PRINCIPLES OF AERODROME WEATHER OBSERVATIONS
Course Notes
BUREAU OF METEOROLOGY TRAINING CENTRE 08AUG2018 Contents
Cloud Observations ...... 1 Definition of a cloud ...... 1 Common terms used when making cloud observations ...... 2 Cloud Classification ...... 3 Table of Classification of Clouds ...... 4 The 11 basic cloud types ...... 5 Observing conditions to which the cloud descriptions apply ...... 6 Cirrus: Ci ...... 7 Cirrocumulus: Cc ...... 8 Cirrostratus: Cs ...... 9 Altocumulus: Ac ...... 10 Altostratus: As ...... 11 Nimbostratus: Ns ...... 12 Stratocumulus: Sc ...... 13 Stratus: St ...... 14 Cumulus: Cu ...... 15 Towering Cumulus: TCu ...... 16 Cumulonimbus: Cb ...... 17 Performing a cloud observation ...... 18 Identifying the types of cloud present ...... 18 Cloud levels – the height range of clouds ...... 19 Cloud composition ...... 21 Optical phenomena associated with clouds ...... 22 Clouds and precipitation ...... 22 Factors affecting the appearance of clouds ...... 23 Determining cloud types at night ...... 24 Estimation of cloud amount ...... 25 Estimation of the height of the cloud base ...... 26 Laser Ceilometer – Cloud Height and Amount ...... 32 Cloud observations - Further considerations ...... 34 Further reading ...... 34 Visibility Observations ...... 35 Definition ...... 35 Factors affecting visibility ...... 35 Weather phenomena and visibility ...... 36 Selection of visibility markers ...... 36 Procedure for making visibility observations ...... 38 Visibility terminology ...... 39 Visibility Meter ...... 40 Weather Observations ...... 42 Definitions ...... 42 Precipitation phenomena ...... 43 Precipitation Intensity - TBRG ...... 44 Drizzle DZ ...... 44 Rain RA...... 46 Freezing Rain FZRA and Freezing Drizzle FZDZ...... 46 Snow SN ...... 47 Hail GR ...... 47 Small Hail/Snow Pellets GS...... 48 Snow Grains SG ...... 48 Ice Pellets PL ...... 48 Weather watch radar and precipitation identification ...... 49 Approximate rainfall rates ...... 49 Rain bands from Altostratus and Nimbostratus ...... 49 Showers from cumuliform clouds ...... 50
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Heavy precipitation from thunderstorms ...... 50 Obscuration phenomena ...... 51 Fog FG ...... 51 Fog Patches BCFG ...... 51 Partial Fog (Fog Bank) PRFG ...... 51 Shallow Fog MIFG ...... 51 Freezing Fog FZFG ...... 52 Mist BR ...... 52 Smoke FU ...... 52 Haze HZ ...... 52 Dust DU ...... 52 Drifting Dust DRDU ...... 52 Drifting Sand DRSA ...... 53 Blowing Dust BLDU ...... 53 Blowing Sand BLSA ...... 53 Drifting Snow DRSN ...... 53 Blowing Snow BLSN ...... 53 Volcanic Ash VA ...... 53 Other phenomena ...... 54 Dust/Sand Whirls (Dust devil) PO ...... 54 Funnel Cloud FC ...... 54 Squall SQ ...... 54 Thunderstorm TS ...... 54 Duststorm DS ...... 54 Sandstorm SS ...... 54 Present Weather Sensor – Vaisala FD12P ...... 55
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Cloud Observations
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.
Definition of a cloud
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.
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Common terms used when making cloud observations
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.
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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 cloud. A single cloud may simultaneously bear multiple varieties and supplementary features and accessory clouds.
The following is an example of the concept:
The various cloud classifications are summarised in the WMO Table of Classification of Clouds on the next page.
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Table of Classification of Clouds
Supplementary Accessory Mother-clouds and Species Varieties Genera features clouds special clouds
(listed by frequency of observation) Genitus Mutatus Cirrus fibratus intortus mamma Cirrocumulus Cirrostratus uncinus radiatus fluctus Altocumulus Homo spissatus vertebratus Cumulonimbus castellanus duplicatus Homo floccus Cirrocumulus stratiformis undulatus virga - Cirrus lenticularis lacunosus mamma Cirrostratus castellanus cavum Altocumulus floccus Homo Cirrostratus fibratus duplicatus - Cirrocumulus Cirrus nebulosus undulatus Cumulonimbus Cirrocumulus Altostratus Homo Altocumulus stratiformis translucidus virga Cumulus Cirrocumulus lenticularis perlucidus mamma Cumulonimbus Altostratus castellanus opacus cavum Nimbostratus floccus duplicatus fluctus Stratocumulus volutus undulatus asperitas radiatus lacunosus Altostratus - translucidus virga pannus Altocumulus Cirrostratus opacus praecipitatio Cumulonimbus Nimbostratus duplicatus mamma undulatus radiatus Nimbostratus - - praecipitatio pannus Cumulus Altocumulus virga Cumulonimbus Altostratus Stratocumulus Stratocumulus stratiformis translucidus virga Altostratus Altocumulus lenticularis perlucidus mamma Nimbostratus Nimbostratus castellanus opacus praecipitatio Cumulus Stratus floccus duplicatus fluctus Cumulonimbus volutus undulatus asperitas radiatus cavum lacunosus Stratus nebulosus opacus praecipitatio Nimbostratus Stratocumulus fractus translucidus fluctus Cumulus undulatus Cumulonimbus Homo Silva Cataracta Cumulus humilis radiatus virga pileus Altocumulus Stratocumulus mediocris praecipitatio velum Stratocumulus Stratus congestus arcus pannus Flamma fractus fluctus Homo tuba Cataracta Cumulonimbus calvus - praecipitatio pannus Altocumulus Cumulus capillatus virga pileus Altostratus incus velum Nimbostratus mamma flumen Stratocumulus arcus Cumulus murus Flamma cauda Homo tuba
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The 11 basic cloud types 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.
