1 ii.4 - orographic influences on clouds 2 4 9 · pdf fileleeward side form lenticular...

II.4 - OROGRAPHIC INFLUENCES ON CLOUDS 1

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II.4.1 3

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In airflow crossing a hill, mountain or ridge, orographic clouds may occur below, at or above the 5 top of the obstacle. Orographic clouds in the troposphere may differ notably from the usual 6 characteristics of any of the ten cloud genera; regardless, tropospheric orographic clouds are 7 always classified as one of these genera. 8 9

Orographic influence on airflow leads to the development or enhancement of clouds on the 10 windward side, which generally dissipates on the leeward side due to downward motion. 11 12 A cross barrier flow may generate mountain waves on the leeside depending on atmospheric 13 conditions and the characteristics of the topography. Sometimes the oscillations of air on the 14 leeward side form lenticular clouds on the crests of such waves, a signature of airflow alteration 15 due to relief. 16 17 The most common orographic clouds belong to the genera Altocumulus, 18 Stratocumulus and Cumulus. 19 20 It is important to observe clouds in mountainous terrain because they 21 may provide an indication of changes in the weather that may have 22 implications on safety. 23 24 25 II.4.2 Orographic influence on the windward side. 26

27 As airflow encounters a mountain or hill it is forced to rise, this is 28 referred to as orographic lift. If the flow is sufficiently moist, clouds form 29 on the windward side of mountains and are called orographic clouds. 30

The type of cloud that forms depends on the air stability and moisture 31 content. Air also rises up a slope due to daytime heating so both 32 orographic and thermal lifting often work together to produce tall, 33 vertically developed cumulus clouds. Therefore, hilly areas are often 34 cloudier than nearby lower land. 35 36 Existing clouds arriving over mountainous or hilly areas, situated at 37 heights comparable with that of the obstacle, may change in shape and 38 structure as a result of the topography. The clouds become denser, the 39 size and concentration of water droplets or ice particles increase and 40 frequently develop greater vertical extent. These clouds may crown the 41 ridge. Precipitation may start or increase in intensity. Quite often, heavy 42 orographic precipitation occurs on the upwind side of the barrier, 43 particularly for barriers located near the sea. This process, referred to as 44 orographic enhancement of precipitation, requires specific synoptic 45 conditions not related to topography. 46 47 Clouds thin out and dissipate on the leeward side, where the relief 48 causes descending motions and precipitation is notably less (rain 49 shadow). In the case of an isolated mountain, orographic clouds often 50

Figure 1 Orographic cloud in stable conditions and high moisture windward

Figure 2 Orographic cloud in stable conditions with low moisture windward

Figure 3 Orographic cloud in unstable conditions and high moisture windward

have the form of a collar surrounding the mountain or that of a cap covering the peak, both of 51 which are both fairly symmetrical. These clouds give little or no precipitation. In the case of a 52 mountain barrier, observed from the leeward side, cap clouds indicate likely wave activity 53 downstream. Sometimes the clouds resemble a bank or wall that follows mountain contours. It is 54 important to remember that their absence does not mean that waves are absent. Under drier 55 conditions, waves may be present without cap clouds. 56 57 58 When the wind is strong, the orographic clouds formed near the summit may be observed 59 streaming away from the mountain on the leeward side. This is a banner cloud and should not be 60 confused with snow blown from the crest or peak. 61 62 63 II.4.3 64

Orographic influence on the leeward side. 65 66 In a stable environment strong winds blowing perpendicular to a 67 barrier (such as a mountain) are forced to rise on the windward 68 side and descend along the downwind slopes. The disturbed 69 airstream starts to oscillate in a series of waves as it moves 70 downstream, generating mountain waves. 71 72 If the wave remains essentially stationary while the air moves 73 through it, they are referred to as non-turbulent stationary or 74 standing waves (also known as trapped lee waves). When the air 75 is sufficiently moist, orographic clouds may appear in the 76 ascending air at the wave crests. They most often form above or 77 downwind from mountain ranges, and remain stationary, 78 usually for some hours (seldom for more than a day). To an 79 observer on the ground these clouds move very slowly, if at all, 80 although the wind at the cloud level may be strong. In certain 81 cases, the speed of the wind is revealed by markings in the 82 cloud, as for instance by separate elements that move from one 83 end of the cloud to the other. These lenticularis shaped clouds 84 produced by mountain waves are an indication of strong winds 85 in stable levels of the atmosphere. They do not produce 86 precipitation. 87 88 Sometimes these waves propagate long distances in "lee wave 89 trains" so the effects may also be felt a great distance away. 90 They can be seen oriented in long bands parallel to the mountain 91 range, at regular intervals of several kilometers. 92

On satellite images they form a streamline pattern. 93 94 Wave clouds may also appear at different levels simultaneously. 95 Often, one or a pile of several orographic shaped lenticularis 96 clouds appear above the hill or mountain, sometimes slightly up-97 wind or down-wind. The orographic influence on the airflow may 98 be significant at levels many times exceeding the levels of peaks 99 or crests, even reaching the stratosphere. 100

Figure 4 Mountain waves

Figure 6 Trapped lee waves

Figure 5 Satellite image of lee wave trains (A - alignment of the ranges: B= wind direction)

A

B

101 In broad mountain ridges when there is high atmospheric stability through a depth of atmosphere 102 and marked wind shear above the mountain top, vertically propagating waves might occur where 103 the energy propagates upwards. These are called untrapped lee waves and cirriform cloud formed 104 due to orographic influence is indicative of turbulence near the top of the troposphere. 105 Sometimes the wave tops may extend beyond the high level into the stratosphere. 106 107 Where there is clear gap (foehn gap) between the hill and the cloud, turbulence is likely to be 108 severe. Where there is no clear gap between the hill and the cloud, then any turbulence is likely to 109 be weak. 110 111 Although the evidence of clouds becomes a signature of air 112 movement and turbulence, this sometimes occurs without 113 visual indicators. Clear Air Turbulence (CAT) often occurs near 114 the top of the troposphere due to vertically-propagating waves 115 in dry conditions. 116 117 At certain times the amplitude of mountain waves may reach 118 high values and the wave energy propagates downwards 119 immediately upwind of ridge producing significant weather 120 events such as breaking waves, strong-to-extreme turbulence, 121 rotors and damaging downslope windstorms on the leeside of a 122 mountain barrier. 123

124 Beneath the lee wave cloud, in the lower layers, a large 125 swirling eddy with horizontal axes may form. If the rising air of 126 this large stationary eddy cools enough, a cloud bar called 127 "rotor cloud" (Stratus roll cloud) may appear in the upper part. 128 Rotor or roll clouds are indicative of an area of severe 129 turbulence at or near the surface, with surface winds of highly 130 variable direction and/or speed, presenting a hazard to 131 aviation. 132 133 134 135 136 137 138

.5 CLOUDS AS SEEN FROM AIRCRAFT 139

II.5.1 140

Issues for Observation of Clouds from AircraftCloud observations from an airborne observer 141

can provide different information than from an observer at the Earth's surface. An 142

airborne observer can have more complete knowledge of the vertical distribution of the 143

clouds, their amounts and altitudes, their structure and the appearance of their upper 144

parts or surfaces, as well as of their constituent particles. 145

The appearance of the clouds depends on their position relative to the aircraft, so it is 146

necessary to state the conditions under which observations are made. Descriptions of 147

Figure 7 Vertically propagating wave

Figure 8 Rotor cloud

appearance in this section correspond to the most frequent appearance of clouds when 148

observed 500 - I000 metres (I650 - 3300 feet) below their base or above their upper 149

surface, or observed from within. 150

II.5.1.1 151

FIELD OF VISION 152

The field of vision of an airborne observer increases with altitude, and more distant views 153

are usually possible at higher altitude because the air is more transparent. Thus, when 154

airborne, an observer is often able to see more extensive cloud ensembles. 155

II.5.1.2 156

APPEARANCE OF CLOUDS 157

(a) The effect of perspective 158

Due to the effect of perspective, an observer flying at or near cloud level may see clouds 159

as a fairly continuous layer, even if they are in fact detached. 160

(b) Apparent width of cloud elements 161

While ground based observers can distinguish between cloud genera (Cirrocumulus, 162

Altocumulus, Stratocumulus) partly based on the apparent width of cloud elements, this 163

criteria is of little use to an airborne observer. In some cases, the cloud altitude may be 164

the only useful criterion to determine their genus. 165

(c) Outlines of clouds 166

As the airborne observer approaches clouds, their outlines appear less distinct and more 167

ragged. 168

(d) Base of clouds 169

The appearance of the cloud base changes with distance, becoming more diffuse and more ragged 170 as the observer approaches. At close range, relief is difficult to distinguish. Because of this, for 171 example, the base of an opaque Altocumulus layer can appear very similar to that of an 172 Altostratus. 173

(d) Upper surface of clouds 174

Observations of the upper surface of clouds are very useful, since they provide indirect 175 information about the degree of instability of the atmosphere. 176

Clouds of different genera can appear similar when viewed from above. This may make it difficult 177 for an airborne observer to identify clouds from their upper surface. 178

The upper surface of clouds is usually better defined than their base. It may appear smooth or 179 rough, clear-cut or diffuse. It is also brighter and has greater variation in luminance. 180

The upper surface of a cloud layer may be flat or may have well-defined undulations of varying 181 width. Undulations may have scales of 10 to 1000 metres (33 to 3300 feet), suggesting ocean 182 waves. It may also have shallow rounded projections, bulges, or domes, sometimes arranged in 183 rows and with a fleecy appearance. Well-developed domes or towers may be visible, coming from 184 within the layer or penetrating from below it. If there are many of these, it can become difficult to 185 detect the surface from which they emerge. A cloud veil (velum) may cover the shallow domes or 186 the sides of the well-developed towers. Occasionally, such veils are extensive enough and thick 187 enough that the underlying clouds are partially or totally masked. 188

II.5.1.3 189

ICING 190

An ice deposit may form on different parts of an aircraft when flying in supercooled clouds or 191 precipitation. The intensity and characteristics of the icing vary, but they depend primarily on the 192 degree of supercooling, the droplet diameters and concentration, and characteristics of airflow 193 around the aircraft. The main types of icing are soft rime, hard rime, clear ice, and glaze. (These 194 are described in Part III Meteors other than Clouds.) 195

II.5.1.4 196

TURBULENCE IN CLOUDS AND IN THEIR VICINITY 197

Vertical currents (ascending and descending) may occur in or around clouds. The successive jolts 198 felt in an aircraft when it passes from one such current to another constitute what aviators call 199 "turbulence". The severity of this turbulence depends on the speed and dimensions of the vertical 200 currents and also on the characteristics of the aircraft. 201

II.5.1.5 202

VISIBILITY IN CLOUDS 203

Within clouds, even if they are very thin, visibility is always less than in the surrounding clear air. 204 Some clouds are dense enough to reduce visibility almost to zero. 205

II.5.1.6 206

PHOTOMETEORS ASSOCIATED WITH CLOUDS 207

Certain photometeors (haloes, coronae, etc.) may be visible to an airborne observer inside a cloud 208 and close to its upper surface. An observer above a cloud patch, sheet or layer, may see a glory or 209 a fog bow if the cloud is composed of water droplets, or may observe halo phenomena if the cloud 210 is composed of ice crystals. 211

II.5.2 212

Descriptions of clouds as observed from aircraft 213

II.5.2.1 214

CIRRUS 215

Cirrus usually occurs at altitudes between 3 and 8 kilometres (10000 and 25000 feet) in polar 216 regions, between 5 and 13 kilometres (16500 and 45000 feet) in temperate regions and between 6 217 and 18 kilometres (20000 and 60000 feet) in the tropics. In the temperate zone, the Cirrus of polar 218 air masses occupies lower levels than the Cirrus of tropical air masses. 219

Below the cloud. Viewed from below, Cirrus usually has no distinct structure. However, it 220 sometimes has white, delicate filaments, or patches or narrow bands that are white or mostly 221 white. Cirrus is distinguishable from Cirrocumulus by the absence of regularly arranged rounded or 222 granular cloudlets, and from Cirrostratus by the fact that it consists of separate elements. 223

Within the cloud. Cirrus is composed almost exclusively of ice crystals. The observer often sees the 224 glitter of these crystals in the sunlight. Halo phenomena, when present, are generally confined to 225 the small halo. 226

Above the cloud. Cirrus is very bright when seen from above in full sunlight. Thin Cirrus may 227 resemble the upper surface of a layer of haze, while dense Cirrus has a milky appearance. Other 228 clouds or the ground are frequently visible through Cirrus. Neither a glory nor a subsun (undersun) 229 will be present, but a small slightly brighter area is often visible around the aircraft's shadow. 230

II.5.2.2 231

CIRROCUMULUS 232

Cirrocumulus most frequently occurs above 3 kilometres (10000 feet) in polar regions, 5 233 kilometres (16500 feet) in temperate regions, and 6 kilometres (20000 feet) in tropical regions. 234

