIATION WEATHER
CHAPTER FIVE
BKN at 5000 feet, OVC at 10,000 feet, where the altitudes refer to the bases of the cloud layers
in feet AGL.
Reportable
Meaning
Amount of
Contractions
Sky Cover
SKC or CLR1
Sky Clear
0/8
FEW2
Few
> 0/8 - 2/8
SCT
Scattered
3/8 - 4/8
BKN
Broken
5/8 - 7/8
OVC
Overcast
8/8
3
VV
Obscured
8/8 (surface based)
1. The abbreviation CLR is used at automated stations when no clouds at or below
12,000 feet are reported; the abbreviation SKC is used at manual stations when no clouds
are reported
2. Any amount less than 1/8 is reported as FEW.
3. The last 3 digits report the height of the vertical visibility into an indefinite ceiling.
Figure 5-16 Sky Coverage Contractions
A ceiling is the height AGL ascribed to the lowest broken or overcast layer or the vertical
visibility into an obscuring phenomenon (total obscuration).
Vertical visibility is the distance seen directly upward from the ground into a surface-based
obscuring phenomenon. This term is used when the celestial dome is totally hidden from view
(8/8ths) by some surface based obscuration, and the reported ceiling is determined by measuring
the vertical visibility upward as seen from the ground. In this type of situation, the base of the
obscuration is less well defined, but it may still be possible to see upwards into the moisture (or
other obstruction) for a short distance. While this does constitute a ceiling, it is sometimes
referred to as an "indefinite" ceiling, and the distance seen upward into the phenomenon is then
given as the vertical visibility. For example, if the sky were totally hidden by fog which touched
the ground, but a ground observer could see a weather balloon ascend upward into the fog for
200 feet, he/she would report a vertical visibility of 200 feet.
It is important to realize the vertical visibility of 200 feet in the foregoing example is very
different from a cloud ceiling of 200 feet. With a low cloud ceiling, a pilot normally can expect
to see the ground and the runway once the aircraft descends below the cloud base. However, in
the case of vertical visibility, the obscuring phenomenon also reduces the slant range visibility.
Therefore, a pilot will have difficulty seeing the runway or approach lights clearly even after
descending below the level of the reported vertical visibility.
If the weather observer on the ground is able to see part of the celestial dome or some clouds
through an obscuring phenomenon (a partial obscuration) it is reported as few, scattered, or
broken as appropriate, and assigned a height of 000 to indicate it is a surface based phenomenon.
If clouds are present, their bases and amount or coverage are also reported.
Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds
5-23


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