The notes will refer to the 10 cloud genera plus Towering Cumulus as the 11 basic cloud types. These 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 level of knowledge required 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 ways depending on the reporting format being used. For instance:
• Twenty-seven variations and combinations of clouds can be reported in a synoptic cloud observation
• The 11 basic cloud types are reported in an aerodrome weather report
• Some reports transmit 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. The ATIS broadcast is such an example.
• 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. The development of these skills will be assisted with further knowledge of some of the more common species and varieties.
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Observing conditions to which the cloud descriptions apply
The general description of each of the cloud types on the following pages, unless otherwise specified, assumes the observations are 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. Thorough knowledge of the relationship between clouds and precipitation is vital for accurate cloud observations. Further details of precipitation types are covered in a later topic.
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Cirrus: Ci 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 fibratus
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 that has originated from the remains of the icy anvil of a Cumulonimbus cloud is known as Cirrus spissatus cumulonimbogenitus.
Distinguishing Ci from other genera
Cirrus is distinguished from Cirrostratus by its Cirrus spissatus 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 uncinus
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Cirrocumulus: Cc 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, 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 with other cirriform cloud 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
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Cirrostratus: Cs 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 Cirrostratus nebulosis 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 fibratus 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 fibratus (sun low on the horizon)
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Altocumulus: Ac 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. Altocumulus stratiformis perlucidus
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 Altocumulus castellanus 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 lenticularis 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.
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Altostratus: As 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. Altostratus opacus 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 translucidus 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, Altostratus opacus rolls, etc.
Associated precipitation
The precipitation associated with Altostratus is (non-showery) rain or snow or ice pellets, often of an intermittent nature.
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Nimbostratus: Ns 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 Nimbostratus 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 Nimbostratus 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
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Stratocumulus: Sc 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 Stratocumulus cumulogenitus (with Cumulus)
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. Stratocumulus stratiformis perlucidus
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 estimated 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 (from above)
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 – a non-WMO convention).
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Stratus: St 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 Stratus nebulosus ragged shreds. This species is known as Stratus fractus. It usually forms beneath the base of higher precipitating cloud. In such as 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 nebulosus
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 fractus beneath Altostratus
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Cumulus: Cu 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 humilis 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. Cumulus mediocris 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 Cumulus congestus congestus; precipitation is more common though when the cloud is of great vertical extent (Towering Cumulus).
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Towering Cumulus: TCu 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 Towering Cumulus 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. A continuous weather watch will sometimes reveal the transformation from Cumulus to Towering Cumulus to
Cumulonimbus. Towering Cumulus Associated precipitation Precipitation in the form of showers of rain, snow, or snow pellets may occur with Towering Cumulus.
Towering Cumulus
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Cumulonimbus: Cb 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. Cumulonimbus capillatus
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 Cumulonimbus capillatus 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 Cumulonimbus calvus 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, hail or snow, often heavy in nature.
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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 report, 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 certain 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 and poor observing conditions, e.g. low visibility or lack of illumination on a moonless night.
A systematic approach to identifying the observed clouds will avoid confusion, particularly when there are several 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 have been observed at the time • Other factors affecting appearance
The following pages expand upon these points.
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Cloud 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) Cirrus High Cirrocumulus Above 20,000 ft Cirrostratus Nimbostratus1,2 Middle Altostratus2 8,500 ft - 20,000 ft Altocumulus Stratocumulus Stratus Low Cumulus2 Below 8,500 ft Towering Cumulus2 Cumulonimbus2
1 Nimbostratus frequently occurs with a base below 8,500 ft.
2 These clouds may extend through two or all three levels
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
Cirrus High Cirrocumulus 10 000 – 25 000 ft 16 500 – 45 000 ft 20 000 – 60 000 ft Cirrostratus Altocumulus Middle Altostratus 6 500 – 13 000 ft 6 500 – 23 000 ft 6 500 – 25 000 ft Nimbostratus Stratus Stratocumulus Low Cumulus Surface – 6 500ft Surface – 6 500ft Surface – 6 500ft Towering Cumulus Cumulonimbus
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The allocated levels of the cloud types are summarised in the diagram below.
Source: WMO Cloud Atlas. Note this diagram does not show TCu.
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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 (or 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 following table is derived from the WMO Cloud Atlas.
Cloud Composition
Cirrus Almost exclusively ice crystals.
Almost exclusively ice crystals; strongly supercooled water droplets may occur Cirrocumulus but are usually rapidly replaced by ice crystals.
Cirrostratus Mainly ice crystals.
Almost invariably water droplets; when the temperature is very low, ice crystals Altocumulus may form.
Water droplets and ice crystals. In the most complete case, three superposed parts may be distinguished: Altostratus • 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 Mainly water droplets; ice crystals may form in those parts with a temperature Towering Cumulus well below 0°C.
Water droplets and, especially in its upper portion, ice crystals; the water Cumulonimbus droplets may be substantially supercooled.
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Optical phenomena associated with clouds
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 never give precipitation.
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
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Factors affecting the appearance of clouds
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.
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Determining cloud types at night
Cloud Moon Present Moon Absent
Ci May be seen against the moon, level of Some stars bright, others hazy; illuminated moon light not reduced. Halo (ring around before sunrise and after sunset and has a the moon with red on the inside) possible reddish glow. Do not confuse hazing of stars with thick cirrus. with mist or smoke.
