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Site: Bureau of Meteorology Training Centre
Course: Cloud Observations Study Guide
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Date: Monday, 16 September 2019, 4:56 PM

Introduction

Cloud Definition:

A cloud is a hydrometeor consisting of minute particles of liquid water or ice, or of both,suspended in the free air and usually not touching the ground. It may also include larger particles of liquid water or ice as well as non-aqueous liquid or solid particles such as those present in fumes, smoke or dust.

- (WMO-407 International Cloud Atlas)

Different processes in the atmosphere produce clouds in varying forms and are responsible for much of the weather we experience. Identifying the cloud types indicates what processes in the atmosphere are taking place, allowing an assessment of the present and expected weather to be made. An incorrect identification of cloud type can lead to an incorrect assessment of the weather situation. The observer's responsibility in this respect cannot be over emphasised.

Altocumulus catellanus
Altocumulus castellanus: the presence of this cloud is a sign of instability at its level. When it aquires a considerable vertical extent, it becomes Cumulus congestus, and sometimes Cumulonimbus (the cloud associated with thunderstorms).

Common Terms

Celestial Dome

  • 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.

Height (of the cloud base)

  • The vertical distance from the ground at the observation point to the base of the cloud. Often referred to as height above ground level (AGL). Cloud heights are expressed in feet.

Elevation

  • The vertical distance to a point on the surface of the earth, measured from mean sea level.

Altitude

  • The vertical distance of a point measured from mean sea level. Commonly refers to the height of an aircraft above mean sea level (AMSL).

Vertical Extent (of the cloud)

  • Vertical distance between the cloud base and the cloud top.

Common terms
Common terms

Cloud Classification

Clouds appear in an infinite variety of forms; observations will show them to be in a continuous process of evolution, and at first it may appear almost impossible to identify them. However, there are several principal ways in which clouds form, and each of these processes produces a cloud with certain distinctive features or characteristics.

Cloud classification is based on the recognition of these features, from which it is possible to identify 10 main cloud groups known as cloud genera. Often clouds of the same genera can vary greatly in appearance. To account for this, most of the genera are further subdivided into species.

The species of a cloud is determined by its shape or internal structure. A cloud observed in the sky of a particular genera, may bear the name of only one species at any given time.

Additional characteristics possessed by a cloud may determine that it is of a particular variety. The variety of a cloud considers characteristics such as its transparency and the arrangement of its observable elements.

Any supplementary features and accessory clouds, as well as an indication of the mother-cloud can be used to further classify a particular cloud. A single cloud may simultaneously bear multiple varieties and supplementary features and accessory clouds.

Here is an example of the concept:

classification example

In addition to the 10 cloud genera, a cloud known as Towering Cumulus is reported for aeronautical purposes due to its significance as a hazard and an indicator of extensive convection occurring in the atmosphere. This section will refer to the 10 cloud genera plus Towering Cumulus as the 11 basic cloud types.

The 11 basic cloud types are:

  • Cirrus

  • Cirrostratus

  • Cirrocumulus

  • Altocumulus

  • Altostratus

  • Nimbostratus

  • Stratocumulus

  • Cumulus

  • Towering Cumulus

  • Cumulonimbus

  • Stratus

“What about the different cloud species and varieties? Just how much do I need to know to perform an accurate observation?”

The required level of knowledge of the observer will largely depend upon the purpose of the observations they perform. A particular cloud observed in the sky may be described in several different ways depending on the reporting format being used. For instance:

  • Twenty-seven variations and combinations of clouds are considered when performing a synoptic cloud observation

  • An aerodrome weather report requires an observer to consider just the 11 basic cloud types.

  • Take-off and landing reports via the ATIS broadcast include the cloud types of Cumulonimbus and Towering Cumulus only; for all other clouds the type is not identified – just the amount and height is reported.

  • Often a cloud may not be deemed ‘significant’ in an observation and no mention of its existence will be reported whatsoever.

Regardless of how a cloud observation is reported, the specific relationship between the cloud types and the weather they produce, such as rain, drizzle or hail, requires that all observers be able to identify at least the 11 basic cloud types if an accurate assessment of the weather is to be made.

Observing conditions to which the cloud descriptions apply

The following pages give a general description of each of the cloud types. These descriptions, unless otherwise specified, assume an observation is carried out under the following conditions:

  • The observer is at the earth’s surface, either on land in areas without mountainous relief or at sea;

  • The air is clear – no obscuring phenomena such as mist, haze, dust, smoke, etc, are present;

  • The sun is sufficiently high to provide the usual luminance and colouration;

  • The clouds are high enough above the horizon such that effects of perspective are negligible.

There will always be the need to adapt the descriptions to other observing conditions. Some guidance is given throughout this section to assist observers when these conditions cannot be met.

In addition to the cloud types, this section describes the precipitation type associated with each cloud. Further details of precipitation types are covered in Weather Observations.

Cirrus

General description

Detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a hair-like (fibrous) appearance, or a silky sheen, or both. Cirrus is composed almost exclusively of ice crystals.

Common subdivisions

The species Cirrus fibratus is comprised of nearly straight or irregularly curved fine, white filaments, that are, for the most part, distinct from one another.

Cirrus uncinus appears as filaments shaped like a comma, terminating in a hook or tuft.

Cirrus in patches, sufficiently dense to appear greyish (unlike other cirrus species) when viewed towards the sun is the species Cirrus spissatus. It can sometimes be thick enough to obscure the sun’s outline, or even hide it. Cirrus spissatus that has originated from the remains of the icy anvil of a Cumulonimbus cloud is known as Cirrus spissatus cumulonimbogenitus.

Cirrus floccus appears in the form of more or less isolated, small, rounded tufts, often with trails.

Distinguishing Ci from other genera

Cirrus is distinguished from Cirrostratus by its discontinuous structure or, when in patches or bands, by its small horizontal extent or the narrowness of its continuous parts. Owing to perspective, Cirrus near the horizon may be difficult to distinguish from Cirrostratus.

Cirrus clouds are distinguished from Cirrocumulus by their mainly fibrous or silky appearance and by the absence of small cloud elements.

Thick Cirrus clouds are distinguished from Altostratus patches by their smaller horizontal extent and their mostly white appearance.

Associated precipitation

Cirrus is not associated with any precipitation.


cirrus fibratus
Cirrus fibratus
cirrus spissatus and floccus
Cirrus spissatus and Cirrus floccus (Photo: P. Toomey)
cirrus uncinus
Cirrus uncinus

Cirrocumulus

General description

Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the form of grains, ripples, merged or separate, and more or less regularly arranged.

Most of the elements have an apparent width of less than one degree (when observed at an angle greater than 30 degrees above the horizon). This is about the width of the little finger at arm's length.

Common subdivisions

A common species is Cirrocumulus stratiformis, showing a relatively extensive sheet or layer, sometimes with gaps, breaches or rifts.

Other species include lenticularis, castellanus and floccus.