Below the cloud. Viewed from below, Cirrocumulus appears as a thin patch, sheet or layer, 235 composed of very small, rounded elements. The elements may be merged or separate, and their 236 horizontal bases are all at the same level. They appear white with without shading. 237

Within the cloud. Cirrocumulus is almost exclusively composed of ice crystals. Supercooled water 238 droplets may form, but these are usually rapidly replaced by ice crystals. The observer has the 239 impression of flying in a thin fog. Light turbulence may be encountered, except in Cirrocumulus 240 castellanus where it can be stronger. The only halo phenomenon which may be observed is the 241 small halo. 242

Above the cloud. Viewed from above, Cirrocumulus elements have soft outlines resembling cotton 243 wool. They may appear similar to Cumulus humilis clouds in shape and size. If Cirrocumulus 244 castellanus, the elements have a common base and are more vertically developed. No subsun 245 (undersun) is observed. 246

II. 5.2.3 247

CiRROSTRATUS 248

Cirrostratus most frequently occurs above 3 kilometres (10000 feet) in polar regions, 5 kilometres 249

(16500 feet) in temperate regions and 6 kilometres (20000 feet) in tropical regions. 250

Below the cloud. Viewed from below, Cirrostratus appears as a transparent, fairly 251

homogeneous whitish veil, totally or partly covering the sky. It usually produces halo 252

phenomena and it is difficult to identify the base. 253

Within the cloud. Cirrostratus often occurs in several layers. Its constituent ice particles often 254 glitter in the sunlight. Many kinds of halo phenomenon may be observed. Slight turbulence may 255 be encountered in the lower portion of the cloud, particularly near the base. 256

Above the cloud. Viewed from above, Cirrostratus appears practically the same as Cirrus, except 257 for its greater breadth of continuous sky cover. The upper surface may be well defined and flat, or 258 can be diffuse with bulges resembling Cirrocumulus. The ground is usually visible through a thin 259 Cirrostratus veil, though seldom through a thick cloud veil. A subsun (undersun) may be observed, 260 but other halo phenomena are rare. 261

II. 5.2.4 262

ALTOCUMULUS 263

Altocumulus most frequently occurs at altitudes between 2 and 4 kilometres (6500 and 13000 264 feet) in polar regions, between 2 and 7 kilometres (6500 and 23000 feet) in temperate regions and 265 between 2 and 8 kilometres (6500 and 25000 feet) in tropical regions. 266

Altocumulus may be observed in several forms, the most important of which are described below. 267

(a) Altocumulus stratiformis occurs as a sheet or layer with detached elements. It is 268

generally less than 500 metres (1650 feet) thick. 269

Below the cloud. Viewed from below, this cloud appears as a broken sheet or layer. It may be 270 either translucent everywhere or partly translucent and partly opaque. It is white, sometimes with 271 grey. 272

Within the cloud. This cloud is composed of small water droplets, sometimes accompanied by ice 273 crystals. Light aircraft icing may occur. Turbulence is weak to moderate. 274

Above the cloud. When viewed from above, this cloud appears smooth and undulated, or may 275 appear fleecy. There are distinct gaps through which lower clouds or the ground can be seen. 276 Well-developed cumuliform clouds, which formed at a lower level, may penetrate through the 277 cloud layer. Occasionally, thin sheets or patches are found about 100 to 300 metres (330 to 1000 278 feet) above the main layer (Altocumulus duplicatus). A glory, sometimes accompanied by a fog 279 bow, may be observed on the cloud elements. A subsun (undersun) may appear, usually in the 280 hazy, ice crystal-filled parts between the cloud elements. 281

(b) "Solid" Altocumulus stratiformis occurs in a sheet or layer with merged 282

elements. This type of Altocumulus is usually less than 500 metres (1650 feet) thick. 283

Occasionally it occurs in two or more thin sheets or patches, and then it has a dark 284

appearance suggesting a considerably greater depth. The total thickness from the base of 285

the lowest to the top of the highest layer is usually less than 2000 metres (6500 feet). 286

Below the cloud. Viewed from below, this cloud appears as sheet or layer, entirely grey or 287 sometimes white and grey, and with varying opacity. When observed from nearby, the elements 288 appear large and dark, and look exactly like Stratocumulus. 289

Within the cloud. This cloud is composed of small water droplets, sometimes accompanied by ice 290 crystals. Variations in visibility are fairly distinct, particularly at night when the aircraft lights are 291 on. There may be considerable icing. Turbulence is usually weak, but may be moderate. 292

Above the cloud. When seen from above, this cloud usually appears continuous except for crevices 293 which mark the thinner borders of the elements. The upper surface may be smooth and undulated 294 or may have a fleecy appearance. Glory, fog bow and subsun (undersun) may be observed, 295 sometimes simultaneously. 296

(c) Altocumulus lenticularis appears as lens-shaped patches of Altocumulus. 297

The vertical extent of Altocumulus lenticularis is usually not more than 200 metres (660 feet), 298 although orographic Altocumulus lenticularis may be much deeper. 299

Below the cloud. Viewed from below, this cloud appears diffuse. It is frequently partly translucent, 300 occasionally with irisation. It is either completely white or white and grey. 301

Within the cloud. Turbulence is usually weak, but it may be moderate. 302

Above the cloud. Viewed from above, a thin Altocumulus lenticularis can be transparent enough to 303 reveal the ground, but still looks fairly dark. Thick Altocumulus lenticularis looks white, and bright 304 glories may be observed on it. 305

(d) Altocumulus castellanus has cumuliform tops rising from a common base. 306

Below the cloud. Viewed from below, this cloud has a horizontal and fairly extensive base, 307 resembling a layer of Altocumulus. Just below the cloud, visibility is reduced by haze and virga may 308 fall from the cloud. Turbulence increases near the base. 309

Within the cloud. Within the higher towers of the upper part of this cloud, turbulence is generally 310 strong and electrical discharges (St. Elmo’s fire) may be observed. Visibility is variable, and icing 311 may occur. 312

Above the cloud. Viewed from above, this cloud strongly resembles well-developed Cumulus 313 clouds with their bases buried in a haze layer or a smooth and wavy cloud layer. The vertical 314 extent of the cumuliform towers varies. Some may develop into Cumulus congestus or even into 315 Cumulonimbus, resulting in thunderstorms at high altitudes. 316

317

(e) Altocumulus floccus, or Altocumulus in tufts 318

Below the cloud. Viewed from below, the patches of this cloud are diffuse. They may be whitish or 319 dark, and the bases are not all at exactly the same level. Virga may be observed falling from the 320 clouds. Turbulence is weak to moderate below Altocumulus floccus. 321

Within the cloud light icing may occur, and turbulence can range from weak to fairly strong. 322

Above the cloud. Viewed from above, the cloud elements look like small Cumulus clouds 323 surrounded by, or emerging from, a milky white area. This cloud is usually 500 to 1000 metres 324 (1650 to 3300 feet) thick. The cumuliform tufts at times reach a vertical extent of 2 to 3 kilometres 325 (6500 to 10000 feet), making the full cloud resemble Altocumulus castellanus from above. 326

II.5.2.5 327

ALTOSTRATUS 328

Altostratus usually occurs between 2 and 4 kilometres (6500 and 13000 feet) in polar regions, 329 between 2 and 7 kilometres (6500 and 23000 feet) in temperate regions, and between 2 and 8 330 kilometres (6500 and 25000 feet) in tropical regions. However, the upper parts of Altostratus may 331 extend beyond the indicated upper limits. Altostratus may range from 1000 to more than 5000 332 metres (3300 to more than 16500 feet) in thickness. 333

Below the cloud. Viewed from below, the base of Altostratus is nearly flat and appears diffuse and 334 hazy. This is because rain or snow falls from it, usually without reaching the ground (virga). Some 335 parts of the cloud layer are thin enough so the sun is weakly visible through them. 336

Within the cloud. Depending on the part traversed and the position of the aircraft relative to the 337 freezing level (0°C) (32°F), particles encountered may be water droplets (supercooled or not), 338 raindrops, ice crystals, snow crystals or snowflakes. Where the cloud consists of ice crystals alone, 339 the particle concentration is usually relatively small. 340

The airborne observer can distinguish two types of Altostratus, with very different internal 341 structure. 342

1. A homogeneous layer, often with its upper surface reaching great altitudes. Visibility in this 343 layer is usually fair and the Earth's surface can be seen through a considerable cloud depth. Halo 344 phenomena may be observed and are often bright. 345

2. Many patches, sheets, or layers of water cloud that may be connected by virga or precipitation. 346 The precipitation can obscure the stratified structure, so the Altostratus may appear as a thick 347 single layer with large clear spaces. Therefore, visibility is quite variable within the cloud, and may 348 be less than 100 metres in places. At night, the open spaces can easily be seen when the aircraft 349 lights are on. Altocumulus patches are usually present at the top of this Altostratus. 350

In both types of Altostratus, turbulence is weak and confined to the lower parts, but it may be 351 strong when there is internal convection. Icing is generally light. 352

Above the cloud. Viewed from above, the upper surface of the first type of Altostratus is similar to 353 Cirrostratus and the second type resembles Altocumulus. The optical phenomena observed on the 354 upper surface of Altostratus are the same as those observed on Cirrostratus and Altocumulus. 355

When the air in which Altostratus develops is unstable or becomes unstable, internal 356

convection can produce cumuliform elements which may rise well above the cloud mass 357

and may even develop into Cumulonimbus. When the instability of the air below is 358

sufficient to produce strong convective currents, Cumulus congestus or Cumulonimbus 359

may penetrate through the Altostratus. 360

II.5.2.6 361

NIMBOSTRATUS 362

The main body of Nimbostratus almost invariably occurs at altitudes between 2 and 4 363

kilometres (6500 and 13000 feet) in polar regions, between 2 and 7 kilometres (6500 and 364

23000 feet) in temperate regions, and between 2 and 8 kilometres (6500 and 25000 feet) 365

in tropical regions. However, the base is often below these limits, and the upper surface is 366

above them. Nimbostratus is generally thicker than Altostratus, having a vertical thickness 367

typically 2 to 8 kilometres (6500 to 25000 feet). 368

Below the cloud. Viewed from below, Nimbostratus is grey and often dark. Rain or snow 369

fall from its base, usually reaching the ground. Therefore, its base appears diffuse or 370

indefinite, or may not even be visible through heavy precipitation. Pannus is often 371

encountered under Nimbostratus. Turbulence is stronger in the pannus than in the 372

Nimbostratus immediately above it. 373

Within the cloud. The constituent particles of Nimbostratus are similar to that of 374

Altostratus, but are generally larger and more numerous. This, and the typically great 375

vertical extent of Nimbostratus, causes it to be somewhat dark within the lower parts of 376

the cloud. While Nimbostratus is essentially a layer cloud, cumuliform convective clouds 377

with considerable vertical extent may form within it. In Nimbostratus visibility is poor, 378

often less than 50 metres, and icing may occur. Turbulence is generally moderate, but 379

may become fairly strong when there is internal convection. 380

Above the cloud. Viewed from above, the upper surface of Nimbostratus is often similar to 381

Cirrostratus and Altostratus. It is diffuse and fairly smooth, and it sometimes appears flat, 382

undulated or fleecy. In unstable air masses, Cumulus congestus or Cumulonimbus may be 383

embedded in the Nimbostratus, and may rise above its upper surface. Optical phenomena 384

like glory, fog bow and subsun (undersun) may be visible. 385

II.5.2.7 386

STRATOCUMULUS 387

Stratocumulus usually occurs below 2 kilometres (6500 feet). Its thickness ranges between 388

500 and 1000 metres (1650 and 3300 feet). Like Altocumulus, Stratocumulus may take 389

several forms, with the two primary forms described below. 390

(a) Stratocumulus stratiformis occurs in a sheet or layer with detached elements. 391

Below the cloud. Viewed from below, this cloud appears as a sheet or layer, consisting of 392

fairly extensive elements that are whitish, or grey, or both. Stratocumulus is darker than 393

Altocumulus because of its higher water content and greater vertical extent. 394

Within the cloud. This cloud is composed of water droplets, occasionally interspersed with ice 395 crystals when at low temperatures. The observer has the impression of flying in dense fog, with 396

variations in visibility that may be large or small. Turbulence is generally moderate, but often 397 greater than in the corresponding type of Altocumulus. 398

Above the cloud. Viewed from above, this cloud, like Altocumulus with detached elements, 399 appears somewhat fleecy. Projections, bulges, or domes may be present, either originating as part 400 of the layer or as the upper parts of Cumulus congestus and Cumulonimbus, which have 401 penetrated from below. Open spaces or crevices are often visible in the layer. A glory, fog bow and 402 subsun (undersun) may be observed, sometimes simultaneously. 403

(b) "Solid" Stratocumulus stratiformis occurs in a sheet or layer with merged 404

elements 405

Below the cloud. Viewed from below, the base of this cloud is usually distinct and 406

corrugated. Its true relief is revealed only by differences in luminance. 407

Within the cloud. This cloud is composed of water droplets, occasionally interspersed with ice 408 crystals when at low temperatures. Raindrops, snow pellets, snow crystals and snowflakes may 409 also be present. The observer has the impression of flying in dense fog. There may be moderate 410 icing, and turbulence is generally moderate. 411