Cs Milky appearance around moon; possible All stars more or less dimmed and the halo; stars diffused and those near the outlines diffused. moon possibly invisible.
Cc Thin cloud passing across the moon and As for Cirrus. not causing any blur of outline.
Ac Small pieces of cloud passing across Stars blotted out in patches and disappearing moon, but not obscuring it; edges thinner and re-appearing regularly. than centre; corona (ring around the moon with red on the outside) may be seen.
As If thin, moon vaguely visible and remaining If overcast, stars invisible; if broken, some uniform as cloud moves; if thick, moon stars visible but no regular appearance and invisible, light rain possible. disappearance. If thick and lowering, may be accompanied by rain; this stage will be indicated by preceding observations.
Ns Moon invisible. Usually continuous rain. No stars visible, usually with continuous rain.
Sc Moon obscured for intervals. Thin edges As for AC, but sometimes drizzle may be with moon visible through them; sides may noticed. Winds usually light. Over towns/cities be discernible. Lower surface may be under-surface often illuminated by lights. illuminated from below over towns.
Cu Normally easily seen due to distinctive Can be difficult to distinguish from AC or form. May be hard to discern if a layer of broken SC. Variations in shading from SC just above the cumulus cloud fills the reflected city lights can assist. intervals between the cumulus. Large Cu may be accompanied by showery precipitation.
TCu Readily seen and distinctive form is As for CU. visible. May be accompanied by showery precipitation.
Cb Not discernible unless at a distance; may As for TCU, except that thunder/lightning/hail be confused with TCU or layers of AS, may be observed. unless accompanied by thunder and/or lightning.
St If thin, moon may be visible; May be If broken, stars are visible. Cannot be discerned as a thin cloud moving rapidly distinguished from AS unless accompanied across the moon, usually with cloud by thick drizzle and light or calm winds; then above. Light winds, light drizzle; will it must be distinguished from SC. May reflect reflect the lights of towns. If thick, moon lights of towns, etc., and appears uniform. invisible; drizzle, light winds. Stratus of bad weather is usually indiscernible because of cloud above.
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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. 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.
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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.
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
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 droplets or ice crystals.
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.
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Visual estimation of the height of the cloud base An Observer is generally required to make visual estimates of the cloud base height in feet rounded down to the nearest hundred feet. For example a cloud base height of 960 feet would be rounded down and reported with a base at 900 feet.
With regular practice the height may be determined with a high degree of accuracy, especially for low clouds. The beginner, however, may experience some difficulty in estimating height, and the following paragraphs outline various methods which will help.
Local aids to cloud height estimation - 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 known, the heights of other layers can be estimated. It is important that the observer references all heights back to that above the observation station level.
Example:
A topographical chart indicates the height of a nearby hill top to be 2000 ft above mean sea level. The aerodrome elevation is 500 ft which means the hill top is 1500 feet above the aerodrome. 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.
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Pilot report Cloud height can be obtained via a pilot report. Keep 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.
More about altimetry An aircraft’s altimeter operates using similar principles to that of 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 seal level pressure (QNH) setting 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 a 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.
Altimeter showing barometric subscale
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Diurnal variations of cloud base height 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 of cloud base height 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
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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 = (22.3 – 11.3) x 400
= 11 x 400
= 4,400 feet
Explanation: 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.
Stratiform cloud heights (layer clouds) – 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. The following guide relates the dew point depression with typical observed 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 over for an example.
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Aerological diagram showing the cloud base height for several layers
Approx cloud base: 30,000ft
Temperature colder than - 40°C → cirriform cloud (ice crystals)
Approx cloud base: 13,000ft
Temperature - 5 to - 12°C, a few thousand feet thick → likely Altocumulus
Low cloud base – Height of cloud determined
by its thickness beneath the inversion, may be below 1,000ft; lower ragged patches also possible
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/
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Laser Ceilometer – Cloud Height and Amount
This section describes the Vaisala CT25K, however other units generally employ a similar operating function.
The Vaisala 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. Vaisala CT25K Laser Ceilometer
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.
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• 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.
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:
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:
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
In the above message, the CL30 data is decodes as: 2 oktas at 2800ft, 6 oktas at 3400ft, 8 oktas at 6000ft.
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
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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.
Cloud observations - 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 The WMO International Cloud Atlas describes all aspects of cloud identification in great detail. The latest edition is an online resource:
• https://cloudatlas.wmo.int/home.html
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Visibility Observations
The accurate reporting of visibility is a vital component of an observation. Pilots use information on reported visibility to assist in making critical operational decisions. The observer's responsibility in this respect cannot be over emphasised.
Definition
International Meteorological Vocabulary, WMO No. 182:
The greatest distance at which a black object of suitable dimensions (located on the ground) can be seen and recognised against the horizon sky, or, in the case of night observations, could be seen and recognised if the general illumination were raised to the normal daylight level.
ICAO Annex 3 - Meteorological Service for International Air Navigation offers some additional information regarding visibility at night:
The greatest distance at which:
a) a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background; and b) lights in the vicinity of 1000 candelas can be seen and identified against an unlit background.
The key concept for visibility observations is that it is a measure of the transparency of the atmosphere and does not depend on the general illumination level. Variations in visibility between day and night do not occur except when a change occurs in the transparency of the atmosphere i.e. in the number of 'obscuring particles' between the observer and visibility markers being used.
When determining the visibility, it is not sufficient merely to see the object; it must be recognised as well. An observer may, because of their knowledge of the locality, recognise an object which can just be seen, but which would not be identified by an observer unfamiliar with the surroundings. Such recognition should not be allowed to affect the visibility observation.
Factors affecting visibility The distance at which an object can be seen depends on:
(a) the transparency of the atmosphere, i.e., the number of obscuring particles present; (b) the position of the sun; (c) the contrast between the object and its background (by day); (d) the general level of illumination (by night); and (e) the size of the object.