Distinguishing Cc from other genera

Cirrocumulus differs from Altocumulus in that most of its elements have an apparent width of less than one degree, and it is without shading. A cloud should not be called Cirrocumulus if it consists of a patch of incompletely developed small elements, such as when observed on the edges of a patch of Altocumulus, or if present in separate patches at the same level as Altocumulus.

Cirrocumulus differs from Cirrus and Cirrostratus in that it appears rippled or grainy; it may include fibrous or smooth portions, but these do not constitute its greater part. In middle or high latitudes, Cirrocumulus is usually associated with other cirriform clouds; less often in low latitude regions. In synoptic observations, Cirrocumulus must dominate the cirriform clouds for it to be reported. This practice would also be considered reasonable for aviation observations.

Associated precipitation

Cirrocumulus is not associated with any precipitation.


cirrocumulus stratiformis
Cirrocumulus stratiformis
cirrocumulus stratiformis
Cirrocumulus stratiformis (Photo: NOAA)
cirrocumulus
Cirrocumulus with other cirroform cloud

Cirrostratus

General description

Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena (a coloured ring or partial ring around the sun or moon with red on the inside and white on the outside).

At high angles Cirrostratus is never thick enough to prevent objects on the ground from casting shadows. However, when the sun is low on the horizon (below about 30 degrees), the longer light path through the cloud may reduce the light intensity such that shadows do not exist. Also at low sun angles, grey shading within the cloud may be apparent.

Common subdivisions

The species Cirrostratus fribratus shows a fibrous veil in which thin striations can be observed. This species may develop from Cirrus fribratus.

The nebulous veil of Cirrostratus nebulosis will show no distinct detail at all. It may be so thin that it is barely visible - the presence of a halo may be only indication of its existence. Conversely, it may also be relatively dense.

Distinguishing Cs from other genera

Cirrostratus is distinguished from Cirrus in that it occurs as a veil usually of great horizontal extent.

Cirrostratus is distinguished from Altostratus by its thinness, and that it may show halo phenomena. Except when the sun is low on the horizon, Cirrostratus does not prevent shadows being cast.

Associated precipitation

Cirrostratus is not associated with any precipitation.


cirrostratus nebulosus
Cirrostratus nebulosus
cirrostratus fibratus
Cirrostratus fibratus (Photo: P. Leigh)
cirrostratus fibratus
Cirrostratus fibratus with the sun low on the horizon
cirrostratus fibratus
Cirrostratus fibratus with an aerological diagram showing its vertical extent - approx 20,000ft (Photo: P.Toomey)

Altocumulus

General description

White or grey, or both white and grey, patch, sheet or layer of cloud, generally with shading, composed of layers laying on top of each other, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or may not be merged.

Most of the regularly arranged small elements usually have an apparent width of between one and five degrees (when observed at an angle greater than 30 degrees above the horizon). This is between the approximate width of one to three fingers at arm's length.

A corona (ring around the sun or moon with red on the outside) may sometimes (albeit rarely) be seen. Irisation may appear along the thinner edges of the elements.

Common subdivisions

The most common species is Altocumulus stratiformis, occurring as an extensive sheet or layer of separate or merged elements.

The species Altocumulus castellanus is in the form of sproutings or small towers having a common base, or appears as small cumuliform tufts. When Altocumulus castellanus acquires a considerable vertical extent, it becomes a high based Towering Cumulus, and can even transition to Cumulonimbus.

Altocumulus lenticularis is a lens or almond shaped, often elongated cloud, commonly associated with mountain wave activity. Mountain waves can present a significant hazard to aviation.

Distinguishing Ac from other genera

Altocumulus differs from Cirrocumulus in that some of the Altocumulus clouds have shading. However, if the clouds are without shading but most of the elements have an apparent width of between one and five degrees, the cloud is to be called Altocumulus.

Altocumulus is distinguished from Stratocumulus by its smaller elements.

Associated precipitation

WMO technical notes do not associate any precipitation with Altocumulus, however an Australian convention associates showery precipitation with the castellanus species.


Altocumuls stratiformis
Altocumuls stratiformis (Photo P. Toomey)
Altocumuls lenticularis
Altocumulus lenticularis (Photo P. Toomey)
altocumuls castellanus
Altocumulus castellanus (Photo P. Leigh)

Altostratus

General description

Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly covering the sky and having parts thin enough to reveal the sun at least vaguely, as if looking through ground glass.

Altostratus prevents objects on the ground from casting shadows, and it does not show halo phenomena.

Altostratus is sometimes a result of thickening and lowering Cirrostratus.

Common subdivisions

Due to its uniform appearance, Altostratus is not subdivided into species. It does have several varieties though. Two of note are:

Altostratus translucidus – thin Altostratus, the greater part semi-transparent to reveal the position of the sun or moon; and

Altostratus opacus – thick Altostratus, the greater part sufficiently opaque to mask the sun or moon completely.

Distinguishing As from other genera

Altostratus differs from Cirrostratus in that Altostratus prevents objects on the ground from casting distinct shadows. The sun may appear vague as through ground glass. If halo phenomenon is observed, the cloud is Cirrostratus.

Thick Altostratus (opacus) is distinguished from Nimbostratus by the presence of occasional thinner parts through which the sun’s position is vaguely revealed. It is also lighter grey in colour. At night, when it is difficult to distinguish between Altostratus and Nimbostratus, it is called Altostratus if no precipitation is falling.

Altostratus differs from Altocumulus and Stratocumulus in that even if it shows gaps, breaches or rifts, it can be distinguished by its more uniform appearance, and lack of rounded masses, rolls, etc.

Associated precipitation

The precipitation associated with Altostratus is (non-showery) rain or snow or ice pellets, often of an intermittent nature.


Altostratus translucidus
Altostratus translucidus
Altostratus opacus
Altostratus opacus (Photo P. Leigh)
Altostratus with Altoculumus beneath
Altostratus with Altocumulus beneath (Photo: P. Toomey)

Nimbostratus

General description

Grey cloud layer, often dark, the appearance of which appears diffuse by more or less continuously falling rain or snow, which in most cases reaches the ground.  It is thick enough throughout to blot out the sun. It is often formed by a thickening and generally lowering Altostratus layer.

Although a middle level cloud, a Nimbostratus base is frequently observed in the low level.

Common subdivisions

Nimbostratus does not present any species or varieties, however low, ragged clouds frequently occur below the layer and can merge with the Nimbostratus base.

Distinguishing Ns from other genera

Nimbostratus differs from thick Altostratus (opacus variety) by the absence of thinner parts through which the sun is vaguely revealed. It is also darker grey in colour. An observer will have no indication of the position of the sun or moon with Nimbostratus cloud. If on dark nights it is difficult to distinguish between Nimbostratus and Altostratus, the cloud is by convention called Nimbostratus if rain or snow is reaching the ground.

Nimbostratus is distinguished from thick Stratus in that its base is more diffuse than that of Stratus, and that it produces rain, whereas the precipitation associated with Stratus is drizzle.