Above the cloud. Viewed from above, the upper surface sometimes appears flat. Most of the time, 412 however, it is undulated or has long parallel bands. Projections, bulges or domes may be visible. 413 Often, the air immediately above this type of Stratocumulus is hazy. A glory, fog bow and subsun 414 (undersun) may be observed, sometimes simultaneously. 415

The cloud sheet or layer often closely follows the shape of the terrain. Lighter bulges and darker 416 depressions can give a good indication of topographic features such as rivers, lakes, coasts, and 417 hills. The terrain may also be seen through breaks in the cloud. 418

II.5.2.8 419

STRATUS 420

Stratus usually occurs between the Earth's surface and 2 kilometres (6500 feet). Its thickness 421 ranges between ten and several hundred metres (ten and several hundred feet). 422

Below the cloud. Viewed from below, a Stratus layer or patch is generally grey, and at times shows 423 variation in luminance. Its base may be clearly defined, diffuse, or ragged. When the sun is visible 424 through Stratus its outline is not blurred (there is no ground glass effect). 425

Within the cloud. Stratus is composed of small water droplets and sometimes of ice crystals. 426 Drizzle droplets, ice prisms and snow grains may also be present. The denseness of the cloud 427 gradually increases towards the top where the high concentration of very fine water droplets may 428 reduce visibility almost to zero. Variations in denseness and visibility are also observed in the 429 horizontal. Light to moderate icing may occur, and turbulence is light to moderate. 430

Above the cloud. Viewed from above, the upper surface generally shows undulations (usually of 431 short wavelengths) and may displays projections. In strong winds the undulations are more 432 pronounced, and bulges and depressions that reflect the irregularities of the ground, may be 433 observed (compare Stratocumulus). Often the air immediately above the upper surface is hazy. A 434

glory, fog bow and subsun (undersun) may be observed, sometimes simultaneously. 435

II.5.2.9 436

CUMULUS 437

Cumulus occurs in various sizes and degrees of development, from Cumulus humilis with a vertical 438 extent ranging from some tens to hundreds of metres (tens to hundreds of feet), to Cumulus 439 mediocris which ranges from a few hundred metres to about two kilometres (from a few hundred 440 feet to about 7000 feet), to Cumulus congestus which sometimes exceeds 5 kilometres (16500 441 feet). 442

(a) Cumulus humilis 443

Below the cloud. Viewed from below, this cloud usually has a horizontal base. Turbulence is 444 generally moderate. 445

Within the cloud. Cumulus humilis is composed of water droplets (sometimes supercooled). An 446 observer flying through it has the impression of being in dense fog, with large variations in 447 visibility. Ascending currents of about 2 to 5 metres (7 to 17 feet) per second may be encountered. 448 Turbulence is sometimes severe, especially during the formation and growth of the cloud, and it 449 diminishes when the cloud is mature. 450

Above the cloud. Viewed from above, this cloud often appears to be floating in a hazy layer from 451 which its rounded tops emerge. Most of the tops extend to nearly the same level. The individual 452 clouds may be widely spaced, or they may be close together and sufficiently flat to resemble 453 patches of Stratocumulus. There is usually no turbulence above Cumulus humilis. 454

(b) Cumulus mediocris 455

Below the cloud. The usually horizontal base of Cumulus mediocris is a little darker than the base 456 of Cumulus humilis, and turbulence is often strong. 457

Within the cloud. This cloud is composed of water droplets (sometimes supercooled). Visibility is 458 variable, and often very poor or even zero. There may be light to moderate icing. Ascending 459 currents may exceed 5 metres (17 feet) per second. Turbulence is fairly severe. 460

Above the cloud. Viewed from above, these Cumulus clouds show slight or moderate projections 461 or domes, which may vary in size from one cloud to another. White cloud veils (pileus, velum) may 462 be observed over Cumulus mediocris. Cumulus mediocris clouds may occasionally be arranged in 463 rows oriented in the direction of the wind. These "cloud streets" may look like Stratocumulus 464 when viewed from a considerable distance. 465

Note that Cumulus mediocris includes cumuliform clouds with variable vertical 466

development (pre thunderstorm convection sky), which usually have ragged borders and 467

torn tops. They rapidly reach the stage of Cumulonimbus after a short passage through 468

that of Cumulus congestus. 469

(c) Cumulus congestus 470

Below the cloud. Cumulus congestus has great contrasts of luminosity. Viewed from below it has a 471 relatively dark base, which is nearly horizontal and fairly often ragged. Under the base visibility is 472 good, except during precipitation. Turbulence is usually strong. 473

Within the cloud. Cumulus congestus is composed mainly of water droplets, though ice crystals 474 may form where the temperature is well below 0°C (32°F). Raindrops may occasionally be 475 observed. Visibility is generally very poor, but varies considerably. There may be considerable 476 icing. Ascending currents sometimes exceed 10 metres (33 feet) per second and turbulence is 477 often severe. Electrical discharges may occur. 478

Above the cloud. Viewed from above, sunlit Cumulus congestus is more dazzling than the other 479 types of Cumulus. The upper parts, with well-defined and strongly shaded projections and domes, 480 have the shape of large cauliflowers, huge chimneys, or towers. Their tops, which may reach 481 widely differing levels, sometimes emerge from a layer of haze or from a fairly continuous layer of 482 cloud. 483

Veils (pileus, velum), sometimes connecting several clouds, may frequently be observed. 484

Il.5.2.10 485

CUMULONIMBUS 486

The base of Cumulonimbus is usually found below 2 kilometres (6500 feet). The top often reaches 487 higher than 10 kilometres (35000 feet). The vertical extent of Cumulonimbus ranges from 3 to 488 rarely more than 15 kilometres (10000 and 50000 feet). 489

Below the cloud. Viewed from below, Cumulonimbus generally looks dark. Under the base, which 490 is often ragged, pannus clouds in the form of ragged shreds are frequently observed. They 491 occasionally constitute a kind of dark roll (arcus) under the forward and lower outer edge of the 492 Cumulonimbus. Visibility may be poor because of the precipitation (heavy showers of rain, snow 493 or hail). Turbulence is often severe. 494

Within the cloud. Cumulonimbus is composed of water droplets and, especially in its upper 495 portion, ice crystals. It also contains large raindrops and, often also snow crystals, snowflakes, 496 snow pellets, ice pellets or hailstones. The water droplets and raindrops may be substantially 497 supercooled, and can lead to rapid aircraft icing, particularly when the supercooled water drops 498 are interspersed with ice crystals. 499

Within the lower and middle portions of the cloud, it is dark and the visibility is very low, often 500 zero. In the upper portions the illumination may be strong but the visibility is poor. Vertical 501 currents (ascending and descending) often exceed 15 metres (50 feet) per second, with the 502 downdrafts mostly in areas of heavy precipitation. Turbulence is severe. 503

Electrical discharges (St. Elmo’s fire) may occur. They seem to be most frequent where the 504 temperature is between 0°C and -2°C (32°F and 28°F). 505

Above the cloud. Depending on its stage of development, Cumulonimbus can appear similar either 506 to Cumulus congestus, with its strong contrasts in luminance, or to dense Cirrus, often in the 507 shape of huge plumes or anvils, with wavy or bulging parts. When sunlit, it is dazzling with very 508 great contrasts in luminance. The main body of a Cumulonimbus sometimes emerges from a layer 509

of stratiform clouds. Cloud veils of various dimensions (pileus, velum) may surround the cloud. 510 Usually no haloes are observed. 511

II. 5. 3 512

Fog and haze as seen from aircraft 513

Fog and haze are considered in this chapter because they frequently resemble certain types of 514 cloud. 515

II.5.3.1 516

FOG 517

Fog is composed of very small water droplets (and sometimes minute ice particles) in suspension 518 in the atmosphere, reducing the horizontal visibility at the Earth's surface to less than 1000 519 metres. The vertical extent of fog ranges from a few metres to several hundred metres. 520

Within the fog. Flying within fog, the airborne observer has low visibility. Icing, when it occurs, is 521 generally very light. In the case of relatively shallow fog turbulence is absent or light, while in 522 deeper fog turbulence may be light to moderate. 523

Above the fog. Viewed from above, fog appears similar to a smooth layer of Stratus. It may be flat, 524 slightly undulated, or display rounded domes of various size. 525

II.5.3.2 526

HAZE ALOFT 527

Haze aloft is composed of extremely small particles that scatter light. The scattering increases with 528 the concentration of the particles. Layers of haze aloft may be encountered by aircraft up to about 529 5 kilometres (16500 feet). 530

Below the haze. Viewed from below, haze aloft appears as a veil with a dark blue or blackish tint. 531 An airborne observer entering such a haze layer from below experiences a gradual reduction in 532 visibility. 533

Within haze aloft, it is often difficult to determine whether the aircraft is in haze alone or in clouds 534 embedded within haze. Flying upward out of a layer of haze, a rapid improvement in horizontal 535 visibility is usually observed. 536

Above the haze. Viewed from above, haze aloft tends to hide the landscape. Scattered light is 537 especially strong in the direction of the sun. In this direction, ground features are impossible to 538 distinguish, except perhaps when the landscape includes very bright areas such as water surfaces. 539 In the direction away from the sun, visibility toward the ground is better. 540

The upper limit of haze aloft forms a horizon. Flying immediately above haze and looking down 541 obliquely, it is almost impossible to distinguish any clouds that may be embedded within it, unless 542 the tops of these clouds emerge above the haze layer. As with many stratiform clouds, the upper 543 limit of the haze coincides with the base of a stable air layer (often a temperature inversion). 544

II.6 UPPER ATMOSPHERIC CLOUDS 545

Clouds visible in the upper atmosphere include Polar Stratospheric Clouds (PSCs), which form in 546 the stratosphere between about 15 and 30 km in high latitudes of both hemispheres during 547 winter, and Noctilucent clouds (NLC), which form in the mesosphere at altitudes of about 80 to 85 548 kilometres, at or near the mesopause in summer. NLC are also known as Polar Mesospheric 549 Clouds. 550

PSCs only occur at very low temperatures, which typically occur in the stratospheric polar vortices 551 that form in winter. They are less common in the Arctic because of a less stable polar vortex. PSCs 552 are the only atmospheric cloud that exists both above and below the ice frost point. 553

PSCs that form above the ice frost point are created through the co-condensation of nitric acid and 554 water. Clouds that form below the ice point are composed primarily of ice, because of the 555 abundance of water vapor compared to nitric acid. Due to their optical signature, these ice PSCs 556 are also known as Nacreous (mother-of-pearl) clouds. The two types of clouds were first detected 557 with remote sensing instruments and were called type 1 (nitric acid and water) and type 2 (ice) 558 clouds. This nomenclature is no longer used in the light of better understanding of the particles in 559 the different types of PSCs. 560

II.6.1 Nacreous clouds (MORN 1893) 561

Ice Polar Stratospheric Clouds, also known as mother-of-pearl clouds 562

II.6.1.1 DEFINITION: Clouds resembling Cirrus or Altocumulus lenticularis and showing very marked 563 irisation, similar to that of mother-of-pearl; the most brilliant colours are observed when the sun is 564 several degrees below the horizon. 565

II.6.1.2 PHYSICAL CONSTITUTION 566

Ice Polar stratospheric clouds (Nacreous clouds) form at temperatures below the ice frost point, 567 typically near minus 85 C, which is colder than the average lower stratosphere temperature. The 568 characteristic bright iridescent colours, resulting from diffraction and interference of light waves, 569 suggest that the clouds are composed of similarly sized spherical crystals of about 10 micrometers 570 in diameter. 571

II.6.1.3 EXPLANATORY REMARKS 572

Ice PSCs, or nacreous clouds, occur mainly at high latitudes during the winter when temperatures 573 in the stratosphere fall below the frost point. They are most common in Antarctica, but have also 574 been observed in the Arctic, Scotland, Scandinavia, Alaska, Canada and northern Russia. On rare 575 occasions they have been reported from other parts of northern Europe. Nacreous clouds are 576 often lenticular wave clouds and thus found downwind of mountain ranges which induce gravity 577 waves in the stratosphere. Their formation may also be associated with severe tropospheric 578 storms. 579

By day, nacreous clouds often resemble pale Cirrus. After sunset, they are characterized by 580 brilliant iridescent colours, which are more extensive and more intense than the localized irisation, 581 or iridescence which often appears on the edges of thin tropospheric clouds (e.g. Altocumulus 582 lenticularis). Iridescence is most brilliant when the sun is several degrees below the horizon. Later, 583 with the sun further below the horizon, the various colours are replaced by a general coloration 584

which changes from orange to pink and contrasts vividly with the darkening sky. 585

