Sky condition is a description of the appearance of the sky. Sky condition may be evaluated either automatically by instrument or manually with or without instruments.

9.2 Scope

This chapter prescribes the standards for observing and reporting sky condition.

9.3 Sky Condition Parameters

Sky condition parameters are:

  a. Sky cover.  The amount of the celestial dome hidden by
     clouds and/or obscurations.

  b. Summation layer amount.  A categorization of the amount of
     sky cover at and below each reported layer.

  c. Layer height.  The height of the bases of each reported
     layer of clouds and/or obscurations; or the vertical
     visibility into an indefinite ceiling.

  d. Ceiling.  The height above the earth's surface of the lowest
     layer that is reported as broken or overcast; or the
     vertical visibility into an indefinite ceiling.

  e. Type of clouds.  The variety of clouds present.

9.4.1 Sky Condition Observing Standards. Sky condition shall be evaluated at all stations with this capability. Automated stations shall have the capability to evaluate sky condition from the surface to at least 12,000 feet. Observers at manual stations shall evaluate all clouds and obscurations visible; the 12,000 foot restriction shall not apply.

  a. Layer Opacity.  All cloud layers and obscurations shall be
     considered as opaque.

  b. Surface.  The surface shall be the assigned field
     elevation of the station.  At stations where the field
     elevation has not been established, the surface shall be
     the ground elevation at the observation site.

  c. Sky Cover.  Sky cover shall include any clouds or
     obscurations detected from the observing location (see
     Table 9-1).

  d. Stratification of Sky Cover.  Sky cover shall be separated
     into layers with each layer containing clouds and/or
     obscurations (i.e., smoke, haze, fog, etc.) with bases at
     about the same height.

  e. Evaluation of Interconnected Layers.  Clouds formed by the
     horizontal extension of swelling cumulus or cumulonimbus,
     that are attached to a parent cloud, shall be regarded as
     a separate layer only if their bases appear horizontal and
     at a different level from the parent cloud.  Otherwise,
     the entire cloud system shall be regarded as a single
     layer at a height corresponding to the base of the parent
     cloud.

  f. Sky Condition Range, Accuracy, and Resolution.  The
     required range, accuracy, and resolution for sky condition
     is listed in Appendix C.

9.4.2 Sky Cover

  a. Clear Skies.   When no clouds or obscurations are observed
     from the point of observation.

  b. Layer Amounts.  The amount of sky cover for each layer shall
     be the eighths (or oktas) of sky cover attributable to
     clouds or obscurations (i.e., smoke, haze, fog, etc.) in the
     layer being evaluated.

  c. Summation Layer Amount.  The sky cover summation amount for
     any given layer is the sum of the sky cover for the layer
     being evaluated plus the sky cover of all lower layers
     including obscurations.  Portions of layers aloft detected
     through lower layers aloft shall not increase the summation
     amount of the higher layer.  No layer can have a summation
     amount greater than 8/8ths.

  d. Variable Amounts of Sky Cover.  The sky cover shall be
     considered variable if it varies by one or more reportable
     values (FEW, SCT, BKN, or OVC) during the period it is being
     evaluated.

9.4.4 Vertical Visibility. Vertical visibility shall be either:

  a. The distance that an observer can see vertically into an
     indefinite ceiling;

  b. The height corresponding to the top of a ceiling light
     projector beam;

  c. The height at which a ceiling balloon completely disappears
     during the presence of an indefinite ceiling; or

  d. The height determined by the sensor algorithm at automated
     stations.

9.4.6 Significant Clouds and Cloud Types. Cloud types shall be identified in accordance with the WMO International Cloud Atlas-Volumes I and II, the WMO Abridged International Cloud Atlas, or agency observing aids for cloud identification. Cumulonimbus, including cumulonimbus mammatus, towering cumulus, altocumulus castellanus, standing lenticular, or rotor clouds are significant clouds.

9.4.7 Height of Sky Cover. A ceilometer, if available, or ceiling light, or known heights of unobscured portions of abrupt, isolated objects within 1 1/2 statute miles of a runway shall be used to measure the height of layers aloft. Otherwise, an alternative method shall be used to estimate the heights. The height may be estimated by using a ceiling balloon, pilot report, other agency guidelines, or observer experience.

  a. Indefinite Ceiling Height (Vertical Visibility).  The height
     into an indefinite ceiling shall be the vertical visibility
     measured in hundreds of feet.

  b. Height of Layers.  The height of a layer shall be the height
     of the cloud bases or obscurations for the layer being
     evaluated.  Layers of clouds that are 50 feet or less above
     the surface shall be observed as layers with a height of
     zero.  When the height of a ceiling layer increases and
     decreases rapidly by the amounts given in Table 9-2, during
     the period of evaluation, it shall be considered variable
     and the ascribed height shall be the average of all the
     varying values.  At mountain stations, clouds below the
     level of the station may be observed.

9.5 Sky Cover Reporting Standards

9.5.1 Frequency for Sky Cover. Sky cover shall be included in all reports.

9.5.2 Layer Amount.

The amount of sky cover reported for each layer shall be based on the summation layer amount for that layer. The amount shall be reported using the reportable contractions given in Table 9-3.

Automated stations shall report no more than three layers. Manual stations shall report no more than six layers. The selection of layers reported shall be made in accordance with Table 9-4. Additionally, all layers with associated cumulonimbus or towering cumulus shall be identified by appending the contractions CB and TCU, respectively.

Sky condition shall be reported in an ascending order up to the first overcast layer. Layers above 12,000 feet are not reported by automated sky condition sensors. At mountain stations, if the cloud layer is below station level, the height of the layer shall be reported as ///.

1. The abbreviation CLR shall be used at automated
   stations when no layers at or below 12,000 feet are reported; the abbreviation
   SKC shall be used at manual stations when no layers are reported.
2. Any layer amount less than 1/8 is reported as FEW.

9.5.4 Reportable Values for Sky Cover Height. The reportable values of sky cover height are hundreds of feet. The reportable value increments are given in Table 9-5.

9.5.6 Obscuration. When a portion of the celestial dome is obscured, the obscuration (amount of sky cover hidden by the weather phenomena) shall be reported (see paragraph 9.4.3 and Table 9-2). The obscuration shall also be reported as a remark (see paragraph 12.7.1.q).

9.5.7 Variable Ceiling. When the height of the ceiling layer is variable, and the ceiling layer is below 3,000 feet, a remark shall be included in the report giving the range of variability (see paragraphs 9.4.7.b and 12.7.1.p).

9.5.8 Ceiling Height at a Second Location. When automated stations use meteorological discontinuity ceilometer(s), remarks shall be added to identify ceiling height conditions at the second location which differ from the ceiling height in the body of the report (see paragraph 12.7.1.t).

9.5.9 Variable Sky Condition. Variable sky conditions shall be indicated in the remarks of the report (see paragraph 12.7.1.r).

9.5.10 Significant Cloud Types. Significant cloud types shall be indicated in the remarks of the report (see paragraph 12.7.1.s).

9.6 Summary of Sky Condition Observing and Reporting Standards

Table 9-6 summarizes the sky condition observing and reporting at each category of station.