Nimbostratus differs from Cumulonimbus in that Nimbostratus is never associated with lightning and thunder or showery precipitation, including hail.

Associated precipitation

The precipitation associated with Nimbostratus is (non-showery) rain or snow or ice pellets, often of a continuous nature and greater in intensity to that from Altostratus.


Nimbostratus
Nimbostratus (Photo: P. Leigh)
Nimbostratus
Nimbostratus (Photo: NOAA)

Stratocumulus

General description

Grey or whitish, or both grey and whitish, patch, sheet or layer of cloud which almost always has dark parts, composed of tessellations, rounded masses and rolls, and which may or may not be merged.

Most of the regularly arranged small elements have an apparent width of more than five degrees, when observed at an angle greater than 30 degrees above the horizon. This is the approximate width of three fingers held at arm's length.

Common subdivisions

The most common species is Stratocumulus stratiformis, being rolls or large rounded masses arranged in an extended sheet or layer. The elements are more or less flattened.

Less common are Stratocumulus lenticularis and castellanus. These exhibit similar characteristics to the Altocumulus version of these species.

Stratocumulus sometimes forms from the spreading out of Cumulus. When the top of a Cumulus cloud reaches a higher stable (warmer) layer, it may spread out to form a patch of Stratocumulus; Stratocumulus cumulogenitus (Cumulus being the mother-cloud) is the resultant cloud.

Another form of Stratocumulus cumulogenitus can occur in the evening if convection ceases leading to the domed cumulus summits flattening out.

Distinguishing Sc from other genera

Stratocumulus differs from Altocumulus in that most of the regularly arranged elements of Stratocumulus have an apparent width of more than five degrees, and its height usually does not exceed 8500 ft.

Stratocumulus differs from Cumulus in that its elements usually occur in groups or patches and generally have flat tops (stratiformis species). If the tops are in the form of shallow domes they rise, unlike those of Cumulus, from merged bases.

Stratocumulus differs from Stratus, Altostratus and Nimbostratus in that it shows the presence of non-fibrous elements, either merged or separate.

Associated precipitation

Stratocumulus rarely produces precipitation; in the event it does, it will be very light rain or snow, or drizzle – (the association of drizzle with Stratocumulus is a non-WMO convention).


Stratocumulus stratiformis
Stratocumulus stratiformis (Photo: P. Toomey)
Stratocumulus
Stratocumulus stratiformis (Photo: P. Leigh)
Stratocumulus cumulogenitus
Stratocumulus cumulogenitus, with Cumulus (Photo: J. Darnley)

Stratus

General description

Generally grey cloud layer with a fairly uniform base.  It usually occurs below about 2000ft AGL. When the sun is visible through the cloud, its outline is clearly discernible. Stratus does not produce halo phenomena except possibly at very low temperatures.

Common subdivisions

The species of Stratus described above is Stratus nebulosus.

Sometimes stratus appears in the form of irregular ragged shreds. This species is known as Stratus fractus. It usually forms beneath the base of higher precipitating cloud. In such an instance it is known as Stratus fractus of bad weather. Despite the name, Stratus fractus of bad weather does not itself produce precipitation.

Distinguishing St from other genera

Stratus is distinguished from Altostratus by the fact that when the sun is visible it does not blur its outline.

Thick Stratus differs from Nimbostratus in that its base is more clearly defined and uniform, and it can produce drizzle, as opposed to rain from Nimbostratus.

Stratus is distinguished from Stratocumulus in that it shows no evidence of elements, either merged or separated.

Stratus fractus is less white and less dense, with smaller vertical development, than Cumulus fractus.

Associated precipitation

The precipitation associated with Stratus nebulous is drizzle when sufficiently thick. It can also produce snow and snow grains.


Stratus nebulosus
Stratus nebulosus (Photo P. Toomey)
Stratus fractus
Stratus fractus beneath Altostratus (Photo: P. Toomey)
Stratus nebulosus
Stratus nebulosus (Photo: P. Leigh)

Cumulus

General description

Separated clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes or towers. The upper parts of larger Cumulus can resemble a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their bases are relatively darker and nearly horizontal.

Common subdivisions

Small Cumulus clouds with very ragged edges and with outlines that are continuously undergoing rapid changes are known as Cumulus fractus. Cumulus fractus sometimes forms in or near precipitation from other cloud types. It is distinguished from Stratus fractus by its generally greater vertical extent and its usually whiter and less transparent appearance.

Very small, rather flattened and isolated Cumulus is the species Cumulus humilis.

Cumulus with a moderate vertical development is the species Cumulus mediocris.

Larger Cumulus is known as Cumulus congestus. If this species reaches a ‘great vertical extent’ it becomes known as Towering Cumulus for aviation reporting purposes.

Distinguishing Cu from other genera

Cumulus differs from Altocumulus and Stratocumulus in that Cumulus tops are dome-shaped and the bases are not merged; caution must be exercised when viewing Cumulus from a distance as the bases may appear merged due to the effect of perspective.

Associated precipitation

Cumulus humilis clouds never give precipitation.

Cumulus mediocris clouds generally give no precipitation.

Showers of rain or snow are possible with Cumulus congestus; precipitation is more common though when the cloud is of great vertical extent (Towering Cumulus).


Cumulus humilis
Cumulus humilis (Photo: P. Toomey)
Cumulus mediocris
Cumulus mediocris (Photo: P. Leigh)
Cumulus congestus
Cumulus congestus (Photo: P. Leigh)

Towering Cumulus

General description

Cumulus clouds with considerable vertical growth in the form of rising mounds, domes or towers. They are of great vertical extent; their bulging upper part frequently resembles a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their base is relatively dark and nearly horizontal.

Common subdivisions

Towering Cumulus is an aviation specific cloud description. It is itself a subdivision of the Cumulus genus. WMO technical notes describe Towering Cumulus as Cumulus congestus of great vertical extent.

Distinguishing TCu from other genera

Towering Cumulus differs from Cumulonimbus in that the sprouting upper parts are sharply defined throughout, with no fibrous or striated texture apparent. Towering Cumulus is not accompanied by lightning and thunder.

Towering Cumulus is distinguished from Cumulus by its greater vertical extent. There is no internationally agreed criteria for TCu but the following guides can be useful:

  • The cloud will be at least 10000 feet tall from base to top; or
  • The height of the cloud is at least twice its width.

A continuous weather watch will sometimes reveal the transformation from Cumulus to Towering Cumulus to Cumulonimbus.

Associated precipitation

Precipitation in the form of showers of rain, snow, or snow pellets may occur with Towering Cumulus.


Towering Cumulus
Towering Cumulus (Photo: NOAA)
Towering Cumulus
Towering Cumulus (Photo: P. Leigh)
Towering Cumulus
Towering Cumulus (Photo: P. Toomey)

Cumulonimbus

General description

Heavy and dense cloud with considerable vertical extent in the form of a mountain or huge tower. At least part of its upper portion is usually fibrous or striated, often appearing as an anvil or vast plume. This appearance is due to the formation of ice particles in its upper part.