If Cirrus and nacreous clouds co-exist after sunset, the high altitude of the nacreous clouds cause 586 them to show bright colours after the Cirrus has already turned grey. Nacreous clouds which are 587 also lenticular will be stationary at the crest of a gravity wave, although the air flows through the 588 cloud. Non-lenticular nacreous clouds may appear to move slowly, due to their distance from the 589 observer, as they appear above the setting sun. 590

II.6.2 Nitric acid and water Polar Stratospheric Clouds 591

II.6.2.1 DEFINITION: Stratospheric clouds with smaller and less numerous particles than an ice PSC, 592 which exist at temperatures up to 6oC above the ice point and contain solid hydrates or liquid 593 solution droplets of nitric acid and water. 594


Polar Stratospheric Clouds composed of water and nitric acid may appear at temperatures below 596 about minus 78 oC. These clouds contain two types of particles from the co-condensation of nitric 597 acid and water. The particles which can exist at minus 78 oC are nitric acid trihydrate (NAT) 598 particles, where each molecule of nitric acid is connected with 3 molecules of water; however, 599 similar to supercooled tropospheric clouds, NAT may not form at minus 78 oC due to a nucleation 600 barrier. At 3 oC below the NAT temperature nitric acid and water co-condense on stratospheric 601 sulfuric acid aerosol to form liquid particles supercooled with respect to NAT, thus supercooled 602 ternary solution (STS) droplets. There is no nucleation barrier to the formation of STS droplets. 603

The size of the NAT particles is of order 1 to 10 micrometres (microns) diameter, but with a very 604 low number density. The number density of STS particles is much higher, but sizes are typically less 605 than 1 micrometre. Due to the small particle size of the STS particles and the low number density 606 of the NAT particles these PSCs are much harder to observe by eye and do not display the bright 607 colours of ice PSCs. Various type categorizations of these clouds in the past reflect differences in 608 the mixtures of NAT and STS as measured by lidar, but in fact these mixtures of particle types form 609 a large continuum. 610

Ice, NAT, and STS PSCs are all linked to destruction of polar stratospheric ozone and the formation 611 of the 'ozone hole'. They provide surfaces to convert benign forms of chlorine into more reactive 612 forms that catalytically destroy ozone. In addition, the formation of the clouds removes gaseous 613 nitric acid from the stratosphere that could otherwise interrupt ozone loss by reforming benign 614 forms of chlorine. 615

II.6.2.3. EXPLANATORY REMARKS 616

Polar Stratospheric Clouds, occur only in high latitude regions during the winter, or near winter, 617 when temperatures in the lower and mid stratosphere fall below about minus 78 oC for NAT, 618 minus 81 oC for STS PSCs and below about minus 85 oC for ice PSCs. They are best viewed before 619 sunrise and after sunset during the period of civil twilight when the sun is between about 1 and 6 620 degrees below the horizon. 621

NAT and STS Polar Stratospheric Clouds are more diffuse and do not display the bright colours of 622 nacreous cloud. Nitric acid and water PSCs may appear as a thin yellowish veil over much of the 623 sky. This may be confused with either cirrostratus clouds or a layer of haze, but fine horizontal 624 structure may be visible near the horizon, and if they are in the stratosphere will be viewed well 625

after sunset. 626

II.6.3 Noctilucent clouds (Polar Mesospheric Clouds) (JESSE 1 1890) 627

II.6.3.1 DEFINITION: Clouds resembling wispy Cirrus, but usually with a bluish or silvery colour; 628 they stand out against the dark night sky well past summer sunset since they are in the 629 mesosphere. 630


Noctilcucent clouds (NLC) are composed of very small water ice crystals formed on minute 632 particles of dust, possibly of cosmic origin from micrometeors,. The average diameter of NLC 633 particles is thought to be about 0.3 micrometres (microns). NLC form only during the summer 634 when temperatures in the mesosphere are coldest. Temperatures lower than about minus 120 oC 635 are required for formation of NLC. 636

II.6.3.3 EXPLANATORY REMARKS 637

Noctilucent clouds (NLC) occur near the altitude of the mesopause. These 'night-shining' clouds 638 are only visible in the twilight or nighttime sky for a few months in summertime. They are only 639 visible to a ground observer in mid to high-latitudes, approximately 50 to 65 degrees North and 640 South. They are rarely seen from latitudes less than 45 degrees, and the summer daylight or bright 641 twilight prevents observation at latitudes greater than 75 degrees. In the northern hemisphere, 642 the observing season is typically mid-May through mid-August in the northern hemisphere, and 643 mid-November through mid-February in the southern hemisphere. The incidence of NLC tend to 644 peak just prior to and for a few weeks after the summer solstice in each hemisphere. 645

It is thought that noctilucent clouds are the 'ragged-edge' of a much more pervasive layer of Polar 646 Mesospheric Clouds. Although noctilucent clouds cannot be observed from the earth's surface at 647 polar latitudes, Polar Mesospheric Clouds can be observed from satellites and manned spacecraft 648 in orbit around the earth. They are observed edge-on against the dark sky background, with their 649 brightness and frequency increasing with latitude towards the poles. 650

Noctilucent clouds are seen against the background of a dark sky when the sun is well below the 651 horizon, but while these high altitude clouds are still illuminated by sunlight. They become visible 652 at about the same time as first magnitude stars appear. The observer should have an 653 unobstructed view of the horizon towards the twilight sector of the sky. Generally NLC will be seen 654 close to the horizon, typically extending up to around 15 to 20 degrees above the horizon along 655 the twilight arch. They can extend to higher elevations, sometimes to beyond the zenith especially 656 at the beginning of displays and near their end. Around local midnight, when the cloud is poorly 657 illuminated, NLC will diminish in brightness and recede closer to the northern horizon in northern 658 hemisphere, and southern horizon in the southern hemisphere. 659

NLC are generally observed during the twilight periods when the sun is between 6 and 16 degrees 660 below the observer's horizon, with the best visibility when the sun is around 10 degrees below the 661 horizon. When the sun is less than 6 degrees, the sky background is too bright for a moderate 662 display to be observed, and when the sun is greater than 16 degrees below the horizon, the level 663 where the clouds are formed is no longer illuminated by the sun's rays. Bright NLC may just be 664 detectable when the sun is between 2 and 6 degrees below the horizon, but it is difficult to 665 distinguish them from high cirrus at such times. 666

When bright, well-developed NLC displays are observed under clear-sky conditions, identification 667 is easy, even by an inexperienced observer. Difficulty may arise, however, when identifying NLC 668 that are faint and lack structural detail. The identification is made more difficult when obscured by 669 tropospheric clouds. In cases of doubt, the observer should make sure that the object is not an 670 aurora or a sunlit or moonlit tropospheric cloud or contrail. 671

What sets noctilucent clouds visually apart from tropospheric clouds is their visibility in the night, 672 their obvious blue-white colour and their disappearance into the dawn, close to onset of civil 673 twilight. Bright displays can be awe-inspiring and very obvious to the casual observer. In twilight 674 after sunset, NLC are at first, greyish or pale blue and, as time advances, they become more and 675 more brilliant; appearing bluish-white like tarnished silver, pearly-white or electric-blue. 676 Sometimes there are golden, reddish or greenish tints to the colour when the clouds are near the 677 horizon. On some occasions there may be a red upper edge to the clouds. 678

At midnight, noctilucent clouds may be seen rather low on the northern horizon (in the northern 679 hemisphere), or low in the south (in the southern hemisphere). As sunrise approaches they can 680 appear in more of the sky, before disappearing at dawn. 681

The following comments will serve as useful aids in identifying NLC: 682

Noctilucent clouds are always brighter than the twilight sky, therefore clouds which stand out 683 as dark silhouette against the background sky cannot be noctilucent clouds. 684

Tropospheric clouds when illuminated by the moon, city light, or light scattered from the 685 bright part of the sky, may appear brighter than the sky background if the sky is fairly dark. 686 However, these clouds can usually be distinguished from noctilucent cloud by their colour and 687 form. They are milky-white, whereas noctilucent clouds shine bluish-white, and unlike NLC 688 these clouds will continue to be visible during civil twilight and after sunrise. 689

Cirrus clouds illuminated by the sun when it is below the horizon are usually coloured yellow, 690 orange and pink. Clouds having these colours usually are not NLC. 691

Binoculars can assist in the identification of NLC. Under magnification, finer detail than can be 692 seen with the naked eye can be sharply resolved. This is not the case with cirrus cloud which 693 tends to be nebulous when viewed through binoculars. 694

The brightness of noctilucent clouds (NLC) can be estimated according to the following 5-point 695 scale: 696

Scale Description

1 Very weak NLC, barely visible against the background of the twilight sky; detected only through very careful examination of the sky.

2 NLC easily detected, but having low brightness.

3 NLC clearly visible, standing out sharply against the twilight sky.

4 NLC very bright, attracting the attention of casual observers.

5 NLC extremely bright and noticeably illuminating objects facing them.

The forms of NLC are classified as follows: 697

Type I. Veils: These are very tenuous, lack well-defined structure and are often present as a 698 background to other forms. They resemble cirrus clouds, occasionally contain faintly fibrous 699 structure, and often exhibit a flickering luminosity. Veils are the simplest form of NLC and often 700 precede (by about half an hour) the appearance of NLC with well-defined structure. 701

Type II. Bands: These are long streaks, often occurring in groups arranged roughly parallel to each 702 other or interwoven at small angles, but occasionally an isolated band is observed. Two groups of 703 this type occur: 704

IIa: are comprised of streaks with diffuse, blurred edges. 705 IIb: have sharply defined edges. 706

Bands show little change in location at low elevation angles, but changes in brightness can occur in 707 periods of 20 to 60 minutes within the general structure. Blurred bands with little mobility are 708 often the predominant structure in the NLC field, particularly when the brightness is low. Smaller 709 streaks with twists or bends may lie across the bands or branch out from them. Distances of 10 to 710 60 kilometres separating successive bands are common and values of over 100 kilometres have 711 been reported. 712

Type III. Billows: These are arrangements of closely spaced, roughly parallel short streaks. The 713 distance separating adjacent billows ranges from about 1 to 10 kilometres. Billows sometimes lie 714 across the long bands, giving the appearance of a comb or feather. At other times they appear 715 alone against the veil background. The billows may change their form and arrangement, or appear 716 and disappear within several minutes, much more rapidly and frequently than the long bands. This 717 NLC type may also be divided into two groups: 718

IIIa: are comprised of short, straight and narrow streaks. 719 IIIb: exhibit a wave-like structure with undulations 720

Type IV. Whirls: These are partial or, on rare occasions, complete rings of cloud with dark centres. 721 They are sometimes seen in veil, band and billow forms. Three sub-groups may be observed: 722

IVa: are comprised of whirls of small radius of curvature (0.1 to 0.5 degrees), and may 723 appear as small bright crests looking somewhat like light ripples on a water surface. 724 IVb: have the form of a simple bend of one or several bands with a radius of curvature of 3 725 to 5 degrees. 726 IVc: have a large-scale ring structure. 727

Complex structures: 728

In complex displays, two or more forms may be seen simultaneously. It is not unusual for two 729 intersecting groups of long bands to occur, and these give rise to bright knots where waves cross. 730 Complex structures can be categorised as follows: 731

O - a form which does not fit into types I to IV above. 732 S - a NLC with bright knots in the structure. 733 P - Billows crossing a band. 734 V - a net-like structure. 735

Artificial NLC can be created by rocket launches into space, where exhaust products of water 736 vapour and solid particles are introduced into the mesosphere. These NLC can be observed at 737 latitudes below 45 degrees. 738

739 II.7 – OBSERVATION OF CLOUDS FROM THE EARTH’S SURFACE 740

741

II.7.1 742

Introduction 743

744 Observational conditions to which definitions of clouds apply 745 746 The definitions of clouds given in the present Atlas apply, unless otherwise specified, to observations 747

carried out under the following conditions: 748

(a) The observer is at the earth's surface, on land in areas without mountainous relief or at sea; 749

(b) The air is clear; no obscuring phenomena such as fog, haze, dust, smoke, etc., are present; 750

(c) The sun is sufficiently high to provide the usual luminance and coloration; 751

(d) The clouds are so high above the horizon that effects of perspective are negligible. 752

753

It will be necessary to adapt the definitions to other conditions. In many cases this can easily be 754

done; for example, by night, 755

when the moon is in its brighter phases, it may play, with regard to the illumination of clouds, a 756

role analogous to that of the sun. 757

758

759

Steps in performing a cloud observation: 760

Estimate or measure total cloud amount( cover) 761 Identify all clouds in the sky by genus, and where possible species, varieties, supplementary 762 features and accessory clouds, mother cloud, and any meteors associated with the cloud 763 Estimate or measure cloud amounts of the individual cloud genera and cloud layers 764 Estimate or measure cloud height 765 Estimate direction of movement 766

767

II.7.2 768

Identifying Clouds 769

Clouds originating in the troposphere are identified by genus and where possible species, 770 varieties, supplementary features and accessory clouds, the mother-cloud, and any associated 771 meteors. 772 773 When identifying clouds, observers must: 774

