The ceiling is the height above the earth's surface (field elevation or ground elevation) ascribed to the lowest non-surface-based layer that is reported broken or overcast, or the vertical visibility into a surface-based obscuration that totally hides the sky.

12-5.   CELESTIAL DOME

The celestial dome is that portion of the sky that would be visible if all human-made structures were removed and there was an unobstructed view of the horizon in all directions from the observation site(s).

12-6.   CLOUD

A cloud is a visible accumulation of minute water droplets and/or ice particles in the atmosphere above the earth's surface. Cloud differs from ground fog, fog, or ice fog only in that the latter are, by definition, in contact with the surface.

12-7.   CLOUD MOVEMENT

When reported in remarks of a surface aviation observation, cloud movement is the direction toward which a cloud is moving.

12-8.   FIELD ELEVATION

Field elevation is the officially designated elevation (H_a) of an airport above mean sea level. It is the elevation of the highest point on any of the runways of the airport. The field elevation for an airport can be found in the United States Government Flight Information Publication, Airport/Facility Directory or the Chart Supplement for Alaska or the Pacific.

12-9.   HORIZON

For the purposes of these instructions, the horizon is the actual lower boundary (local horizon) of the observed sky or the upper outline of terrestrial objects, including nearby natural obstructions. It is the distant line along which the earth, or the water surface at sea, and the sky appear to meet. The local horizon is based on the best practical point of observation near the earth's surface and selected to minimize obstruction by nearby buildings, towers, etc.

12-10.   INTERCONNECTED CLOUD 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 that of the base of the parent cloud.

12-11.   LAYER

A layer consists of clouds or obscuring phenomena, not necessarily all of the same type, whose bases are at approximately the same level. A layer may be either continuous or composed of detached elements.

12-12.   LAYER AMOUNT

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. All cloud layers and obscuring phenomena aloft shall be considered. Only that portion of surface-based obscuring phenomena that hide a portion of the sky is considered.

12-13.   LAYER HEIGHT

The height, in feet, of the layer's base above the surface or field elevation is the layer height.

12-14.   MULTIPLE LAYERS

The existence of a layer or layers above a lower layer constitutes multiple layers.

12-15.   OBSCURING PHENOMENA

Any collection of particles aloft or in contact with the earth's surface, dense enough to be discernible to the observer, shall be considered obscuring phenomena.

12-16.   SKY COVER

Sky cover is a term used to denote the amount (to the nearest eighth) of the sky that is:

a.  Covered by clouds and/or obscuring phenomena aloft

b.  Hidden by surface-based obscuring phenomena, or

c. A combination of paragraphs a and b above

a. VERTICAL VISIBILITY (VV) is:

(1)  the distance that an observer can see vertically upward into surface-based obscuring phenomena that totally hide the sky, or

(2)  the height corresponding to the top of a ceiling light projector beam, or the height at which a balloon completely disappears during the presence of surface-based obscuring phenomena that totally hide the sky, or

(3)  the height determined by the sensor algorithm at automated stations into the surface-based obscuring phenomena that totally hide the sky.

b.  CLEAR (SKC or CLR). SKC is the abbreviation used for manual reports to indicate that no clouds are present, and CLR is the abbreviation used for automated reports to indicate that no clouds are detected at or below the design limit of the ceilometer.

c.  FEW (FEW) (few clouds) represents sky cover of more than zero to 2/8ths. Any layer amount less than 1/8 is considered 1/8.

d.  SCATTERED (SCT) represents sky cover of 3/8ths to 4/8ths at and below the level of a layer aloft.

e.  BROKEN (BKN) represents sky cover of 5/8ths up to, but not including, 8/8 at and below the level of a layer aloft.

f.  OVERCAST (OVC) represents sky cover of 8/8ths at and below the level of a layer aloft.

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, including that portion of surface-based obscuring phenomena that hides the sky. 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.

The summation principle states that the sky cover at any level is equal to the summation of the sky cover of the lowest layer, plus the additional sky cover present at all successively higher layers up to and including the layer being considered. No layer can be assigned a sky cover less than a lower layer, and no sky cover can be greater than 8/8ths. This concept is applicable for the evaluation of total sky cover.

For height determinations, the term "surface" denotes the horizontal plane whose elevation above sea level equals the field elevation. At stations where the field elevation has not been established, "surface" will refer to the ground or elevation at the observation site. At sea-plane bases, the mean high-tide mark may be regarded as the surface.

Total amount is the amount, in eighths, of the entire sky covered, not necessarily hidden, by all layers present. This amount cannot be greater than 8/8ths.

Variable ceiling describes a condition in which a ceiling rapidly increases and decreases during the period of evaluation.

12-23.   VARIABLE SKY CONDITION

Variable sky condition is a sky condition that has varied between reportable conditions (e.g., SCT to BKN, OVC to BKN) during the period of observation (normally the past 15 minutes).