The base of the cloud appears dark and stormy. Low ragged clouds are frequently observed below the base and generally other varieties of low cloud, (Cu) and (Sc) are joined to or in close proximity to the Cumulonimbus. Lightning and thunder are characteristic of Cumulonimbus.

Common subdivisions

Cumulonimbus calvus is a species in which the sproutings of the upper part, whether partially or wholly, are more or less indistinct and flattened and have the appearance of a whitish mass without sharp outlines. No fibrous or striated parts are visible. This species is the transition between Towering Cumulus and the Cb species capillatus.

Cumulonimbus capillatus is characterised by an upper portion having cirriform parts of clearly fibrous or striated structure, a plume or a vast more or less disorderly mass of hair.

When in the shape of an anvil, the term incus is appended to the name as a supplementary feature, eg. Cumulonimbus capillatus incus.

Cumulonimbus does not present any varieties.

Distinguishing Cb from other genera

Cumulonimbus differs from Nimbostratus in that the precipitation is in the form of showers which can include hail, and may be accompanied by lightning and thunder. Also, Nimbostratus usually covers the sky for extended periods whereas Cumulonimbus is rarely extensive enough to cover the whole sky for very long.

Cumulonimbus is distinguished from Cumulus and Towering Cumulus in that the upper portion, at least in part, does not have clearly defined edges. It mostly appears fibrous or striated, frequently like an anvil or a vast plume. Lightning, thunder and hail only occur with Cumulonimbus.

Associated precipitation

Precipitation associated with Cumulonimbus is showers of rain, small hail, hail, snow or snow pellets, often heavy in nature.


Cumulonimbus capillatus
Cumulonimbus capillatus (Photo: P. Toomey)
Cumulonimbus capillatus
Cumulonimbus capillatus (Photo: P. Leigh)
Cumulonimbus calvus
Cumulonimbus calvus (Photo: P. Leigh)

Performing a Cloud Observation

A cloud observation consists of: 

  • Identifying the types of clouds present

  • Estimation of the amount of each cloud type

  • Estimation of the height of the cloud base for each cloud type

After the observation is performed, an observer must then consider any specific requirements regarding how the observation is disseminated to end users, i.e., via an aerodrome weather report, ATIS broadcast, plain language radiotelephony, etc.

Note: Cloud observations performed for inclusion in Aerodrome Weather Reports consider all cloud observed under the ‘celestial dome’. Alternative reporting methods such as the ATIS broadcast may exclude cloud observed beyond particular distances and heights based on industry requirements.

Identifying the types of Cloud present

When only one or two distinct types of cloud are observed no great difficulty should be experienced in determining the cloud types. The complexity of cloud observations increases with the number of cloud types present, if clouds are either forming or dissipating or transforming from one type to another, or in poor observing conditions such as low visibility or lack of illumination on a moonless night.

A systematic approach to identifying the observed clouds is needed, particularly when there are a number of types present. By first studying the sky for evidence of low cloud and recording these, then analysing the sky for middle and then high level cloud, a clear picture of the total cloud picture can be reported.

An awareness of the prevailing synoptic situation and the types of cloud patterns associated with various meteorological events will significantly assist in making accurate observations of cloud types.

When positive identification of clouds by their form and other physical characteristics is difficult, consideration should be given to other details that can aid in the identification. Some of these details include:

  • The height of the cloud

  • Cloud composition

  • The type of precipitation (if any) occurring at the time

  • Optical phenomena that may be present

  • Other factors affecting appearance.

The following pages expand upon these points.

Height levels - the height range of clouds

Clouds are generally encountered over a range of heights between the ground and about 60,000 feet.  By convention, the atmosphere is vertically divided into three levels: high, middle and low.  Each level is defined by the range of heights at which clouds of a certain type occur most frequently. When the height of a cloud is known, a choice can usually be narrowed down to the cloud types normally encountered at that height.

The height ranges for the levels in the table below are derived from the Bureau's Surface Observations Handbook.

Level

Cloud Type

Height ranges (Australia)

High

Cirrus

Cirrocumulus

Cirrostratus

Above 20,000 ft

Middle

Altocumulus

Altostratus2

Nimbostratus1,2

8,500 ft - 20,000 ft

Low

Stratocumulus

Stratus

Cumulus2

Towering Cumulus2

Cumulonimbus2

Below 8,500 ft

1              Nimbostratus frequently occurs with a base below 8,500 ft (6,500 ft in Antarctica).

2              These clouds may extend through two or all three étages  

Cloud types can occur outside these height ranges depending on location, season and influencing air mass. It is common, for instance, for the base of ‘low’ cumuliform clouds to occur well above 8500ft in inland Australia during the warmer months.

The WMO Cloud Atlas cloud height table below shows an overlap of the levels with their limits varying between polar, temperate and tropical regions of the world. Observers may find this table helpful in situations where the Australian region levels (above) are inadequate.

Level

Cloud

Polar region

Temperate region

Tropical region

High

Cirrus
Cirrocumulus
Cirrostratus

10 000 – 25 000 ft

16 500 – 45 000 ft

20 000 – 60 000 ft

Middle

Altocumulus
Altostratus
Nimbostratus

6 500 – 13 000 ft

6 500 – 23 000 ft

6 500 – 25 000 ft

Low

Stratus
Stratocumulus
Cumulus

Towering Cumulus
Cumulonimbus

Surface – 6 500ft

Surface – 6 500ft

Surface – 6 500ft


Cloud Composition

The measured temperature of a cloud throughout its vertical extent will indicate its likely composition as either water droplets or ice crystals, or a combination of the two.

Water droplets can be further classified as ordinary (or warm) water droplets, and supercooled water droplets; the latter being droplets colder than 0°C yet still in the liquid state.

The temperature at which the constituents of a cloud will exist as ice crystals depends on a number of complex factors. A simple empirical rule suggests the temperature of -20°C (and colder) can be used as a guide to indicate the predominance of ice crystals within a cloud.

Examination of an aerological diagram will show the temperature, or temperature range within an observed cloud; this temperature will give an indication of its predominant composition, which in turn can help with cloud identification.

The table below is derived from the International Cloud Atlas.

Cloud

Composition

Cirrus

Almost exclusively ice crystals.

Cirrocumulus

Almost exclusively ice crystals; strongly supercooled water droplets may occur but are usually rapidly replaced by ice crystals.

Cirrostratus

Mainly ice crystals.

Altocumulus

Almost invariably water droplets; when the temperature is very low, ice crystals may form.

Altostratus

Water droplets and ice crystals. In the most complete case, three superposed parts may be distinguished:

  • Upper part – wholly or mainly ice crystals

  • Middle part – mixture of supercooled water droplets and ice crystals

  • Lower part – wholly or mainly ordinary or supercooled water droplets.

Nimbostratus

As per Altostratus.