1. Ideally, by day, wear polarized prescription or sunglasses, preferably those with opaque 775 side wings to shut out light coming from the sides, especially when viewing high cloud. If 776 the polarized lenses make an insignificant difference to how you see the sky; it is likely 777 the lenses are not properly orientated. 778

Polarized lenses minimize the dazzling effect of bright sunshine and protect your eyes 779 from UV radiation. They also reveal the presence of cirriform cloud when very thin, such 780 as Cirrostratus nebulosus, by creating greater contrast between cirriform clouds and the 781 blue sky, and when the clouds are veiled by haze. 782

2. At night, perform the observation from as dark a place as possible, well away from lights. 783 Let your eyes adjust to the darkness; this will take at least 5 minutes. Night vision works 784 best when using peripheral vision; moving your head from side to side will reveal more 785 detail than a fixed gaze. 786

3. Maintain a continuous watch. Accurate observing relies on noting the constant evolution 787 of clouds. Has the Altocumulus formed due to the spreading of tops of the Cumulus 788 congestus observed 15 minutes earlier? Distant lightning may be revealed where it has 789 not been visible at routine times of observations. 790

In particular, observe the sky during sunrise and sunset. The systematic changes in colour 791 of cloud in these transition periods may confirm or bring to your attention the presence of 792 multiple cloud layers. See II.1.2.2 COLOUR for more detail. 793

4. Observe the sky in its entirety, including the complete horizon: 794

Look toward the upstream horizon to see if the invading Cirrus uncinus is thickening 795 to Cirrostratus. 796

Looking toward the sun may reveal a 22° halo, confirming the presence of thin 797 Cirrostratus nebulosus that you observed because you were wearing polarized 798 sunglasses. 799

Look all around the horizon.You may have an expectation that thunderstorms 800 always come from the west but on this day they may come from the east. 801

802

Observing the sky in its entirety can assist in determining the general character of the 803 sky. Isolated areas of virga beneath an extensive layer of Altostratus translucidus may 804 indicate slow transition to Altostratus opacus; conversely extensive areas of virga may 805 indicate rapid transition to Altostratus opacus and even transition to Nimbostratus. 806

807 Be aware that clouds of the same genera, species and varieties may appear differently in 808 different meteorological situations. For example, cumuliform (see Glossary) clouds during 809 an outbreak of cold polar air may look different to cumuliform clouds during an incursion 810 of unstable tropical air. During the cold outbreak, the clouds appear sharper in outline, 811 broader and with little vertical extent; they are less sharp, more turreted and of great 812 vertical extent in tropical air. 813 814

Other factors such as vertical wind shear can affect the appearance of clouds. A few 815 examples are: 816

Cumulonimbus capillatus incus tops are often asymmetrical (spread downstream) but 817

areoccasionally symmetrical (spread out in all directions) when there is minimal wind 818

shearatcloud top. 819

Cirrus uncinus usually has the trail (falling ice crystals) falling below and behind the 820 top ofthe tuft (head) as the wind speed usually increases (positive shear) with height 821 from the trail to the tuft (head). When there is no difference in wind speed (nil 822 shear) with height through this layer, the trail appears to fall directly beneath the 823 tuft (head). When the wind speed decreases (negative shear) with height through 824 this layer, the trail falls below and ahead of the tuft (head). The positive and negative 825 sheared Cirrus uncinus are mirror images of each other although even an 826 experienced observer may see them as being completely different. 827

828 II.7.2.1 829 IDENTIFYING THE GENUS 830

The genus of each cloud is identified with the aid of: 831

definitions and descriptions in Part II.3 Descriptions of Clouds; 832

cloud and time lapse photography in Vol II of the International Cloud Atlas; 833

the Tabular Guide for the Identification of the Genus of Clouds (a summary of the key 834 features of each genus) 835

the Cloud Genera Identification Pictorialin II.7.2.1836

837

E

U

P

S

Ci Cc Cs Ac As Ns Sc St Cu Cb

Pdetached P P P P U Uwithacommonbase P P P P

U

U

lessthan1o(<1finger) E

between1oand5o U

morethan5o(>3fingers) E

E E E U

U P P S

U P P P S

U P

U P P P P P

P P

U U U U U P

P U P P

U U E P P P P

P P U U P P P

P U U P U P

U E U P P P P

P P U P P U P U U

P U U P U Pcontinued

thefeaturemayoccuronlyatthesummitorupperportionoftheclouds

thefeatureispossible,occurringsometimesincertainspecies

thefeatureisusual

Shadedthroughout

Silkysheen

Fibrous(hair-like)

Granular

Undulatedorrippled

Ragged

Generainwhichthefeaturesoccur

Spreadoutinapatch,sheet,orlayer,subdivided

intomoreorlessregularlyarrangedthinlayersor

rounedmasses,theapparentwidthofmostofthe

elementsbeing:

Thin.Groupedinsheavesorendinginahookortuft

Moreorlessdevelopedvertically,

Partlyshaded

Gen

eralShap

ean

dGroupingofClouds

andCloudElemen

ts

Structure

andTexture

Optical

Thickn

ess

Shad

ing

Uniformbase

Diffusebase

Thinclouds,throughwhichthediskofthesunormooncanbeseen

Translucentclouds,revealingonlythepositionofthesunorthemoon

Opaqueclouds

Withoutshading

Spreadoutasaveiltotallyorpartiallycoveringthesky

Detachedwithaflattenedappearance

Thin,withdetachedfilaments

TABULARGUIDEFORTHEIDENTIFICATIONOFTHEGENUSOFCLOUDS

Considerthesecriteriainsuccession

DistinctiveFeatures

thefeatureisessentialtothegenus

Legend

838 839

840

841

842

Ci Cc Cs Ac As Ns Sc St Cu Cb

S

P P P P P P

P P U U P P Puniform (intermittent

or continuous)P U P P

in the form of showers P U

P P

P P

P P

P P

P U P U

P P

P

P P P

S S S S S

P

P

P U P P U

P

P P P P P P

P P P

P

P P

P

P U P P S

P P U P P P P

P P P

P P

P

P

P

Praecipitatio (pra)

Virga (vir)

Mamma (mam)

Incus (inc)

TABULAR GUIDE FOR THE IDENTIFICATION OF THE GENUS OF CLOUDS (continued)

Thunder

Thunderstorm

Elec

tro-

met

eors

Consider these criteria in succession

Distinctive FeaturesGenera in which the features occur

Irisation on clouds

Rainbow

Lightning

Supp

lem

enta

ry fe

atur

es a

nd a

cces

sory

clou

dsHy

drom

eteo

rsPh

otom

eteo

rs

Snow pellets

Snow grains

Ice pellets

Hail

Halo phenomena

Corona

Fluctus (flu)

Flumen (flm)

Murus (mur)

Rain

Drizzle

Snow

Cauda (cau)

Cavum (cav)

Asperitas (asp)

Arcus (arc)

Tuba (tub)

Pileus (pil)

Velum (vel)

Pannus (pan)

843 844

845

CLOUD GENERA IDENTIFICATION PICTORIAL 846

847

of 53

848 II.7.2.2 849 IDENTIFYING THE SPECIES 850

The species should be identified based primarily on definitions and descriptions given in this Cloud 851 Atlas (link) and on comparison with theCloud Atlas reference imagery. If no species can be 852 identified by its characteristics, then none should be reported. Also, when several clouds of the 853 same species are present, they do not necessarily all belong to the same species. 854 855 11.7.2.3 856

IDENTIFYING THE VARIETIES 857 The variety or varieties should be identified based primarily on definitions and descriptions given in 858

thisCloud Atlas (link) and 859

on comparison with the Cloud Atlas reference imagery. Only indicate a variety if it is clearly 860

recognized. If the same cloud has 861

characteristics of more than one variety, all observed varieties should be reported. 862

863 Il.7.2.4 864

IDENTIFYING THE SUPPLEMENTARY FEATURES AND THE ACCESSORY CLOUDS 865 Any supplementary features and any accessory clouds should be identified based primarily on 866

definitions and descriptions given in this Cloud Atlas (link) and on comparison with the Cloud Atlas 867

reference imagery. More than one supplementary feature or accessory 868

cloud may be present simultaneously for the same cloud. 869

870

II.7.2.5 871

DETERMINING THE MOTHER-CLOUD 872 Any determination of a mother-cloud, from which the cloud under observation originated, 873

requires knowledge of the evolution of the clouds. Careful observation of the sky over time is 874

necessary. The observer should use the relevant definitions, descriptions and illustrations given in 875

thisCloud Atlas. If there is any doubt of the origin of the observed clouds (“mutatus”), or of the 876

manner of their formation (“genitus”), no “mutatus” or “genitus” indication should be used. 877

878 II.7.2.6 879 IDENTIFYING METEORS ASSOCIATED WITH THE CLOUDS 880 Meteors other than clouds, but associated with clouds, should be identified based primarily on 881

definitions and descriptions given in this Cloud Atlas (link) and on comparison with the Cloud 882

Atlas reference imagery. They should always be recorded together with the associated clouds, 883

since they often provide important information about the physical processes occurring in the 884

clouds. Their presence may even be decisive when identifying certain cloud genera. 885

886 887 II. 7.3 888 889 Total cloud cover and cloud amount 890 Total cloud cover is the fraction of the sky covered by all the visible clouds. Cloud amount refers to 891

the fraction of the sky covered by clouds of a particular type or combination. It can refer to a 892

genus, species, variety, layer, or a certain combination of clouds. 893

894

of 53

Always make an estimate of the total cloud cover and also of the cloud amounts of the various 895

genera present. Cloud amounts of the different species or varieties of clouds belonging to the 896

same genus, and of the different layers, should also be noted. 897

898

The estimate should be made from an open area from which the whole sky can be seen. When the 899

sky is partially hidden, for example by mountains, or by haze, fog or smoke, total cloud cover and 900

the cloud amounts should be estimated from the visible fraction. Also, when the sky is partly 901

veiled by precipitation, this part should be considered covered by the precipitating cloud. 902

903

It may be difficult to estimate cloud amounts if some clouds are only partly visible or temporarily 904

concealed. This is often the case when the clouds occur in superposed layers or patches. In this 905

situation it may be possible to estimate the cloud amount(s) by observing the sky over time, as 906

previously hidden clouds may become visible. When clouds are layered or otherwise superposed, 907

the sum of the observed cloud amounts may exceed the total cloud cover. 908

909

Gaps between clouds near the horizon may not be visible to the observer. Only gaps which are 910

visible from the observer's position should be considered when estimating cloud cover or cloud 911

amount. 912

913

On dark nights, only the total cloud cover can be determined. This should be based on the 914

proportion of the sky in which the stars are dimmed or completely hidden by clouds. 915

916

Il.7.4 917 Height and altitude 918 The observer should measure or estimate the height of the cloud base above the level of the place 919

of observation or the altitude above mean sea level. If possible, the clouds’ vertical extent should 920

also be determined. The basis for the reported height or altitude (estimation, measurement by, 921

ceilometer, etc, should always be stated. 922

923 II. 7.5 924 Direction and speed of movement 925 By convention, the direction of movement of a cloud is the direction from which the cloud moves. 926

For example, if a cloud moves from southwest to northeast, the recorded direction of movement 927

is "southwest". The speed of a cloud is the speed of its horizontal movement. 928

929

930

931

An observation of the sky should report the direction and, whenever possible, the speed of 932

movement of the clouds or their macroscopic elements. In most cases this is also a good 933

approximation of the direction and speed of the wind at cloud level. It is possible for the 934

movement of a cloud as a whole to be very different from the movement of its macroscopic 935

elements, particularly in the case of orographic clouds. When such a difference is observed, it 936

should be reported. 937

938

939

of 53

II.7.6 940 Optical thickness 941 The optical thickness of a cloud is the degree to which the cloud prevents light from passing 942

through it. Optical thickness depends on the physical properties and on the cloud dimensions. The 943

observer should record the optical thickness, and indicate the direction in which the clouds or 944

cloud layers have the greatest thickness. The following table (link) provides a numerical scale for 945

optical thickness. 946

947

948

Scale for Optical Thickness

1 Very weak Blue of the sky is discernible through the cloud.

2 Weak The cloud hides the blue of the sky, but does not prevent the sun from casting shadows. Such a cloud is usually white but may be light grey.

3 Moderate The cloud has a good general luminance, but noticeable shading in places. When present as an extensive sheet or layer, the cloud is light grey.

4 Strong The cloud is strongly shaded. When present as an extensive sheet or layer, the cloud appears dark grey. When the layer is discontinuous or formed of scattered elements, the parts directly exposed to the sun are white and fairly brilliant.

5 Very

strong

The cloud is dark, except for the parts exposed to the sun, which are brilliantly white. The cloud has a threatening appearance.

