Observations of stratification, amount, direction of movement and height of bases of clouds, and the effect of obscuring phenomena on vertical visibility shall be taken from as many locations as are necessary and practical to view the entire sky.

12-25.   LAYER AMOUNTS

All layers visible from the station shall be reported in sky cover reports. The amount of sky cover for each layer shall be the eighths of sky cover attributable to the clouds or obscuring phenomena in the layer being evaluated. Figure 12-1, Reportable Contractions for Sky Cover, shall be used to determine the reported value for each layer visible. The report shall be based on the eighths of sky covered by each layer in combination with any lower layers. Additionally, all layers with associated cumulonimbus or towering cumulus shall be identified as such using the contractions CB and TCU, respectively. Automated stations shall report no more than three layers of clouds; the layers reported shall be selected in accordance with Figure 12-2, Priority for Reporting Layers. At manual stations, a maximum of six layers of clouds or surface based obscuring phenomena shall be reported. If more than six layers are observed, they shall be selected in accordance with Figure 12-2.

12-26.   SUMMATION LAYER AMOUNT

The summation amount of sky cover for any given layer is the sum of the sky cover of the layer being evaluated, plus the sky cover of all lower layers. 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. (See Figure 12-5, Examples: Summation of Sky Cover.)

12-27.   LAYER HEIGHTS

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 reported as layers with a height of zero. At mountain locations, clouds below the level of the station may be observed and are reported with a height of ///. If available, a ceilometer shall be used to determine the height of layers aloft and vertical visibility into obscuring phenomena. If a ceilometer is not available, layer heights should be obtained by an alternative method; e.g., ceiling light, balloon, pilot report, etc. Known heights of unobscured portions of abrupt, isolated objects within 1 1/2 SM of a runway can also be used to measure the heights of layers aloft. Heights of layers observed at the station shall be reported in hundreds of feet above the surface (not above MSL), rounded to the nearest reportable increment. When a value falls halfway between two reportable increments, the lower value shall be reported. Figure 12-3, Increments of Reportable Values for Layer or Ceiling Heights, shall be used to determine the reportable increments for layer heights.

Figure 12-3. Increments of Reportable Values for Layer or Ceiling Heights

Observers should supplement layer data obtained from ceilometers or ceiling lights by visual observations to determine that the instrumental values are representative of the layers to which they are ascribed. Figure 12-6, Convective Cloud-Base Height Diagram, presents a convective cloud-base height diagram.

12-28.   EVALUATION OF MULTIPLE LAYERS

Frequent observations are necessary to evaluate stratification. A series of observations will often show the existence of multiple layers. Through thin lower layers it may be possible to observe higher layers. Differences in the directions of cloud movements often aid in observing and differentiating cloud layers. Ceilometer returns and, at night, ceiling light indications may also be used to determine the existence of multiple layers. Observers should be aware of and use these guidelines to determine and evaluate multiple layers.

12-29.   AMOUNT OF OBSCURATION

If a portion of the sky is not visible because of surface-based obscuring phenomena, the observer shall determine the portion of sky (in eighths) that is not visible. The amount of sky obscured shall be indicated as FEW, SCT or BKN, as appropriate, followed by three zeros (000). In remarks, the obscuring phenomena shall precede the amount of obscuration and three zeros. For example, if 5/8ths of the sky is obscured by fog, BKN000 would be in the body of the observation, with FG BKN000 in the remarks section.

12-30.   DETERMINING AMOUNT OF SKY COVER

The summation amount of sky covered at and below each layer shall be determined. Also, the amount of sky cover at and below the layer under evaluation shall be determined. Surface-based obscuring phenomena shall not be considered sky cover if the sky, higher clouds or obscuring phenomena aloft, or the moon or stars are visible through it.

12-31.   EVALUATION OF SKY COVER AMOUNTS

Sky cover amounts shall be evaluated:

a.  In eighths of coverage of the entire sky area above the horizon, and

b.  In terms of the total amount of sky cover, and

c.  With reference to an observation site as near as possible to the earth's surface.

12-32.   SKY COVER CLASSIFICATION

Select the appropriate sky cover contraction or combination of contractions to be reported after evaluating the following:

Step 1. Estimate (to the nearest eighth) the amount of sky covered by the lowest layer present. If this layer is a surface-based obscuring phenomenon, determine only the amount of sky that is hidden. Transparent surface-based atmospheric phenomena do not constitute sky cover.

Step 2. Determine if additional layers of clouds and/or obscuring phenomena aloft are present above the lowest layer. Estimate the eighths of sky covered by each of these layers in combination with the lower layers. Do not add to the total coverage amounts visible through transparencies in lower layers, except those amounts of upper layers visible through transparent surface-based atmospheric phenomena.

Step 3. Repeat the evaluation in step 2 for each additional layer present in ascending order of height. Estimate the summation (in eighths) of sky covered by each layer, in combination with all lower layers.

12-33. VARIABLE SKY COVER

The sky cover shall be considered variable if it varies by one or more reportable classifications during the period it is being evaluated, e.g., SCT V BKN. When a layer amount varies between reportable values during the time the amount is being evaluated, a variable sky condition remark shall be included in the observation. The format of the remark is given in paragraph 15-37, Variable Sky Condition.