Stratocumulus

Water droplets; ice crystals may be present in extremely cold weather.

Stratus

Usually small water droplets; ice particles at low temperatures.

Cumulus

Towering Cumulus

Mainly water droplets; ice crystals may form in those parts with a temperature well below 0°C.

Cumulonimbus

Water droplets and, especially in its upper portion, ice crystals; the water droplets may be substantially supercooled.

Optical Phenomena

The following optical phenomena may aid in the identification of certain cloud types:

  • Halo - a luminous ring usually white in colour, with the sun or moon at its centre. Sometimes the inner edge of the ring is faint red in colour, and in rare cases, the outer edge is a faint violet colour. Inside the ring the sky is darker than outside it. This feature is usual for Cirrostratus, and may occur with other types in certain species under specific conditions.

  • Corona - one or more sequences of coloured rings of relatively small diameter, centred on the sun or moon. The outer edge is red, the inner blue. This feature can occur with Altocumulus, and with other types in certain species under specific conditions.

  • Irisation - colours appearing on clouds, sometimes in the form of bands nearly parallel to the margin of the cloud. Green and pink predominate, often with pastel shades. This feature can occur with Cirrocumulus, Altocumulus and Stratocumulus.

Clouds and Precipitation

Identifying the type of precipitation falling from a cloud can help identify the type of cloud present and vice versa. The following table summarises the associated precipitation for each cloud type.

In interpreting this table, the precipitation types listed indicate what the cloud is capable of producing. It must be remembered that some of the clouds that are capable of producing precipitation rarely ever do so; or that a particular species of a cloud genera may not give precipitation but another species of the same genera will.

Cloud

Associated Precipitation

Cirrus

Nil

Cirrostratus

Nil

Cirrocumulus

Nil

Altocumulus

Nil, unless castellanus; then light showers of rain or snow

Altostratus

Rain or snow or ice pellets

Nimbostratus

Rain or snow or ice pellets

Stratus

Drizzle or snow or snow grains

Stratocumulus

Rain or snow, of very light intensity; or drizzle

Cumulus

Showers of: rain or snow

Towering Cumulus

Showers of: rain, snow or snow pellets

Cumulonimbus

Showers of: rain, small hail, hail, snow or snow pellets

Factors affecting cloud appearance

The appearance of a cloud is affected by the amount of light that is reflected, scattered and transmitted by the cloud. This light comes mainly from the sun or moon or from the sky; it may also come from the surface of the earth, and is particularly strong when sunlight or moonlight is reflected by ice-fields or snow-fields.

Haze

When there is haze between the observer and the cloud it generally diminishes cloud brilliance. Haze reduces the contrasts which reveal the shape, structure and texture of a cloud. Haze makes distant clouds look yellow, orange or red.

Night-time

On a moonlit night, clouds are visible when the moon is more than a quarter full. In its darker phases the moon is not bright enough to reveal clouds far from it, especially thin clouds. On a moonless night, clouds are generally invisible but their presence may be deduced from some or all stars being concealed, or from artificial lighting (illumination from large cities, towns, fires, etc which tend to give the base of clouds an orange glow).

Sun or moon behind cloud

The further thin cloud is away from the sun or moon the darker the cloud will appear. With thick clouds there is only a slight change in appearance with distance.  Sometimes the edges of a thick cloud may be brilliantly illuminated. Thick Cirrus type clouds are always brilliantly white unless the sun or moon is behind them when they will show shading.

Cloud opposite sun or moon

Light is reflected from the cloud to the observer. The thicker the cloud the more light is reflected and the more brilliant the cloud appears. When sufficiently thick and deep, clouds can reveal shades of grey revealing more of the cloud profile.

Thick Cirrus type clouds are an exception to the above. They will appear brilliantly white and show no shading with the sun or moon opposite.

Sun high above the horizon

Clouds or portions of clouds in direct sunlight appear white or grey. Parts receiving light mainly from the blue sky (those closer to the ground) are bluish-grey. With weak illumination the clouds tend to take the colour of the surface below them.

Sun approaching the horizon

The colour of the sun may change from yellow through orange to red, and the sky in the vicinity of the sun and the clouds may show a corresponding colouration. The colours may still be influenced by the blue of the sky and the surface colour below, and the effect also varies with the height of the cloud.

Sun close to or below the horizon

High clouds may still look white whilst middle level clouds exhibit a strong orange or red coloration, and very low clouds, in the shadow of the earth are grey. These differences help to obtain an idea of the relative heights of the clouds.  Note, however, that clouds at the same level appear more red when they are seen away from the sun than when viewed toward it.

Determining cloud types at night

The following information may assist with cloud identification at night.

Cloud

With the Moon Present

With the Moon Absent

Cirrus

May be seen against the moon, level of moon light not reduced. Halo (ring around the moon with red on the inside) possible with thick cirrus.

Some stars bright, others hazy; illuminated before sunrise and after sunset and has a reddish glow. Do not confuse hazing of stars with mist or smoke.

Cirrostratus

Milky appearance around moon; possible halo; stars diffused and those near the moon possibly invisible.

All stars more or less dimmed and the outlines diffused.

Cirrocumulus

Thin cloud passing across the moon and not causing any blur of outline.

As for Cirrus.

Altocumulus

Small pieces of cloud passing across moon, but not obscuring it; edges thinner than centre; corona (ring around the moon with red on the outside) may be seen.

Stars blotted out in patches and disappearing and re-appearing regularly.

Altostratus

If thin, moon vaguely visible and remaining uniform as cloud moves; if thick, moon invisible, light rain possible.

If overcast, stars invisible; if broken, some stars visible but no regular appearance and disappearance. If thick and lowering, may be accompanied by rain; this stage will be indicated by preceding observations.

Nimbostratus

Moon invisible. Usually continuous rain.

No stars visible, usually with continuous rain.

Stratocumulus

Moon obscured for intervals.  Thin edges with moon visible through them; sides may be discernible.  Lower surface may be illuminated from below over towns.

As for AC, sometimes (but rarely) precipitation may be noticed. Winds usually light. Over towns/cities the under-surface often illuminated by lights.

Cumulus

Normally easily seen due to distinctive form. May be hard to discern if a layer of SC just above the cumulus cloud fills the intervals between the cumulus. Large Cu may be accompanied by showery precipitation.

Can be difficult to distinguish from AC or broken SC.  Variations in shading from reflected city lights can assist.

Towering Cumulus

Readily seen and distinctive form is visible. May be accompanied by showery precipitation.

As for CU.

Cumulonimbus

Not discernible unless at a distance; may be confused with TCU or layers of AS, unless accompanied by thunder and/or lightning.

As for TCU, except that thunder/lightning/hail may be observed.

Stratus

If thin, moon may be visible; May be discerned as a thin cloud moving rapidly across the moon, usually with cloud above.  Light winds, light drizzle; will reflect the lights of towns. If thick, moon invisible; drizzle, light winds.