949

950

II .7.7 951

Observations of clouds made from mountain stations 952

When a mountain station is at a level lower than the base of the clouds, the procedure for 953

observing clouds is the same as at low level stations. Mountainous country often provides clear 954

vertical reference points so information about cloud height or altitude can often be quite accurate. 955

When clouds are observed below the station level, they should be indicated separately. A 956

description should be given of the upper surface of these clouds, including features such as a flat 957

or undulated surface, or the presence of towering cumuliform clouds above the top of the layer. 958

When estimating the cloud amount, locations where mountains protrude through a patch, sheet 959

or layer should also be considered as covered with clouds. 960

961 962 II. 7.8 963

Observation of Upper Atmospheric Clouds 964

965

II.7.8.1 966

NACREOUS AND NOCTILUCENT CLOUDS 967 The observation of nacreous and noctilucent clouds are covered in Part II.6 968

969

When these upper atmospheric clouds are observed, an exact record must be kept of the dates 970

and times and the location of the observation. The position of the clouds in the sky should be 971

recorded, and photographs should be made, whenever possible. 972

973

of 53

In the case of noctilucent cloud, the azimuth (both left and right extent) of the display, and the 974

elevation above the horizon of the upper edge of the cloud (and lower edge if there is one) should 975

be recorded. The brightness of the cloud, on a 5-point scale and, if possible, its structure and 976

classification should also be determined - (see Part II.6.3.3 for specific details of the brightness 977

scale and NLC classifications). Variations in brightness and classification through the period of 978

observation should also be recorded. 979

The general observing conditions should also be noted; for example, the presence of haze, mist 980

and any tropospheric clouds. The presence of aurora occurring simultaneously with NLC should 981

also be noted. 982

983

If the observer is unable to determine the azimuth or elevation of noctilucent clouds at the time of 984

the observation, a careful note should be made of the extent of the cloud relative to topographic 985

features, and the elevation relative to any bright stars that are visible in the vicinity of the clouds. 986

By providing such reference marks, the extent and elevation of the clouds may be determined 987

later. 988

989

II.8 – CODING OF CLOUDS IN THE CODES CL, CM AND CH 990

AND CORRESPONDING SYMBOLS 991

992

II.8.1 INTRODUCTION TO THE CODING OF CLOUDS 993

994

The codes CL, CM and CH presented in this Atlas, provide a convenient way of 995

describing clouds in meteorological reports, by means of figures selected from 996

tables of specifications. 997

998

In section II. 8.2, the code specifications and coding procedures are discussed in 999

detail. 1000

The information listed below is given for each code figure: 1001

(a) A technical specification. 1002

1003

(b) An explanation which enlarges the technical specification on the appearance and 1004

evolution of the clouds in question and sometimes relations or differences to other 1005

clouds. 1006

1007

In section II.8.3 the coding procedure for the selection of the correct code figure is 1008

explained. 1009

1010

1011

The selection of the correct code figures requires the following and must be 1012

noted:- 1013

1. Observation of the sky as a whole: 1014

of 53

The first requirement ar ises from the fact that certain code specifications apply 1015

not only to the particular genera, species or varieties of the individual clouds, but 1016

also to the aspect of the sky as a whole. 1017

In addition, there are situations in which the aspect of the sky as a whole is 1018

immediately recognizable, whereas the cloud forms present are difficult to 1019

identify. 1020

1021

2. An almost continuous watch on the sky: 1022

The second requirement stems from the fact that certain code specifications are 1023

directly related to the evolution and development of individual clouds or of the total 1024

cloud cover. An almost continuous watch on the sky is also necessary in situations 1025

when the appearance of the sky at the time of observation is so confusing that it is 1026

impossible to select the correct code figure, except by relating the existing 1027

transitional cloud forms with the characteristic forms from which they evolved. 1028

1029

CL code indicates clouds of the genera Stratocumulus, Stratus, Cumulus 1030

and Cumulonimbus, 1031

CM code indicates clouds of the genera Altocumulus, Altostratus and 1032

Nimbostratus. 1033

CH code indicates clouds of the genera Cirrus, Cirrocumulus and 1034

Cirrostratus. 1035

1036

1037

1038

1039

II.8.2 CODE SPECIFICATIONS AND CODING PROCEDURES 1040

1041

II.8.2.1 CL -clouds of the genera Stratocumulus, Stratus, Cumulus and Cumulonimbus 1042

1043

II.8.2.1.1 CL = 0 1044 1045 a) TECHNICAL SPECIFICATION 1046

No CL-clouds (Stratocumulus, Stratus, Cumulus or Cumulonimbus). 1047 1048

1049

II.8.2.1.2 CL = 1 1050

1051

a) TECHNICAL SPECIFICATION 1052

Cumulus humilis or Cumulus fractus of dry weather or both. 1053

humilis: little vertical extent and seemingly flattened 1054

fractus : ragged 1055

("Dry weather" denotes conditions without precipitation). 1056

of 53

1057

b) EXPLANATION 1058

The clouds corresponding to the code figure CL = 1 include the following: 1059

(i) Cumulus in the initial stages of formation or in the last stages of 1060

dissipation. 1061

(ii) Cumulus completely formed but frayed by a fairly strong and sufficiently 1062

turbulent wind; these Cumulus fractus are well separated and generally 1063

look white. The difference between these clouds and Cumulus fractus of 1064

wet weather is indicated in the explanation on specification CL = 7. 1065

(iii) Cumulus completely formed with clear-cut horizontal bases; these clouds 1066

have either a flattened or deflated form, or show somewhat rounded tops 1067

without a cauliflower appearance. 1068

1069

1070

II.8.2.1.3 CL = 2 1071

1072


Cumulus mediocris or congestus, with or without Cumulus of species fractus or 1074

humilis or Stratocumulus, all having their bases at the same level. 1075

1076

b) EXPLANATION 1077

Cumulus mediocris or congestus are Cumulus of moderate or strong 1078

vertical extent, generally with protuberances in the form of domes or 1079

towers. 1080

i) On days with fresh or strong wind, these Cumulus have irregular bases 1081

and may be ragged in places. 1082

ii) In middle latitudes hot days with a thundery tendency, and also 1083

frequently in low latitudes (trade wind zones), Cumulus are generally of 1084

the species congestus. They have a clear-cut horizontal base and a 1085

bulging upper part resembling a cauliflower; these clouds are sometimes 1086

in the form of a tower, sometimes in the form of a complex mass of 1087

protuberances. 1088

iii) Cumulus congestus may sometimes give precipitation in the form of 1089

showers. 1090

iv) Sometimes, the turrets of Stratocumulus castellanus develop so strongly that 1091

they reach the stage of Cumulus mediocris or congestus; the coding is then CL 1092

= 2, and not CL = 5. An analogous evolution may occur in Altocumulus 1093

castellanus; the coding is then again CL = 2, and not CM = 8 1094

1095

1096

II.8.2.1.4 CL = 3 1097

of 53

1098


Cumulonimbus calvus, with or without Cumulus, Stratocumulus or Stratus. 1100

1101

b) EXPLANATION 1102

i) The principal characteristic of calvus (bald) is the summits of the 1103

Cumulonimbus lack sharp outlines and none of the Cumulonimbus present 1104

have yet reached the stage of Cumulonimbus capillatus. 1105

ii) Cumulonimbus calvus evolve from Cumulus congestus and usually develop 1106

rapidly into Cumulonimbus capillatus. Cumulonimbus calvus generally 1107

constitutes a very short term intermediate stage of development. 1108

iii) They are distinguished from Cumulus congestus by the at least partial 1109

disappearance of clear-cut outlines and cauliflower appearance; both 1110

characteristic of the upper part of Cumulus congestus. 1111

iv) They are distinguished from Cumulonimbus capillatus by no portion of 1112

their upper part having a clearly fibrous or striated appearance, or any 1113

development in the form of an anvil, a plume or a mass of hair. 1114

v) The smooth part of a Cumulonimbus calvus may become hidden by new 1115

domes produced by other convective updrafts. Although the cloud mass 1116

may temporarily assume the appearance of Cumulus congestus, it is still to 1117

be identified as Cumulonimbus calvus and coded CL = 3. 1118

vi) Sometimes a cloud identified as Cumulus congestus is accompanied by 1119

lightning, thunder or hail. The cloud is identified as Cumulonimbus calvus 1120

and coded CL = 3. 1121

1122

1123

II.8.2.1.5 CL = 4 1124

1125


Stratocumulus cumulogenitus; Cumulus may also be present. 1127

1128

b) EXPLANATION 1129

i) Stratocumulus cumulogenitus most often results from the spreading out 1130

of Cumulus when its vertical development (extent) reaches a stable layer. 1131

ii) When this layer is very stable, convection ceases and the whole cloud 1132

mass spreads out. 1133

iii) On some occasions, the stable layer is either not strong or deep enough to 1134

completely stop convection. Cumulus, after temporarily spreading out, 1135

then resume their growth above the stable layer, at least in some places. 1136

Thus, Stratocumulus cumulogenitus may occur at any level between the 1137

base and top of Cumulus clouds. 1138

of 53

iv) The transformation of Cumulus clouds into Stratocumulus cumulogenitus 1139

is a continuous process, generally marked by gradual widening of the 1140

Cumulus clouds towards the stable layer where they spread out. 1141

v) Stratocumulus cumulogenitus may form by the spreading out of the upper 1142

part of Cumulus clouds as a result of strong wind shear. 1143

vi) Stratocumulus cumulogenitus often occurs in the late afternoon and 1144

evening when convection ceases and the domed summits of Cumulus 1145

clouds flatten, assuming the appearance of patches of Stratocumulus. 1146

vii) Stratocumulus cumulonimbogenitus and Stratocumulus cumulogenitus is 1147

coded as CL = 3 or 9 while Cumulonimbus is observed. 1148

viii) If the Cumulonimbus has disappeared, the presence of Stratocumulus 1149

cumulonimbogenitus requires the coding CL = 4. 1150

ix) When pre-existing Stratocumulus is penetrated (entered or entered and 1151

passed through) by Cumulus, the latter does not widen upwards towards 1152

the Stratocumulus and a thinned or even a cleared zone may surround the 1153

Cumulus tower. The coding is CL = 8. 1154

1155

1156

1157

II.8.2.1.6 CL = 5 1158

1159


Stratocumulus non-cumulogenitus (not resulting from the spreading out of Cumulus). 1161

1162

b) EXPLANATION 1163

i) Stratocumulus, which can occur at one or more levels, usually consist of 1164

grey or whitish sheets or layers which almost always have dark parts. 1165

ii) They are composed of fairly large elements, separate, merged or broken 1166

up into patches. 1167

iii) Wind shear and turbulence may give the Stratocumulus a ragged 1168

appearance in places. 1169

iv) Sometimes it can produce precipitation in the form of very weak rain, 1170

snow or snow pellets. 1171

v) When turrets of Stratocumulus castellanus develop strongly, they may 1172

reach the stage of Cumulus mediocris or congestus; the coding is then CL= 1173

2, not CL= 5. 1174

vi) Stratocumulus non-cumulogenitus broken up into patches (CL= 5) can look 1175

similar to, but should not be confused with Stratocumulus formed by the 1176

spreading out of Cumulus or Cumulonimbus. In this latter case the coding 1177

is CL= 4. 1178

vii) Sometimes a layer of Stratocumulus assumes a menacing appearance and 1179

of 53

,

its base becomes diffuse in places, indicating a process of transformation 1180

into Nimbostratus. When the transformation is complete in a substantial 1181

continuous portion of the layer, as is evidenced by the absence of 1182

elements, this portion is identified as Nimbostratus and is reported as CM = 1183

2. 1184

1185 1186

II.8.2.1.7 CL = 6 1187

1188


Stratus nebulosus or Stratus fractus of dry weather, or both. 1190

1191

b) EXPLANATION 1192

i) Stratus nebulosus generally consists of a continuous single sheet or layer 1193

with a fairly uniform base, usually grey but occasionally dark or menacing. 1194

ii) Stratus fractus are a transitory stage during the formation or the 1195

dissipation of a layer of Stratus. 1196

iii) Differences between this Stratus fractus and Stratus fractus of wet 1197

weather are pointed out in the explanation on CL = 7. 1198

iv) When Stratus fractus occur beneath a layer of Stratus nebulosus, they may 1199

be either fragments which become merged with the base of the layer 1200

when the latter is in the process of thickening, or fragments detached 1201

from the base when the layer is in the process of breaking up. 1202

1203

1204

II.8.2.1.8 CL = 7 1205

1206


Stratus fractus or Cumulus fractus of wet weather, or both (pannus), usually below 1208

Altostratus or Nimbostratus. 1209

Wet weather denotes the conditions which generally exist during precipitation and a 1210

short time before and after 1211

1212

b) EXPLANATION 1213

i) Stratus fractus of wet weather or Cumulus fractus of wet weather, or both 1214