An observer must eliminate the effects of (b) to (e) above so that the reported visibility will depend solely on atmospheric transparency (a).
The effect of the sun is significant at sunrise and sunset and can be eliminated by viewing objects at 90 degrees or more from the sun’s position. Suitable selection of visibility makers will eliminate issues regarding contrast, illumination and size.
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Weather phenomena and visibility The following phenomena will affect visibility to a varying extent depending on the intensity:
• Mist • Fog • Haze • Smoke • Dust • Duststorms and Sandstorms • Volcanic Ash • Precipitation
Some weather phenomena are defined by their effect on visibility.
For example: Fog - A suspension of very small, usually microscopic water drops in the air, reducing visibility to less than 1000 metres at the Earth’s surface.
Selection of visibility markers Daylight observations Markers should be well distributed in azimuth so that visibility in different directions can be estimated, and should comply with the following requirements:
• Markers should be black or dark with a horizon background. If there are insufficient objects to comply with this, dark objects having a terrestrial background may be selected. The object however, must stand out from the background for a distance of at least half that between the object and the observer;
• The object size should subtend an angle of not less than ½ a degree and not more than 5 degrees from the observer’s position. (The width of half a finger and three fingers held at arm’s length subtend angles of approximately ½ and 5 degrees respectively).
Night observations The most suitable markers for determining the visibility at night are unfocussed light sources of ‘moderate’ intensity at known distances. The following details are specified:
• Unfocussed incandescent lights of approximately 40 watts and never greater than 100 watts are suitable for estimates up to 2000 m; lights between 100 and 250 watts are suitable for estimates in excess of 2000 m;
• White lights should be selected where practicable, but green and red may be used as an alternative;
• Avoid selecting lights in a direction which have excessive background glare from the illumination of towns and cities.
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Plan of visibility markers When making observations for visibility, an observer should have access to a plan of the location of suitable visibility markers relative to the observing location. The Bureau uses the F401 (below) for this purpose.
If a plan of visibility markers is not available, a topographic chart or satellite image may help determine the distance from the observing point of suitable markers. When using these methods, special care must be taken to ensure the accuracy of the information.
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Procedure for making visibility observations Daylight observations The following procedure is to be followed when making daylight observations:
• Study the whole horizon and select the furthest object that can be seen and identified in each direction with the aid of a plan of markers.
• If a visibility marker is just recognisable, then the distance to that marker is the visibility in that direction. In practice though, the visibility may lie somewhere between the distances of two markers; in this instance an estimate of the visibility must be made of the distance at which an imaginary suitable marker could be seen and recognised, based on the relative clarity and distance of the two known markers.
• The estimated visibility is not to be limited by the distance of the furthest available marker. If for example, the furthest visibility marker is clear with sharp outlines and relief, with little or no blurring of colours, the visibility may be determined to be twice the distance to that marker.
• When the visibility varies with direction, any sector reductions may also be included depending on the reporting type.
Night observations The procedure for night observations is the same as for day observations, with the following additional requirements:
• Allow the eyes to become accustomed to the dark for at least two minutes. Direct vision becomes fully adjusted after this time whereas indirect vision (looking a little to one side of a light) will continue to increase in sensitivity for several hours while exposed to the dark. Observers must make sure they regard a marker (light source) as visible only if they can see it when looking directly at it.
• Where the number of lights is limited, silhouettes of hills or mountains and the brilliance of stars near the horizon are useful aids.
• When available, use the output from the visibility meter to assist.
Units for reporting visibility
The unit of measurement for reporting visibility varies depending on the reporting mechanism. The following generalisations can be made with respect to reporting visibility for aviation purposes:
• An Aerodrome Weather Report indicates visibility in metres, with 9999 indicating a visibility of 10km or greater. • In a Take-off and Landing Report, the visibility is typically broadcast in metres up to and including 5000m, and in kilometres above that value. A visibility ‘greater than 10km’ may be indicated when appropriate.
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Visibility terminology
Depending of the purpose of the observation it may be necessary to determine and report more than one type of visibility. Following is an example that describes the procedure used to establish the minimum, maximum and prevailing visibility.
Minimum Visibility Minimum visibility is the least sector visibility around the horizon. In the example, 2000 m
Maximum Visibility Maximum visibility is the greatest sector visibility around the horizon. In the example, 20 km
Prevailing Visibility Prevailing Visibility is the greatest sector visibility that prevails over half or more of the horizon.
To determine Prevailing Visibility, first establish the sector with the maximum visibility. Does this sector prevail over half or more of the horizon?
If not, then add the sector with the next- greatest visibility. (They do not have to be adjacent to one another). Do these two sectors together cover half or more of the horizon?
If not, keep adding the sector with the next- greatest visibility until the sectors cover half or more of the horizon.
The Prevailing Visibility is lowest sector visibility within these combined sectors.
In this case the prevailing visibility is 08 km.
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Visibility Meter
This section describes the Vaisala FD12, however other models of forward scatter visibility meters generally employ a similar operating function.
The Vaisala FD12 Visibility Meter measures air clarity using the principle of forward scattering of light. Infra- red light is transmitted into a scattering volume (approximately 1 litre) which is viewed by a receiver. The amount of light received is expressed as a measure of visibility, which is output by the instrument every 15 seconds.
The transmitter and receiver showing the principle of forward scattering
Vaisala FD12 Visibility Meter
An algorithm collects a set of 40 observations taken at 15 second intervals over a 10 minute period to produce a visibility estimate. This visibility estimate is biased towards the lowest reports of visibility received during the 10 minute sampling period.