12-34.   NON-UNIFORM SKY COVER

Observers shall be alert to variations in sky condition that are not reflected in the sky cover reported in the body of the observation. When non-uniform sky conditions are observed (for example, a significant lower ceiling in a particular direction from the station), the observer shall describe the condition in the remarks section. Unless a height is available from a reliable source, the height shall be described in relation to the heights reported in the body of the report. For example, CIG LWR N would indicate that ceilings are lower to the north.

12-35.   ESTIMATED CEILING HEIGHTS

Ceiling heights may be estimated by any of the following methods:

a.  Use of height reported by a pilot (converted from height above mean sea level to height above surface).

b.  Use of ceiling or pilot balloons of known ascension rate with the following procedures to estimate ceiling heights.

(1)  Choose and inflate the appropriate colored balloon; red balloons are usually preferred with thin clouds and blue or black balloons under other conditions.

(2)  Release and watch the balloon continuously to determine, with a watch, the length of time that elapses between release of the balloon and its entry into the base of the layer. The point of entry, for layers aloft, will be considered as midway between the time the balloon begins to fade until the time the balloon completely disappears.

(3)  Determine the height above the surface corresponding to the elapsed ascent time, using Figure 17-1, 10-Gram Balloon Ascension Rates, or Figure 17-2, 30-Gram Balloon Ascension Rates. The accuracy of the height obtained by the balloon will be decreased when the balloon:

(a)  Does not enter a representative portion of the cloud base, or

(b)  Is used at night with a light attached, or

(c)  Is used during the occurrence of hail, ice pellets, any intensity of freezing rain, or moderate to heavy rain or snow

c.  Use of the Convective Cloud-Base Height Diagram (Figure 12-6). Use this diagram only to estimate the height of cumulus clouds formed in the vicinity of the station. It cannot be used at stations in mountainous or hilly terrain, or to determine the height of other than cumulus clouds. This diagram is most accurate when used to determine the height of cloud bases below 5,000 feet. Use the dry-bulb temperature and dew point to obtain the height of cloud bases above the point of observation as follows:

(1)  Locate the point of intersection of the vertical line corresponding to the observed dew point temperature, and the curve (sloping upward to the left) corresponding to the observed dry-bulb temperature. (Note: Use Celsius or Fahrenheit scales, as appropriate.)

(2)  From the point of intersection, move horizontally to the height scale printed, in feet, on the right side of the diagram. This value is a good estimate of the height of the cloud base.

d.  Use of known heights of unobscured portions of abrupt, isolated objects within 1 1/2 miles from any runway of the airport.

e.  Use of observational experience; provided that other guides are lacking or, in the opinion of the observer, are considered to be unreliable.

f.  Use of a ceiling light where heights indicated are less than or equal to 10 times the baseline.

g.  Determine the ceiling height, where practicable, by balloons if the ceiling cannot be determined with a ceilometer, ceiling light, or pilot report, and:

(1)  the ceiling is at, or below, the minimum height for VFR operations, in the airport traffic area, or

(2)  the ceiling height is 2,000 feet or less and the presence of a stratus-type cloud layer makes estimation difficult.

Rapid fluctuations of the ceilometer indications, or the spot from a ceiling light projector, will indicate an irregular base whose height is measured, but also variable. When the height of a ceiling layer increases and decreases rapidly during the period of evaluation by the amounts given in Figure 12-4, Criteria for Variable Ceiling, and the ceiling height is below 3,000 feet, it shall be considered variable and the ascribed height shall be the average of all the values. A remark shall be included in the observation giving the range of variability (see paragraph 15-35, Variable Ceiling Height). Variable ceilings at or above 3,000 feet may be reported as variable only if considered operationally significant.

Observers shall be alert for the occurrence of cumulonimbus, towering cumulus, altocumulus castellanus, standing lenticular, or rotor clouds and report them whenever they occur. These clouds may be reported by entering a remark in METAR and SPECI observations. The remark shall contain the identification of the cloud, and (insofar as known) the direction and distance from the station and, for cumulonimbus clouds, the direction of movement. See paragraph 15-38, Significant Cloud Types, for detailed instructions on coding these remarks. Cumulonimbus (CB) or towering cumulus (TCU) shall be appended to the appropriate layer in the body of the observation. When TCU or CB is appended to the layer report accompanied by the remark, "TCU NW" or "CB NW MOV E", it is implied that the TCU or CB is associated with the layer and within 10 SM. When TCU or CB is outside 10 SM, a DSNT remark is appropriate, for example, "TCU DSNT NW". (In this case, TCU or CB would not be appended to the layer in the body of the METAR.) Also, see paragraph 15-38, Significant Cloud Type.

12-38. CLOUD TYPES (NA LAWRS)

At designated locations, synoptic cloud types shall be reported and coded. See paragraph 15-53, Cloud Types, for detailed instructions on coding these remarks.

12-39. OPERATION OF EQUIPMENT

Practices and procedures for the operation of sky condition instruments and related equipment are presented in Chapter 17, Operation of Equipment.

Figure 12-6. Convective Cloud-Base Height Diagram