(pannus), often form beneath the base of lowering Altostratus or 1215

Nimbostratus. As a rule, they become increasingly numerous and merge 1216

into a more or less continuous layer. The pannus clouds appear dark or 1217

grey against a background of lighter grey formed by the base of the cloud 1218

layer above them which is usually visible through gaps in the pannus layer. 1219

They can also often be present beneath the base of a Cumulonimbus or a 1220

precipitating Cumulus. 1221

of 53

ii) Pannus clouds covering the entire sky are distinguishable from Stratus 1222

nebulosus and Stratocumulus by their ragged base. 1223

iii) Stratus fractus of the specification CL = 7 always occur in conjunction with 1224

clouds of other genera; they are generally numerous and appear dark or 1225

grey against the lighter grey background of the base of the cloud layer 1226

above them. They almost always have a certain character of instability and 1227

they generally move fast and change shape rapidly. They are usually 1228

accompanied by precipitation. 1229

iv) Stratus fractus of the specification CL = 6 may occur alone, in which case 1230

they appear grey when viewed towards the sun and white when viewed 1231

away from the sun. They look similar to Stratus fractus of the specification 1232

CL = 7 when seen against a background of other clouds, such as a Stratus 1233

nebulosus layer; however, they are not accompanied by precipitation. 1234

v) Cumulus fractus of the specification CL = 7 always occur in conjunction 1235

with clouds of other genera; they are generally numerous and stand out 1236

dark or grey against the lighter grey background formed by the base of the 1237

clouds above them. Like Stratus fractus of the same specification, Cumulus 1238

fractus clouds of wet weather almost always have a certain character of 1239

instability. They are frequently accompanied by precipitation. 1240

vi) Cumulus fractus of specification CL = 1 mostly occur alone and are well 1241

separated. They are characteristically white, appearing almost brilliant 1242

when viewed away from the sun and showing shading when viewed 1243

towards the sun. These clouds are frequently observed when the wind at 1244

their level is fairly strong and turbulent. 1245

1246

1247

II.8.2.1.9 CL = 8 1248

1249


Cumulus and Stratocumulus other than Stratocumulus cumulogenitus, with bases at 1251

different levels. 1252

1253

b) EXPLANATION 1254

i) The code figure CL = 8 applies when Cumulus clouds form beneath patches 1255

or a sheet or layer Stratocumulus non-cumulogenitus. 1256

ii) Cumulus may penetrate (enter or enter and pass through) the 1257

Stratocumulus. 1258

iii) Cumulus does not, however, spread out and form Stratocumulus 1259

cumulogenitus. 1260

iv) The code figure CL = 8 also applies when Cumulus are observed above 1261

Stratocumulus. An example is Altocumulus castellanus towers developing 1262

of 53

and transitioning into Cumulus mediocris or congestus. 1263

1264

1265

II .8.2.1.10 CL = 9 1266

1267


Cumulonimbus capillatus (often with an anvil), with or without Cumulonimbus 1269

calvus, Cumulus, Stratocumulus, Stratus or pannus. 1270

1271

b) EXPLANATION 1272

i) Cumulonimbus capillatus evolve from Cumulonimbus calvus. They are 1273

distinguished from Cumulonimbus calvus (CL = 3) by the appearance of their 1274

upper portions: 1275

the upper part of a Cumulonimbus capillatus shows a clearly fibrous or 1276

striated structure and frequently has a form resembling that of an anvil, a 1277

plume or a huge mass of hair; 1278

a Cumulonimbus calvus has no fibrous or striated parts. 1279

ii) Among the numerous possible cases covered by CL = 9, the following two are 1280

frequently observed: 1281

Cumulonimbus with a clear-cut horizontal base which is sometimes partially 1282

or totally hidden by pannus occur during hot, thundery days in middle 1283

latitudes and, frequently, in the humid zones of low latitudes. 1284

Cumulonimbus with their base frayed by a fairly strong wind and 1285

occasionally accompanied by pannus. 1286

iii) The cirriform parts of Cumulonimbus capillatus may become invisible when the 1287 cloud passes over the point of observation. It should nevertheless be classified 1288

as Cumulonimbus capillatus on the basis of its history, coding CL = 9 1289

The same applies when the cirriform parts of Cumulonimbus capillatus become 1290

hidden by other clouds. 1291

iv) The occurrence of lightning, thunder or hail sometimes provides the only 1292 indication of the presence of a Cumulonimbus. In this case it is not possible to 1293 decide whether the cloud belongs to the species calvus or capillatus, the coding 1294 is by convention CL = 9. 1295

v) Sometimes, when the 0°C (32°F) level is low, the fibrous structure of the upper 1296 part spreads through the whole Cumulonimbus capillatus, which then 1297

degenerates into a cirriform cloud mass (CH = 3), the coding CL = 9 is maintained 1298

as long as at least one Cumulonimbus remains in sight or is known to be present. 1299

vi) Cumulonimbus capillatus sometimes produces cloud masses which may 1300

become detached from it and assume an independent identity. Very often they 1301

have the appearance of Cirrus, Altocumulus, Altostratus or Stratocumulus. 1302

1303

1304

II.8.2.1.11 CL = / 1305

of 53

1306


CL clouds invisible owing to darkness, fog, blowing dust or sand, or other similar 1308

phenomena. 1309

1310

II.8.2.2 CM-clouds of the genera Altocumulus, Altostratus and Nimbostratus 1311

1312

II.8.2.2.1 CM = 0 1313

1314


No CM-clouds. 1316

1317

1318

II.8.2.2.2 CM = 1 1319

1320


Altostratus translucidus. 1322

1323

b) EXPLANATION 1324

i) The greater part of this Altostratus, which is of a greyish or bluish colour, is 1325

translucent enough to reveal the position of the sun or moon. This Altostratus 1326

usually forms by the continuous evolution of a gradually thickening veil of 1327

Cirrostratus. Sometimes, especially in the tropics, it may be produced by the 1328

spreading out of the middle or upper part of a Cumulonimbus. 1329

ii) Altostratus does not show halo phenomena. 1330

1331

1332

II.8.2.2.3 CM = 2 1333

1334


Altostratus opacus or Nimbostratus. 1336

1337

b) EXPLANATION 1338

i) Altostratus corresponding to the code figure CM = 2 is of a darker grey or a 1339

darker bluish grey than Altostratus translucidus and is sufficiently dense over 1340

the greater part of its extent to completely mask the sun or moon. It may 1341

occur in several layers. 1342

ii) Altostratus opacus may result from the thickening of a layer of Altostratus 1343

translucidus, from the merging of the elements of a sheet or layer of 1344

Altocumulus, from the spreading out of the middle or upper part of a 1345

Cumulonimbus, from thinning of Nimbostratus or from the horizontal extension 1346

of 53

of Cirrus spissatus. 1347

iii) Nimbostratus, which is also CM = 2, has a denser and darker appearance than 1348

Altostratus opacus; its base is at a comparatively low level and generally has a 1349

diffuse and "wet" appearance. 1350

iv) Nimbostratus results either from the evolution of a thick layer of Altostratus 1351

opacus or from the merging of the elements of a thick sheet or layer of 1352

Altocumulus opacus or rarely, Stratocumulus opacus. It may also evolve from 1353

Cumulonimbus clouds. 1354

v) When pannus clouds accompanying the layer of Altostratus opacus or 1355 Nimbostratus become merged into a continuous layer such that the Altostratus 1356

or Nimbostratus can no longer be seen, the coding CM = 2 should be replaced 1357

by CM = / ; the pannus clouds are coded CL = 7. 1358

1359

II.8.2.2.4 CM = 3 1360

1361


Altocumulus translucidus at a single level. 1363

1364

b) EXPLANATION 1365

i) The coding CM = 3 applies to Altocumulus in patches or sheets at the same 1366

level, or to Altocumulus in a layer; the various elements of these clouds are 1367

neither very large nor very dark. If the cloud elements change at all, they do so 1368

in a hardly perceptible manner and they do not progressively invade the sky. 1369

ii) The sky may contain several Altocumulus patches or sheets at the same level 1370

and of different optical thickness. The definition of the variety translucidus 1371

permits individual patches or sheets to be identified as Altocumulus 1372

translucidus when their greater part is sufficiently translucent to reveal the 1373

position of the sun or moon. However, when the code refers to Altocumulus 1374

translucidus, it relates to the total amount of Altocumulus at the same level. 1375

The coding CM = 3 can only be used when semi-transparent Altocumulus 1376

predominates at that level. Similar convention applies when code 1377

specifications refer to Altocumulus opacus. 1378

iii) When Altocumulus translucidus (not invading the sky) is present at two or 1379

more levels refer to CM = 7. 1380

1381

1382

II.8.2.2.5 CM = 4 1383

1384


Patches (often lenticular) of Altocumulus translucidus, continually changing and 1386

occurring at one or more levels. 1387

1388

of 53

b) EXPLANATION 1389

i) The irregularly arranged elements of Altocumulus patches of the specification 1390

CM = 4 are continually changing in shape; they often appear to be dissolving in 1391

some places and forming in others. The cloud patches are of limited horizontal 1392

extent and their elements are continually changing; as a consequence they 1393

usually belong to the variety translucidus and only rarely to the variety opacus. 1394

The patches as a whole may have the form of large lenses and may occur at 1395

one or more levels. The clouds are not progressively invading the sky. 1396

ii) The coding CM = 4 is applicable not only to the above described patches which 1397

consist of numerous relatively small, continually changing elements but also to 1398

those relatively stable clouds which consist of one single smooth lenticular 1399

element or of a pile of such elements. 1400

iii) These clouds may occur in the form of accessory clouds (pileus, velum) either 1401

near or fairly distant from the upper part of Cumulus or Cumulonimbus. 1402

iv) Lenticular clouds are frequently observed in hilly or mountainous regions. 1403

1404

1405

II.8.2.2.6 CM = 5 1406

1407


Altocumulus translucidus in bands, or one or more layers of Altocumulus translucidus 1409

or opacus, progressively invading the sky; these Altocumulus generally thicken as a 1410

whole. 1411

1412

b) EXPLANATION 1413

i) The main characteristic is Altocumulus invading the sky progressively. The 1414

cloud gradually advances from one part of the horizon toward the zenith, 1415

whereby the cloud amount increases. The border of the cloud system often 1416

passes the zenith and may finally reach the horizon opposite to that from 1417

which the cloud first appeared. When looking at the sky, observers will see 1418

the cloud system extending down to the horizon in the direction in which the 1419

clouds initially appeared; it is also in this direction that the clouds are usually 1420

thickest. The main part of the system consists of one or more layers, wholly or 1421

partially translucent or wholly or partially opaque. The forward portion of the 1422

system, often in the process of dissipation, may consist of small frayed 1423

Altocumulus elements or of rolls or bands, usually observed at a single level 1424

and consisting of semitransparent clouds. This forward portion may cover a 1425

large expanse of the sky. 1426

ii) The coding CM = 5 is not used as soon as the forward edge has reached the 1427

part of the horizon opposite to that where the clouds first appeared, or when 1428

the forward edge has ceased its progress. 1429

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iii) Altocumulus progressively invading the sky may at the same time be 1430

changing, either partially or as a whole, into Altostratus or Nimbostratus. If 1431

the Altocumulus has partly changed into Altostratus or Nimbostratus, i.e. if in 1432

a part of the Altocumulus, the evidence for the existence of elements 1433

(laminae, rolls, rounded masses, etc.) has disappeared, the coding becomes 1434

CM = 7 instead of CM = 5. As soon as the evidence for the existence of 1435

elements has disappeared throughout, the coding is CM = 1 or CM = 2, as the 1436

case may be. 1437

1438

1439

II.8.2.2.7 CM = 6 1440

1441


Altocumulus cumulogenitus (or cumulonimbogenitus). 1443

1444

b) EXPLANATION 1445

i) Altocumulus cumulogenitus results generally from the spreading out of the 1446

summits of Cumulus that reach a stable layer. Occasionally, Cumulus congestus 1447

in vertical development meet stable layers which cannot stop their growth 1448

completely; in this case, the Cumulus, after a temporary spreading out, resume 1449

their growth above the stable layer, at least in places. Thus, the Altocumulus 1450

cumulogenitus may appear on the lateral portion of Cumulus congestus. 1451

ii) Owing to its mode of formation, Altocumulus cumulogenitus occurs in patches. 1452