Limitations and Interpretation of output Automated visibility data can provide guidance to observers with their estimations of visibility. Observers should be mindful of the following when interpreting output from the visibility meter and visibility algorithm:
• The more uniform the weather, the more accurate and representative of the entire airport is the visibility meter measurement; ‘patchy’ visibility reductions may go undetected.
• The algorithm uses data collected over a 10 minute period, resulting in a time lag; there will be a delay in the accurate reporting by the visibility meter when rapid changes in visibility occur.
• The human eye can face physical limitations such as viewing angle, contrasts and individual eye response, in determining the representative visibility. As the visibility meter is not affected by these limitations, there will be times when the automated parameter and the observer’s estimation will be different.
Visibility meters are also installed at non-staffed sites to provide observations where otherwise none would be available.
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Most display consoles allow users to view both one-minute output as well as the 10-minute processed output. Below is an example of a typical display. The one-minute visibility (Vis) is in metres.
The One Minute Data (OMD) message extract also displays visibility information. The visibility meter output for the previous minute is given following the ‘VI:’ group identifier. The algorithm output based on the previous 10 minutes follows the ‘VI10:’ group identifier. Note that in the OMD format, 8888 has the same meaning as 9999 in the TAF/METAR format (visibility 10km or more):
OMD YMML 20141028 010048 DATE:20141028 TIME:1200 CL:05650/05000/04550/04450/99999 CL30:(03,037/04,042/07,064) VI:34135 VI10:8888 SWV:A6B15 MSG:5052/139/999/500
The 10 minute 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:
METARAWS YBDG 280100 29013/19KT //// 17.0/07.6 1014.0 RMK WDM10:285 WSM10:013KT MWG10:019KT RF00.0/000.0/000.0 CLD:BKN039 BKN049 OVC054 VIS:8000 QFF:10139 BV:13.6 IT:32.8 VER:2.4.2 SWV:3.2OID:SYSTEM/CCF1 SNT:201410280101 SP30/10/10/10/5/2/99/00/1695/6000/WAP:10 MSG:9197/283/000/000
METAR YBDG 280100Z AUTO 29013KT 8000 // BKN039 BKN049 OVC054 17/08 Q1014 RMK RF00.0/000.0
(Note: While this section refers to the Vaisala FD12 Visibility Meter, the concepts are generally applicable to other forward scatter visibility meter units. The operating principles of other automated visibility sensors such as transmissometers that measure the extinction coefficient of the atmosphere vary significantly).
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Weather Observations
Definitions Most weather phenomena observed are composed of meteors. The International Cloud Atlas defines a meteor as:
A phenomenon observed in the atmosphere or on the surface of the earth, which consists of a suspension, a precipitation, or a deposit of aqueous or non-aqueous liquid or solid particles, or a phenomenon of the nature of an optical or electrical manifestation.
For weather observations the term weather phenomena refers to the following types of meteors:
• Hydrometeors – consisting of liquid or frozen water particles, either suspended in or falling through the atmosphere, or raised by wind from the ground;
• Lithometeors – consisting of mostly solid and non-aqueous particles, either suspended in the air or raised by the wind from the ground;
• Electrometeors – consisting of a visible or audible manifestation of electricity.
Photometeors, (luminous phenomena), although sometimes associated with weather phenomena, such as a rainbow during a shower of rain, are not themselves defined as a type of weather. Likewise clouds, which are a hydrometeor, are not categorised as weather phenomena for observing purposes. Furthermore, some weather phenomena, such as a squall, do not fall under the definition of a meteor.
This topic places weather phenomena into three broad categories:
• Precipitation phenomena • Obscuration phenomena • Other phenomena
Weather phenomena may also include the use of a descriptor. A descriptor is used to indicate additional characteristics associated with the phenomenon. For example the weather phenomenon fog will include the descriptor shallow if it extends to a height of not greater than two metres above the ground.
The weather phenomena and descriptors in this topic are generally applicable to all forms of Aviation Observations and Reports. For some phenomena and descriptors, observers may need to consider additional requirements for the inclusion of the weather in various reporting or broadcast formats. These requirements, for example, may relate the distance of the phenomenon from the observing point, or the visibility reduction associated with it.
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Precipitation phenomena
Precipitation consists of particles of liquid or frozen water normally formed within a cloud and falling to the ground. The character of precipitation can be described as intermittent, continuous, or as showers. Which of these descriptions is used depends on the cloud type from which the precipitation falls.
Showers are associated with cumuliform clouds (Cu, TCu, Cb, Ac). Precipitation from these cloud types is preceded with the descriptor showers of; Examples: shower of rain, shower of snow.
Due to the isolated nature of these clouds there is usually at least a partial clearing of the sky between the cumuliform clouds so that a break is visible. Showers are generally characterised by rapid changes of intensity and the suddenness with which they start and stop. Showers are also associated with sudden short changes in wind speed and direction (due to the down draft).
Showers are usually short-lived, most often less than 15 minutes. They may also occur in combination with intermittent or continuous precipitation. In these cases the showers are indicated by the sudden increases and decreases precipitation intensity. A visual indication of the associated cumuliform cloud may also be apparent.
Intermittent or continuous precipitation falls from stratiform clouds (As, Ns, St, Sc) which cover the whole sky or nearly so.
• Intermittent applies to precipitation, other than a shower, when:
o it began within the hour before the time of observation whether there has been a break or not; or
o it began an hour or more before the time of observation but there has been, during the last hour, a break of at least ten minutes or a number of breaks which add up to at least ten minutes.
• Continuous applies to precipitation in all other instances not covered by the definitions of showers or intermittent.
The cloud associated with continuous precipitation will usually be more dense than that producing intermittent precipitation. In either case, there may be considerable variation in the density of the layers. Intermittent and continuous precipitation are characterised by gradual changes of intensity.