If broken, stars are visible. Cannot be distinguished from AS unless accompanied by thick drizzle and light or calm winds; then it must be distinguished from SC. May reflect lights of towns, etc., and appears uniform. Stratus of bad weather is usually indiscernible because of cloud above.

Estimation of cloud amount

The total cloud cover is the fraction of the celestial dome covered by all the clouds observed. The term cloud amount in reference to a genus, a species, a variety, a layer, or a certain combination of clouds indicates the fraction of the sky cover by that genus, species, variety, layer or combination.

This fraction is represented by a figure equivalent to how many eighths or oktas of the sky is covered.

Oktas

Description

0         

sky completely clear

1         

from a trace of cloud up to 1/8

2         

more than 1/8 but not more than 2/8

3         

more than 2/8 but not more than 3/8

4         

more than 3/8 but not more than 4/8

5         

more than 4/8 but not more than 5/8

6         

more than 5/8 but not more than 6/8

7         

more than 6/8 but not total coverage i.e. if there is any sky visible then use 7/8

8         

sky completely overcast (no breaks or openings)

Note that cloud amounts are generally round up to the next okta. For example ‘2 and a bit’ oktas is rounded to 3 oktas. The exception is when more than 7 but less than 8 oktas is observed – in this instance cloud amount is rounded down to 7 oktas.

Dividing the sky into oktas (eighths) is fairly easy; one can imagine the sky divided into half, so that each half is equal to 4 oktas.  Dividing into half again gives an area of 2 oktas. It is then easy to see how much of the sky is equal to 1 okta. When the cloud is in a more or less continuous sheet or patch it is not difficult to assess its amount; however, if the cloud consists of several separated elements, it is necessary to imagine the amount of sky covered as if all the separate pieces were joined together.

An animation of the concept: (file size: 14MB )

Owing to the effects of perspective, gaps existing between clouds near the horizon may not be visible. When estimating amount in this instance, take into account only those gaps visible from the observing point.

Observing when the sky is partly obscured by fog, haze, etc

If cloud can be seen, estimate the amount as well as circumstances permit. If there is no evidence of cloud and the sun or stars can be seen through the fog, consider the sky to be completely clear of cloud.

Several layers of cloud

When clouds exist at different heights it may be necessary to determine the amount of cloud present at each level, even though the lower clouds may obscure some of the clouds in a higher layer.

With practice, and knowledge of the nature of the various cloud forms, the estimation of cloud amounts at different heights becomes relatively easy.  For example, if ragged low clouds exist below a layer of Nimbostratus it would be safe to assume the amount of Nimbostratus cloud as 8 oktas.  This can usually be confirmed by watching the sky for a short time, when the movement of the lower cloud will usually reveal any breaks or gaps that may exist in the higher layer. However do not make unconsidered guesses. The amount of cloud (or if necessary, the amount of each individual genera of cloud) at each level is determined as if no other clouds are present.

Assessing the amount during the night hours

Before commencing an observation at night, allow the eyes to become accustomed to the surrounding light.  The observation is then made in the same way as during the daylight hours.  On bright moonlight nights the observations can be made without difficulty.  When there is no moon it is more difficult but after conditioning the eyes to the dark, silhouettes of low cloud can be seen.  The blotting out of stars indicates the presence of clouds and clouds can be assumed with the presence of precipitation.  At times of thunderstorm activity, lightning may provide sufficient illumination to enable an estimate to be made.

Laser Ceilometer

Refer to the Laser Ceilometer section for details on the ceilometer's capability to provide cloud amount information.

Cloud amount for varying reporting requirements

While manual cloud observations performed for inclusion in METAR/SPECI reports consider all cloud within the celestial dome, information contained in other report and broadcast mechanisms may only consider the area associated with the probable arrival and departure flight paths of aircraft operating at the aerodrome. Observers must be familiar with any local requirements regarding their observations.

For dissemination to aeronautical users via the METAR/SPECI code and ATIS broadcast, the following conversions are applied for manually observed cloud amounts:

Oktas

Code

Decode

1 – 2

FEW

Few

3 – 4

SCT

Scattered

5 – 7

BKN

Broken

8

OVC

Overcast

Estimation of the height of the cloud base

The cloud base is the lowest zone in which the obscuration corresponding to a change from clear air or haze to water droplets or ice crystals causes a significant change in the profile of the backscatter extinction coefficient (WMO definition) – or more simply, the lowest level in the atmosphere where the air contains a perceptible quantity of cloud ‘particles’.

Cloud base height

The cloud base height is always expressed in terms of the height above the station level. This station is usually the observation point or the aerodrome reference point (ARP). For all reporting and broadcast mechanisms in Australia the unit used to express the height of cloud is feet.

When clouds are observed over distant hills, the height of cloud base still refers to the height above the observing point, and not the height above the hills.  For example, the cloud base may be only 500 feet above the tops of the hills, but if the tops of the hills are 3000 feet above the station level, then the cloud base is reported at 3500 feet.

With regular practice the height may be determined by visual estimation with a high degree of accuracy, especially for low clouds. The following pages outline various methods to assist with determining cloud height.

Local aids - Hills or high objects

Where clouds are seen over hills or high objects, such as radio masts or high buildings, the height of the cloud base can be compared with the known height of the tops of the hills or other objects. Similarly, when there are several layers of clouds and the height of one layer is estimated or known, the heights of other layers can be estimated.

It is important that all heights are referenced back to that above the observation station level.

In the example below, a topographical chart indicates the height of a nearby hill top to be 2000 ft above mean sea level. If the aerodrome elevation is 500 ft then the hill top is 1500 feet above the aerodrome. Further, the foot of the hill is at a similar elevation to the aerodrome. The observer estimates the distance between the base of the lowest cloud (Cumulus) and the hill top is approximately the same as the distance from the foot of the hill to its peak, 1500 ft. Another layer of cloud (Stratocumulus) is observed above the Cumulus. The base of this higher layer appears to be the same distant again from the Cumulus base, another 1500 ft. It can then be deduced that the Cumulus base is approximately 3000 ft (2 x 1500 ft) above the aerodrome, and the Stratocumulus 4500 ft (3 x 1500 ft) above the aerodrome.

Picture showing the use of visuals cues to determine cloud height
Using visual cues to determine cloud height

Pilot report

An accurate cloud height can be obtained via a pilot report. Bear in mind that aircraft usually operate with the altimeter barometric subscale set to QNH, so any information being relayed may likely be referenced to the height above sea level. Be sure to convert this information to the height above the station (ground level) for reporting/broadcasting where required.

Example:

A pilot reports entering an overcast cloud base on departure at 6500 feet “on aerodrome QNH”. This cloud base is 6500 feet above mean sea level. To convert this to the height above the aerodrome, the observer must subtract the aerodrome elevation. The aerodrome elevation is 1500 feet.

→ 6500ft - 1500ft = Cloud base 5000ft above the aerodrome.