Intermittent and continuous precipitation are sometimes collectively referred to as non-showery precipitation or uniform precipitation.
Note: The terms intermittent and continuous are generally not specified in weather reports made for aeronautical purposes. However, these terms are useful to help distinguish between cloud genera with similar characteristics. For example, continuous precipitation is more common from Nimbostratus than Altostratus.
Note: The ‘start-stop’ nature of showery precipitation must not be mistaken for the breaks associated with intermittent precipitation. An observer must recognise the importance of identifying the cloud type producing the precipitation so that an accurate weather description can be given.
Note: When the falling liquid or frozen water drops evaporate prior to reaching the ground the term ‘virga’ is used.
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Precipitation Intensity - TBRG
The Tipping Bucket Rain Gauge (TBRG) consists of a collector and a funnel fitted with a filter and syphon, mounted on a cylindrical housing containing a measuring unit.
The measuring unit consists of two buckets mounted either side of a pivot. The collected water is funnelled into one of the buckets. Once the equivalent of 0.2mm of precipitation has been collected the bucket will 'tip' due to the weight of the water.
Interpretation of output Observers can view the accumulated precipitation in the previous 10 minute period on most display consoles, which, when multiplied by six, gives an hourly equivalent rainfall rate. This calculated rate can be used to determine rainfall intensity (light/moderate/heavy) for widespread rain, and showers of rain that are observed to be falling over the TBRG.
Some precipitation types, such as drizzle, are very slow to accumulate in the tipping buckets and may not register a tip for a considerable period of time. In such an instance it is not practical to establish an hourly equivalent rate to determine the intensity; other methods (such as the effect on visibility) are used where necessary.
Bear in mind the random nature of showers of rain and the variation in their size often means a shower may occur at the aerodrome but not be recorded by the TBRG. This is heightened when considering the area defined as ‘the aerodrome’ can be quite large – out to a radius of 8km for an Aerodrome Weather Report, for example.
Measuring Unit and Data Logger
Tipping Bucket Rain Gauge
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Drizzle DZ
A fairly uniform precipitation composed exclusively of fine droplets of water very close to one another. Drizzle droplets are so small that their individual impact on a water surface is imperceptible. The diameter of each droplet is usually less than 0.5mm.
Drizzle falls as either intermittent or continuous precipitation from Stratus and Stratocumulus clouds and is classified by intensity. Drizzle is associated with very low cloud bases and poor visibility.
Using rainfall rates to determine intensity of drizzle 1
Description Rate of Fall Light Up to and including 0.2 mm per hour Moderate Greater than 0.2 mm and up to and including 0.4 mm per hour Heavy Greater than 0.4 mm per hour 2
Note 1: Due to the limitations of measuring the slow rate of droplet accumulation for drizzle using a rain gauge/TBRG, drizzle intensity is best determined by the associated visibility reduction rather than hourly equivalent rates of fall (see below).
Note 2: When the precipitation rate exceeds 0.8 mm per hour the precipitation is usually rain.
Visually approximating intensity of drizzle
Description Visual Aids Can be felt on face or seen on windscreen of car but otherwise is not readily visible, produces little run-off on roads or roofs. In the absence of other Light obscuring phenomena, the visibility is generally reduced, but not to less than 1000 metres Windows and roads stream with moisture, drizzle readily visible, generally Moderate moderate drizzle reduces visibility to between 400 and 1000 metres Heavy Generally visibility reduced to less than 400 metres
0m 400m 1000m VISIBILITY > HEAVY MODERATE LIGHT INTENSITY
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Rain RA
Precipitation in the form of liquid water drops, either as drops of appreciable size or of smaller widely scattered drops.
Drops falling on the edge of a rain zone, or during light rainfall, may be as small as drizzle drops, owing to partial evaporation. Rain drops are distinguished from drizzle in that they are more scattered.
Rain (RA) without the descriptor SH refers to non-showery rain (either intermittent or continuous) which falls from Nimbostratus, Altostratus or occasionally Stratocumulus.
Showers of rain (SHRA) fall from Cumulus, Towering Cumulus and Cumulonimbus. Rain drops falling from convective clouds tend to be larger than those from stratiform clouds.
Rain is classified by intensity. The intensity rates apply to both showery and non-showery rain.
Using rainfall rates to determine intensity of rain
Description Rate of Fall Light Up to 2 mm per hour Moderate 2.2 mm to 6 mm per hour Heavy Greater than 6 mm per hour
Visually approximating intensity of rain
Description Visual Aids Individual drops easily identifiable, puddles form very slowly, wets dry surface Light slowly, small streams may flow in gutters and downpipes Individual drops not clearly identifiable, puddles form rapidly, gutters and Moderate downpipes flow freely, some spray visible over hard surfaces Rain seemingly falls in sheets, misty spray visible over hard surfaces, may Heavy make roaring noise on roofs
Freezing Rain FZRA and Freezing Drizzle FZDZ
The descriptor freezing is appended to rain or drizzle with a temperature below 0°C (supercooled water droplets) and which freeze on impact with the ground or with objects on the earth's surface.
Note: Whether or not the supercooled rain precipitation is a shower shall not be specified.
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Snow SN
Precipitation in the form of ice crystals. The crystals are usually branched to form six pointed stars and interlock to form snowflakes.
Showers of snow fall from Cumulus, Towering Cumulus or Cumulonimbus clouds while Altostratus, Nimbostratus, Stratocumulus or Stratus produce intermittent or continuous snow. Snow is classified by intensity.
Using rainfall rates to determine intensity of snow
Description Rate of Fall
Light Melted water equivalent of up to 2 mm per hour
Moderate Melted water equivalent of 2.2 mm to 6 mm per hour
Heavy Melted water equivalent of greater than 6 mm per hour
Visually approximating intensity of snow
Description Visual Aids Snowflakes small and sparse. In the absence of other obscuring phenomena the Light visibility is generally reduced, but not to less than1000 metres Larger more numerous flakes generally reducing the visibility to between 400 and Moderate 1000 metres
Heavy Numerous flakes of all sizes generally reducing the visibility to below 400 metres
Hail GR
Precipitation of small balls or pieces of ice, hard and partly transparent, which fall separately or frozen together into irregular lumps. The size of hailstones varies, but to be reported as hail (GR) the largest must be 5mm or more in diameter.
Hail falls as showers from Cumulonimbus cloud only and is generally associated with thunderstorm activity.
Visually approximating intensity of hail
Description Rate of Fall
Light Sparse hailstones of smaller size, often mixed with rain
Moderate The fall is abundant enough to whiten the ground
Heavy Includes at least a proportion of hail-stones exceeding 6 mm diameter
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Small Hail/Snow Pellets GS
Precipitation of white and opaque grains of ice. They resemble snow crystals but are spherical or sometimes conical and of small size (generally up to 5mm in diameter).
Small Hail consists of snow pellets that are totally or partially encased in a layer of ice. They are not easily crushable. Small Hail falls as showers from Cumulonimbus cloud only.
Snow Pellets are less dense than small hail and are therefore brittle and easily crushed. Snow Pellets fall as showers from Cumulus, Towering Cumulus and Cumulonimbus.
Snow Grains SG
Precipitation of very small opaque white particles of ice which falls from fog or stratus. These particles are fairly flat or elongated; their diameter is generally less than 1 mm.
This precipitation corresponds as it were to drizzle and occurs when the temperature is between 0°C and -10°C approximately.
Ice Pellets PL
Precipitation of transparent or ice particles which are usually spherical or irregular, but rarely conical. They have a diameter of less than 5mm. They are usually not easily crushable.
Ice Pellets generally fall from Nimbostratus or Altostratus.
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Weather watch radar and precipitation identification
Approximate rainfall rates The Bureau's radar images show the location of precipitation in relation to local features such as the coastline, with different colours used to depict rainfall intensity. For example, off-white represents light drizzle, while dark red is used to depict very heavy rain (possibly containing hailstones).
There are fifteen levels of rainfall intensity shown on the images - each level provides an approximate indication of the rainfall rate in millimetres per hour.
The values indicated can be used as a general guide but they are not always accurate.
In addition to the colour, the distribution of the precipitation on the weather watch radar can be used to assist with the identification of varying precipitation types, and the speed and direction of movement.
Rain bands from Altostratus and Nimbostratus Radar echoes from widespread rain are usually extensive and fairly uniform in intensity, with ill-defined edges. The estimated rainfall intensity usually appears as light to medium because of the smaller raindrop size produced in such rain bands.
Rain band signatures
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Showers from cumuliform clouds Radar echoes from showers falling from convective cloud appear as sharp-edged cells scattered around the radar display. The estimated rainfall intensity can be moderate to heavy owing to the high rainfall rates from such clouds.
Showers of rain signatures
Heavy precipitation from thunderstorms Radar echoes from the rain and hail produced in thunderstorms are very sharp-edged cells with intense cores indicating heavy rainfall. Hailstones produce particularly intense echoes because of their large size. Thunderstorm precipitation cells can appear as isolated cells or in clusters or lines. Each cell tends to last for 30 minutes or more.
Orange/red colours showing the location of thunderstorms
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Obscuration phenomena
Fog FG
A suspension of very small, usually microscopic water drops in the air, reducing visibility at the Earth’s surface.
The term fog is used without a descriptor (partial, patches, shallow or freezing) when the observed visibility over half or more of the horizon (prevailing) is less than 1000 metres, the temperature 0°C or higher, and the fog extends to more than 2 metres above ground level.
Fog Patches BCFG
The descriptor patches is appended to the fog phenomenon to describe the occurrence of fog patches at the location.
Fog patches frequently form in preferred spots around aerodromes, particularly in low swampy areas or over lakes and rivers. Fog is also observed in patches when forming and dissipating.
The estimated visibility in a fog patch will be less than 1000 metres, with the fog extending to at least 2 metres above ground level. The visibility over half or more of the horizon will be more than 1000 metres, but the minimum observed visibility could be less than 1000 metres depending on the proximity of the nearest fog patch to the observation site.
Fog patches are less extensive than a fog bank (PRFG).
Partial Fog (Fog Bank) PRFG
The descriptor partial is appended to the fog phenomenon to describe a fog bank, i.e. fog covering part of the local area, but not extensive enough to be considered fog (FG) as described above.
Banks of fog frequently form in preferred spots around aerodromes, particularly in low swampy areas or over lakes and rivers. Fog is also frequently observed in a bank when forming and dissipating.
The estimated visibility in the fog bank will be less than 1000 m, with the fog extending to at least 2 metres above ground level. The visibility over half or more of the horizon will be more than 1000 metres, but the minimum observed visibility could be less than 1000 metres depending on the proximity of the fog bank to the observation site.
A fog bank covers substantially more of the aerodrome than fog patches (BCFG).
Shallow Fog MIFG
The descriptor shallow is appended to the fog phenomenon when the fog extends to a height of less than 2 metres above the ground.
The observed horizontal visibility at 2 metres above ground level will be 1000 metres or more, with the apparent visibility in the fog layer less than 1000 metres.
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Freezing Fog FZFG
The descriptor freezing is appended to the fog phenomenon when the fog (water) droplets are in a supercooled state, i.e., with a temperature less than 0° C.