Picture showing how cloud height is determined from a pilot report
Using a pilot report to determine cloud height

More about altimetry

Altimeter
Altimeter showing barometric subscale

An aircraft’s altimeter uses a similar operating principle to an aneroid barometer. On an altimeter, the pilot will set a known pressure in the barometric subscale (for a particular datum – usually mean sea level), and the pointers will indicate the height above that datum. When the mean sea level pressure (QNH) setting is used, the altimeter will indicate the height of the aircraft in feet above mean sea level (AMSL), or altitude as it is known.

To obtain the correct altitude, an accurate QNH must be entered or dialled on the barometric subscale on the altimeter. An error of 1hPa entered on the subscale will result with an approximate 30 foot error in the height indicated on the altimeter.

If the pilot of an inbound aircraft was notified that the QNH is 1031hPa when in fact it is actually 1013hPa, the altimeter would over-read by 540 feet; that is, the pilot would think the aircraft was 540 feet higher than it actually is. If the aircraft is operating close to terrain without visual reference to the surrounds, the consequences could be disastrous.

Accurate pressure information is vital for the safety of aircraft operations.

Dew point depression

Cumuliform cloud heights (convective clouds) - Surface air temperature and dew point

Clouds are generally formed by the air being cooled in some way to the dew point temperature of the air; just below this temperature the invisible water vapour is condensed into the visible water drops forming the cloud. This cooling is achieved in most cases by the air being lifted to a higher level; the rate of cooling of the air as it ascends is almost constant. It is a simple matter to calculate how far the air must rise for cloud to form, providing the initial temperature and dew point temperature are known.

If cumuliform cloud (Cu, TCu or CB) is forming above the observing location, the approximate cloud base height (in feet) can be calculated by the following formula:

Cumuliform cloud base = (Air temperature – Dew Point temperature) x 400

This method can be used for determining the approximate height of cumuliform clouds on a day when the air at ground level is warmed until it becomes light enough to rise upwards. When it reaches a level where it has cooled to its dew-point temperature, the cloud begins to form.

Example:

The surface air temperature is 22.3°C

The dew-point temperature is 11.3°C

Cumuliform cloud base = (Air temperature – Dew Point temperature) x 400

= (22.3 – 11.3) x 400

= 11 x 400

= 4,400 feet

If the surface air temperature is 22.3°C and the dew-point temperature is 11.3°C then the difference is 11°C. When the surface air is lifted 4400 ft (11 x 400) the air temperature would have fallen to the dew-point temperature of the rising air and condensation will take place.  This would be a good approximation of the cloud base.

The height of cloud base obtained in this way is a guide only. It is most accurate for Cumulus clouds being formed inland and in dry climate zones (especially in the afternoon).  For coastal or humid tropical areas in the morning a ‘x 300’ multiplication factor may be more suitable.

Aerological diagram

The presence of layer clouds on an aerological diagram is indicated by spikes or broader areas with a reduced dew point depression. The height of the base of the cloud can be determined from the vertical axis of the diagram. Further, the actual value of the dew point depression gives an approximation of cloud coverage:

Dew Point Depression

Cloud Cover

0 to 2°C

Overcast layer

3 to 5°C

Broken thick layers

6 to 10°C

Scattered thin layers

> 10°C

No layer cloud

As well as an indication of height, the aerological diagram will show the temperature of the cloud layer which can assist with identifying the cloud type. See the example below.


aerological diagram cloud height information

*Note: The height scale indicates heights above mean sea level in a standard atmosphere. Elevation must be taken into account when converting the height to above station level.

Aerological diagrams are generally available at capital city airports as well as a number of other key locations. Radiosondes are typically launched at 2315hrs UTC and are available on the Bureau’s external website a couple of hours later. Some locations also release a radiosonde at 1115hrs UTC. 

The aerological diagrams are available from the following web address: http://www.bom.gov.au/aviation/observations/aerological-diagrams/

More information about aerological diagrams can be found here: http://www.bom.gov.au/aviation/data/education/skew-t.pdf

Diurnal, seasonal, locational variations

Diurnal variations

The normal day and night variation of temperature affects the height of the cloud base of some clouds, mainly the low clouds.  The following remarks may be helpful:

  • Stratus tends to form at night, and to lift or disperse during the day.  Fog may form overnight and then lift during daylight to form stratus.

  • Stratocumulus usually has the same tendency to form at night and lift or disperse during the day.  However, this cannot be taken as a general rule, as on some occasions Stratocumulus will tend to thicken during the day, and the base becomes a little lower.

  • Cumulus often forms during the day and disperses late in the day or evening.  Cloud base usually gradually becomes higher during the day. Cumulus at night or early morning is not so common, except in tropical areas, or when associated with cold fronts or other weather systems.

Season and location variations

The height of clouds will vary with season and location; the variations are dependent on temperature and the water content of air or surface.  Although generalising here, this may be useful when an Observer moves from one location to another.

For a given latitude, clouds tend to be:

  • Lower in winter and higher in summer

  • Lower over the oceans/near the coast and higher inland

  • Lower over hills and higher over plains

Laser Ceilometer

Principle of operation

Vaisala CT25K Ceilometer

CT25K Ceilometer

This section describes the Vaisala CT25K, however other units generally employ a similar operating function.

The Vaislala CT25K ceilometer transmits a single vertical or near vertical-pointing laser pulse from the unit’s transmitter. This pulse will be reflected or scattered when it encounters cloud. Any reflected signal back towards the ceilometer will be sensed by the unit’s receiver. The height of cloud base can be determined using the speed of light, and the time delay between the launch of the laser pulse and the detection of the reflected signal.

The CT25K produces raw data at 15 second intervals (four pulses per minute), with a measurement range up to 25,000ft.

The raw data collected over a 30 minute period is processed by a Sky Condition Algorithm (SCA) in the AWS to produce estimates of cloud amount and height for up to three cloud layers. The data in the most recent 10 minute period is given a double weighting to produce a better response time in situations when cloud cover is changing rapidly.

Limitations and Interpretation of output

The raw data provided by the ceilometer presents cloud height information to observers as the cloud passes over the unit. The Sky Condition Algorithm reports can provide guidance for estimations of both cloud height and amount.

Due to the limitations of the ceilometer’s sampling procedure, and the algorithm’s processing procedure, staff should be mindful of the following when interpreting the SCA output:

  • When more than one cloud layer is detected, the cloud amount for any higher layer(s) will include the amount(s) of the lower layer(s) because the SCA assumes that any higher cloud will be obscured by lower layers. This will lead to an overestimation of cloud amount at higher layers in some cases.

  • Upper cloud layers will not be reported at all if they are entirely obscured by lower cloud directly above the ceilometer during the half hour sampling period.

  • The SCA can give incorrect observations when cloud of scattered or broken proportions is stationary or is slow moving. If scattered or broken cloud remains directly above the ceilometer for a period of time, it can result in an incorrect overcast report. On the other hand, if the ceilometer is directly under a clear patch of sky for the sampling period, it can result in an incorrect report of a clear sky.