Note: Any fog predominantly consisting of supercooled water droplets shall be reported as freezing fog (FZFG) whether it is depositing rime ice or not.
Mist BR
A suspension in the air of microscopic water droplets. The horizontal visibility will be reduced, but not less than 1000 metres. The obstruction to vision may consist of water droplets or ice crystals. Generally, the relative humidity is greater than 90 precent.
Refer to FOG phenomena when visibility is less than 1000 metres.
Smoke FU
Fine ash particles suspended in the atmosphere. When smoke is present the disc of the sun at sunrise and sunset appears very red and during daytime it has a reddish tinge. Smoke at a distance usually has a light greyish or bluish colour.
Haze HZ
Suspension in the air of extremely small dry particles invisible to the naked eye. Haze resembles a uniform veil over the landscape that subdues its colours. When viewed against a dark background (e.g. a mountain) it has a bluish tinge; against a bright background (e.g. sky, clouds) it has a dirty yellow or orange tinge To distinguish haze from mist in cases of doubt, if the relative humidity is less than 90 per cent the obscurity is likely haze.
Dust DU
Solid fine dust particles in the atmosphere of varying character and size held in suspension.
Drifting Dust DRDU
Drifting dust is an aggregate of fine dust particles raised by the wind, to a height of less than 2 metres above the ground. The drifting dust moves more or less parallel to the ground and does not reduce the visibility appreciably at eye level.
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Drifting Sand DRSA
Drifting sand is an aggregate of sand particles raised at or near the station by the wind, to a height of less than 2 metres above the ground. The drifting sand moves more or less parallel to the ground.
Drifting sand does not reduce the visibility appreciably at eye level.
Blowing Dust BLDU
Blowing dust is dust raised at or near the station and carried by the wind to a height extending above 2 metres above the ground. The concentration of the dust particles may sometimes be sufficient to veil the sky and even the sun.
Visibility in blowing dust is reduced, but not less than 1000 metres (see Duststorm).
Blowing Sand BLSA
Blowing sand is sand raised at or near the station and carried by the wind to a height extending above 2 metres above the ground. The concentration of the sand particles may sometimes be sufficient to veil the sky and even the sun.
Visibility in blowing sand is reduced, but not less than 1000 metres (see Sandstorm).
Drifting Snow DRSN An aggregate of snow particles raised by the wind to a height of less than 2 metres above the ground. The snow particles are blown more or less parallel to the ground.
Drifting Snow does not reduce the visibility as eye level.
Blowing Snow BLSN An aggregate of snow particles raised by the wind to moderate or great heights above the ground. The concentration of the snow particles may sometimes be sufficient to veil the sky and even the sun.
Horizontal visibility is generally very poor. Severe blowing snow is difficult to distinguish from falling snow but blowing snow particles are generally smaller in size.
Volcanic Ash VA A suspension or fall of dust or solid particles in the atmosphere, known to have originated from an active volcano.
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Other phenomena
Dust/Sand Whirls (Dust devil) PO
A whirling column of dust or sand, usually less than 30 metres high (but may extend to 300 metres or more) and of narrow dimensions. Moving with the wind they whirl dust and light objects into the air and they usually subside after traveling a short distance.
Funnel Cloud FC
A violent disturbance of the vortex type with an approximately vertical axis. The cloud resembles an ice cream cone or funnel, hanging from the main Cb cloud base; it may or may not reach the surface. A funnel cloud is the characteristic feature of a tornado or waterspout.
Squall SQ
A sudden increase in the wind speed by 16 knots or more, the wind speed reaching 22 knots or more and lasting for one minute or more.
Thunderstorm TS
A thunderstorm is one or more sudden electrical discharges, manifested by a flash of light (lightning) and a sharp or rumbling sound (thunder). The thunder may be heard for a distance of up to 20km from the source.
Thunderstorms are associated with Cumulonimbus clouds and are often (but not always) accompanied by precipitation. Any precipitation occurring with a thunderstorm is considered showery in nature, however the convention used when describing such precipitation is to use the term ‘thunderstorm with’ in place of ‘showers of’. For example, thunderstorm with heavy rain, or thunderstorm with small hail.
Note: Although thunderstorm (TS) is categorised as a descriptor in aviation weather codes, it can also be used as a weather phenomenon on its own.
Duststorm DS
A duststorm is caused by strong winds raising large quantities of fine dust particles, often to great heights (in excess of 10,000 feet) which may be carried great distances from the source.
A Duststorm reduces the visibility to less than 1000 metres, or in the case of a severe Duststorm, less than 200 metres.
Sandstorm SS
A sandstorm is caused by strong winds raising large quantities of sand into the air resulting in a significant reduction in visibility.
A Sandstorm reduces the visibility to less than 1000 metres, or in the case of a severe sandstorm, less than 200 metres.
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Present Weather Sensor – Vaisala FD12P
The Vaisala FD12P Present Weather Sensor is essentially the FD12 Visibility Meter with the addition of a ‘Present Weather Sensor Kit’. The kit comprises of a rain detecting capacitive sensor, a temperature sensor and associated electronics and software. The capacitive sensing device is used to measure the water content from precipitation. This information is coupled with scatter signal data from the optical sensor to estimate the basic precipitation type and intensity. The temperature sensor also provides input to determine the weather code output. The FD12P can also detect the presence of non- precipitation phenomena such as fog, mist and haze.
Vaisala FD12P Present Weather Sensor
The present weather field in the METAR/SPECI AUTO message at some non-staffed locations throughout the country is being populated by data derived from the FD12P. Note: As of early 2015, the Bureau commenced the installation program for the Vaisala FS11P Present Weather Sensor.
Rain capacitive sensor with wind/bird guard
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