  • Because the reported cloud amount is a temporally-based measurement producing a weighted average amount, there is an effective time lag of approximately 10 minutes built into the system. Thus, in a situation of rapid onset of broken or overcast stratus, the algorithm can take 10 minutes before it produces an output of broken cloud. Similarly, episodes of short-lived (less than 10 minutes) broken cloud may not be reported by the SCA.

  • The ceilometer will function normally in light precipitation, shallow fog and blowing dust or snow. However as these weather phenomena increase in intensity, a point will be reached where the ceilometer can no longer unambiguously identify the cloud base. In these instances the ceilometer will report the vertical visibility and this is processed by the SCA as an effective cloud base.

  • When the sky is dominated by convective (cumuliform) cloud, the bulging sides or sides and tops of leaning cloud masses can be incorrectly interpreted as being a cloud base. This may lead to the SCA providing a report of multiple cloud layers that do not actually exist.

  • The conversion from oktas to the terms FEW, SCT, BKN and OVC when reported via the SCA differ from those derived via a manual cloud observation:

Manual observation: FEW = 1 to 2 oktas; SCT = 3 to 4 oktas; BKN = 5 to 7 oktas; OVC = 8 oktas.

Ceilometer: SCT = 1 to 3 oktas; BKN = 4 to 6 oktas; OVC = 7 to 8 oktas. Note: FEW is not used.

Ceilometers are also installed at non-staffed sites to provide data where there would otherwise be none available.

Most display consoles will allow users to view both the raw ceilometer output (the cloud base height for each individual laser pulse) as well as the 30 minute SCA output. Below is an example of a typical display:

Ceilometer Display

Typical Console Display

The four figures on the top line are the raw data for the most recent minute, with the SCA processed data on the second line.

The One Minute Data (OMD) message extract also displays ceilometer information. The ceilometer output for each individual ‘laser pulse’ is given following the ‘CL:’ group identifier. The SCA output is given following the ‘CL30:’ group identifier. In the message below, the CL30 data is decodes as: 2 oktas at 2800ft, 6 oktas at 3400ft, 8 oktas at 6000ft.

OMD YMML 20141027 235247 DATE:20141028 TIME:1052 CL:03650/03550/03550/03400/99999 CL30: (02,028/06,034/08,060) VI:15404 VI10:8888 SWV:A6B15 MSG:4916/139/999/509

The Sky Condition Algorithm output also appears in the remarks (RMK) section of the METARAWS/SPECIAWS message and in the main body of the report for a METAR/SPECI AUTO. When interpreting cloud information in an automated report, be sure to take into account the variation in oktas conversion as described above.

SPECIAWS YBLT 280035 26021/31KT //// 13.5/09.2 1013.1 RMK WDM10:261 WSM10:021KT MWG10:031KT RF00.0/000.0/000.0 CLD:SCT019 SCT026 BKN036 VIS:9999 QFF:10130 BV:13.5 IT:21.4 VER:2.4.2 SWV:3.2 OID:SYSTEM/CCF1 SNT:201410280035 SP30/10/10/10/5/2/99/00/1500/5000/wg/WAP:10 MSG:1378/286/000/000

SPECI YBLT 280035Z AUTO 26021G31KT 9999 // SCT019 SCT026 BKN036 14/09 Q1013 RMK RF00.0/000.0

The SPECIAWS and OMD messages below from Sydney airport show the variation of the oktas conversion when processed via the SCA compared with a manual observation.

SPECIAWS YSSY 160130 30006/10KT 240V350 5000 2000N -RA 2ST005 3ST010 5CU018 15.5/14.1 1014.2 RMK WDM10:301 WSM10:006KT MWG10:010KT RF01.4/004.2/004.8 CLD:SCT006 SCT010 OVC018 VIS:1900 QFF:10142 BV:13.5 IT:24.1 VIS REDUCED TO N VER:2.4.1 SWV:2.16.4 OID:JBLOGGS/STANDBY SNT:201309160132 SP30/10/10/10/5/2/99/00/1500/7000/UI/VI/WAP:10 MSG:3371/351/000/000

OMD YSSY 20130916 012855 DATE:20130916 TIME:1130 CL:01600/01550/01350/01000/99999 CL30: (01,006/02,010/07,018) VI:02679 VI10:1800 SWV:A6B15 MSG:4855/139/999/492

Safety

The CT25K ceilometer uses an invisible vertical-pointing laser beam to obtain raw data and is classified, according to US and European standards, as a Class 1 laser device. This implies that they present no known biological hazard unless viewed with magnifying optics.

Despite the Class 1 classification, the Bureau takes the view that there is potential danger and accordingly advises staff to always avoid looking into the ceilometer. It is especially important not to look into the ceilometer with any form of magnifying glass, binoculars, telescopes, etc. as this may cause irreparable damage to the viewing eye. Any personnel who inadvertently look into the beam, with or without magnifying optics, should seek immediate medical advice.

(Note: While this section refers to the Vaisala CT25K Ceilometer, the information is generally applicable to other laser ceilometer units).

Further Considerations

  • Observers are advised to wear polarising sunglasses while performing observation during the day. This will increase the accuracy of the observation and reduce the potential for eye damage. Sunglasses make the observation of clouds easier, as well as minimising the dazzling effect of bright sunshine. As well very thin clouds are often invisible against a bright blue sky or a haze, except when viewed with such glasses.

  • At night, a viewing point well away from lights is essential, and the eyes must be given sufficient time to adapt to the darkness. This varies with Observers and circumstances, but it may take 5 minutes or longer to obtain adequate night vision for cloud observations.

  • Because of the continual evolution clouds go through it is necessary to keep an almost continuous watch on the sky. "Difficult" clouds can often be identified by recalling their recent history, during which they may have passed through a more easily recognisable phase. The greater difficulties in cloud observations are usually experienced when an observation is made having no knowledge of previous conditions.

  • Always observe the whole sky. Weather phenomena and their associated cloud could be in only one part of the sky such as a thunderstorm east of the aerodrome or conversely, stratus could be obscuring higher stratocumulus if you only look at part of the sky.

  • Before sunset, the Observer should spend a brief period studying and examining the existing cloud structure of the sky. This will assist considerably in the recognition of clouds after sunset. The clouds will not usually undergo significant change after sunset (unless a frontal system is approaching), however cumulus clouds tend to dissipate at sunset, as generally they require daytime heating to form and be maintained.

  • The first observation after sunrise may require revision of cloud heights, amount and types.

Further Reading

International Cloud Atlas

The WMO International Cloud Atlas describes all aspects of cloud identification in great detail. The latest edition is an online resource:


An older printed edition is still available from the WMO in pdf format.

  • Volume 1 is a text document.

  • Volume 2 is a pictorial guide.

pdf icon International Cloud Atlas - Volume I pdf icon International Cloud Atlas - Volume II

Cloud Atlas Vol 1 and 2 books
Cloud Atlas Vol I and II