Initially, these patches with large and dark elements are fairly thick and 1453

opaque; their under surface may show a rippled relief. Later on, the patches 1454

thin out and finally break into separate elements. The same sky often shows 1455

Altocumulus patches in various stages of evolution. 1456

iii) When the Altocumulus cumulogenitus patches are seen in profile, they may 1457

show, especially at their borders, a cumuliform appearance. Care should be 1458

exercised not to confuse such patches with Altocumulus castellanus. 1459

iv) Altocumulus cumulogenitus should not be confused with the anvil of a 1460

Cumulonimbus or with Cirrus spissatus cumulonimbogenitus, both of which 1461

may show mamma at their lower surface and may resemble Altocumulus. 1462

However, Altocumulus never has the fibrous structure, the silky sheen and the 1463

whiteness of the anvils or of the Cirrus spissatus. 1464

v) Altocumulus accompanying Cumulonimbus (Ac cumulonimbogenitus) is also 1465

coded CM = 6; it often forms whilst the Cumulonimbus is in its Cumulus stage 1466

of development. 1467

1468

1469

II.8.2.2.8 CM = 7 1470

1471

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Altocumulus duplicatus, or Altocumulus opacus in a single layer, not progressively 1473

invading the sky, or Altocumulus with Altostratus or Nimbostratus. 1474

1475

b) EXPLANATION 1476

The specification CM = 7 includes the following skies: 1477

i) Patches, sheets or layers of Altocumulus at different levels (duplicatus); these 1478

patches, sheets or layers may be Altocumulus translucidus usually opaque in 1479

places, or Altocumulus opacus. The elements of this Altocumulus are not 1480

changing continually; the clouds are not progressively invading the sky. 1481

ii) Patches, sheets or a layer of Altocumulus opacus at a single level. The 1482

elements are not changing continually; the clouds are not progressively 1483

invading the sky. 1484

The sky may contain several Altocumulus patches or sheets at the same level 1485

and of different optical thickness. The definition of the variety opacus permits 1486

individual patches or sheets to be identified as Altocumulus opacus when their 1487

greater part is sufficiently opaque to hide the position of the sun or moon. 1488

However, when the code refers to Altocumulus opacus, it relates to the total 1489

amount of Altocumulus at the same level. The case of CM = 7 under discussion 1490

can only be used when opaque Altocumulus predominates at that level. 1491

iii) Altocumulus together with Altostratus or Nimbostratus may be observed in 1492

the following arrangements: 1493

A single or a multiple layer showing partly the characteristics of 1494

Altocumulus, partly those of Altostratus or Nimbostratus. This sky often 1495

results from the transformation when Altocumulus locally changes and 1496

acquires the appearance of Altostratus or Nimbostratus, or Altostratus or 1497

Nimbostratus breaks up into Altocumulus, 1498

Altostratus translucidus or opacus above patches of Altocumulus at one or 1499

several levels. 1500

A rather low veil, often hardly discernible, together with higher 1501

Altocumulus. 1502

1503

1504

II.8.2.2.9 CM = 8 1505

1506


Altocumulus castellanus or floccus. 1508

1509

b) EXPLANATION 1510

i) These two species of Altocumulus have a cumuliform appearance; this feature 1511

is more marked in Altocumulus castellanus than in Altocumulus floccus. 1512

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ii) Altocumulus castellanus is composed of turrets which appear to be arranged in 1513

lines; the turrets generally have a common horizontal base, which gives the 1514

cloud a crenelated (castle battlement) appearance. 1515

iii) Altocumulus floccus occur in white or grey scattered tufts with rounded and 1516

slightly bulging upper parts; they are often accompanied by fibrous trails 1517

(virga). These clouds resemble very small, more or less ragged Cumulus. 1518

iv) When some of the Altocumulus castellanus or floccus present develop into 1519

Cumulus 1520

mediocris or congestus, or into Cumulonimbus, they become subject to the 1521

rules for coding CL-clouds. 1522

1523

1524

II.8.2.2.10 CM = 9 1525

1526


Altocumulus of a chaotic sky, generally at several levels. 1528

1529

b) EXPLANATION 1530

i) The main characteristic of this sky is its chaotic, heavy and stagnant 1531

appearance. The clouds of the middle level consist of superposed, more or less 1532

broken cloud sheets of ill-defined species or varieties, with all transitional 1533

forms from a rather low and opaque Altocumulus to a high, translucent and 1534

fibrous veil of Altostratus. 1535

ii) This sky also generally exhibits a diversity of clouds belonging to the low and 1536

high levels. 1537

1538

1539

II.8.2.2.11 CM = / 1540

1541


CM-clouds invisible owing to darkness, fog, blowing dust or sand or other similar 1543

phenomena, or because of a continuous layer of lower clouds. 1544

1545

1546

II.8.2.3 CH-clouds of the genera Cirrus, Cirrocumulus and Cirrostratus 1547

1548

II.8.2.3.1 CH = 0 1549

1550


No CH-clouds. 1552

1553

1554

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II.8.2.3.2 CH = 1 1555

1556


Cirrus fibratus, sometimes uncinus, not progressively invading the sky. 1558

1559

b) EXPLANATION 1560

i) Cirrus corresponding to CH = 1 most often occur in the form of nearly straight 1561

or more or less curved filaments (Cirrus fibratus); more rarely, they are shaped 1562

like commas topped with either a hook or a tuft which is not rounded (Cirrus 1563

uncinus). 1564

ii) Cirrus fibratus and uncinus commonly appear in the same sky with Cirrus of 1565

other species; the code figure CH = 1 may be used only if the sky cover of Cirrus 1566

fibratus or uncinus or of a combination of these clouds is greater than the 1567

combined sky cover of other Cirrus species. 1568

iii) Cirrus, coded CH = 1, is not progressively invading the sky. 1569

1570

1571

II.8.2.3.3 CH = 2 1572

1573


Cirrus spissatus, in patches or entangled sheaves, which usually do not increase and 1575

sometimes seem to be the remains of the upper part of a Cumulonimbus; or Cirrus 1576

castellanus or floccus. 1577

1578

b) EXPLANATION 1579

i) Cirrus corresponding to CH = 2 are of the species spissatus non-1580

cumulonimbogenitus, or Cirrus castellanus or floccus, or a combination of all 1581

these species. 1582

ii) Cirrus spissatus consist of patches of sufficient optical thickness to appear 1583

greyish when viewed towards the sun. They sometimes have borders of 1584

entangled filaments (variety intortus) and may give the erroneous impression 1585

that they are the remains of the upper part of a Cumulonimbus. 1586

iii) Cirrus castellanus shows small fibrous turrets or rounded protuberances, rising 1587

from a common base; Cirrus floccus has the form of more or less isolated tufts, 1588

often with trails. 1589

iv) The above-mentioned clouds may be accompanied by Cirrus fibratus or 1590

uncinus; however the sky cover of Cirrus spissatus non-cumulonimbogenitus, 1591

Cirrus castellanus or floccus or of any combination of these clouds is greater 1592

than the combined sky cover of Cirrus fibratus and uncinus. 1593

1594

1595

II.8.2.3.4 CH = 3 1596

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1597


Cirrus spissatus cumulonimbogenitus. 1599

1600

b) EXPLANATION 1601

i) The coding CH = 3 is used only when at least one Cirrus cloud present in the sky 1602

provides direct or indirect evidence of having originated from a 1603

Cumulonimbus. This Cirrus spissatus cumulonimbogenitus may be 1604

accompanied by Cirrus spissatus of doubtful origin, by Cirrus castellanus or 1605

floccus, or by Cirrus fibratus or uncinus. 1606

ii) Observers, by keeping a continuous watch on the sky, may be able to witness 1607

the development of Cirrus spissatus from the upper part of a Cumulonimbus. 1608

Often, however, they have no direct information about the origin of Cirrus 1609

spissatus. There may nevertheless be sufficient indirect evidence to indicate, 1610

with reasonable certainty that a Cirrus spissatus present in the sky originated 1611

from a Cumulonimbus. 1612

iii) Cirrus spissatus cumulonimbogenitus frequently reveals its origin by the hairy 1613

or frayed appearance of its edges, by its general anvil-like shape, or by its 1614

optical thickness which is often sufficient to veil the sun, obscure its outlines or 1615

even hide it. 1616

1617

1618

II.8.2.3.5 CH = 4 1619

1620


Cirrus uncinus or fibratus, or both, progressively invading the sky; they generally 1622

thicken as a whole. 1623

1624

b) EXPLANATION 1625

i) The main characteristic of Cirrus corresponding to CH = 4 is they are invading 1626

the sky progressively. The collection of clouds extends to one part of the 1627

horizon and its forward edge is moving towards the opposite part of the 1628

horizon. 1629

ii) The clouds occur most frequently in the form of strands trailing from a small 1630

hook or tuft (Cirrus uncinus); less frequently, they are in the form of straight or 1631

irregularly curved filaments (Cirrus fibratus). 1632

iii) The clouds usually seem to fuse together in the direction of the horizon from 1633

which they first appeared, but no Cirrostratus is present. 1634

1635

1636

1637

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II.8.2.3.6 CH = 5 1638

1639


Cirrus (often in bands) and Cirrostratus, or Cirrostratus alone, progressively invading 1641

the sky; they generally thicken as a whole, but the continuous veil does not reach 45 1642

degrees above the horizon. 1643

1644

b) EXPLANATION 1645

i) Cirrostratus invading the sky progressively but with its continuous part still 1646

less than 45 degrees above the horizon. 1647

ii) The veil of the Cirrostratus may be preceded by Cirrus clouds, often in long 1648

filaments (Cirrus fibratus), or shaped like commas (Cirrus uncinus), frequently 1649

arranged in bands crossing a part of the sky and seemingly converging towards 1650

one point or towards two opposite points of the horizon (variety radiatus). 1651

iii) Cirrus may also have a form resembling a fish skeleton (variety vertebratus). 1652

1653

1654

II.8.2.3.7 CH = 6 1655

1656


Cirrus (often in bands} and Cirrostratus, or Cirrostratus alone, progressively invading 1658

the sky; they generally thicken as a whole; the continuous veil extends more than 45 1659

degrees above the horizon, without the sky being totally covered. 1660

1661

b) EXPLANATION 1662

i) Cirrostratus invading the sky progressively with its continuous part more than 1663

45 degrees above the horizon but not covering the sky completely. 1664

ii) The veil of the Cirrostratus may be preceded by Cirrus clouds often in long 1665

filaments (Cirrus fibratus) or shaped like commas (Cirrus uncinus), frequently 1666

arranged in bands crossing a part of the sky and seemingly converging towards 1667

one point or towards two opposite points of the horizon (variety radiatus). 1668

iii) Cirrus may also have a form resembling a fish skeleton (variety vertebratus) 1669

1670

1671

II.8.2.3.8 CH = 7 1672

1673


Cirrostratus covering the whole sky. 1675

1676

b) EXPLANATION 1677

i) Cirrostratus covering the whole sky usually occurs as a light, uniform and 1678

nebulous veil showing no distinct details (Cirrostratus nebulosus), or as a white 1679

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and fibrous veil with more or less clear-cut striations (Cirrostratus fibratus). 1680

ii) The Cirrostratus veil is sometimes so thin that it is hardly visible and halo 1681

phenomena, especially frequent in thin Cirrostratus, provide the only evidence 1682

for its presence. The Cirrostratus may also be relatively dense. 1683

iii) Cirrus at different levels and Cirrocumulus may accompany the Cirrostratus. 1684

iv) When a veil of Cirrostratus is concealed in places by clouds at a lower level, or 1685

when the horizon is dark or hidden partially or totally by haze, smoke, etc., 1686

observers shall not report CH = 7 unless they are sure (for instance from 1687

continuous observation) that the Cirrostratus really covers the whole sky. If 1688

any doubt exists, the coding should be CH = 8, unless it is known that the veil 1689

was invading the sky progressively, in which case the coding CH = 6 is used. 1690

v) If there are gaps or clear intervals in the veil, through which it is possible to 1691

distinguish the blue of the sky, the coding should be CH = 8. 1692

vi) When by a process of continuous transition, a thin layer of Altostratus 1693

translucidus follows upon a complete veil of Cirrostratus, the two together 1694

covering the whole sky, the code figure CH = 7 should be used simultaneously 1695

with the coding CM = 1 (if no Altocumulus is present) or CM = 7 (if Altocumulus 1696

is present). 1697

1698

1699

II.8.2.3.9 CH = 8 1700

1701


Cirrostratus not progressively invading the sky and not entirely covering it. 1703

1704

b) EXPLANATION 1705

i) A veil of Cirrostratus which is not (or no longer) invading the sky progressively 1706

and which does not completely cover the whole sky; the edge of the veil may 1707

be clear cut or frayed. 1708

ii) The code figure CH = 8 also applies to patches of Cirrostratus, increasing in 1709

amount or not. 1710

iii) Cirrus and Cirrocumulus (not predominant) may also be present. 1711

1712

1713

II.8.2.3.10 CH = 9 1714

1715


Cirrocumulus alone, or Cirrocumulus predominant among the CH-clouds. 1717

1718

b) EXPLANATION 1719

i) The code figure CH = 9 may be used only if Cirrocumulus is either the only CH-1720

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cloud present, or if its amount is greater than the combined sky cover of any 1721

coexisting Cirrus and Cirrostratus. 1722

ii) When Cirrocumulus is the only CH-cloud in the sky, its elements are frequently 1723

grouped into more or less extensive patches with very characteristic small 1724

wavelets. 1725

iii) When Cirrocumulus occurs together with Cirrus or Cirrostratus, these clouds 1726

are often associated in composite patches, usually in a process of continual 1727

internal transformation. 1728

1729

1730

II.8.2.3.11 CH = / 1731

1732


CH-clouds invisible owing to darkness, fog, blowing dust or sand or other similar 1734

phenomena, or because of a continuous layer of lower clouds. 1735

1736

1737 1738 II. 8. 3 CODING INSTRUCTIONS OF CLOUDS IN THE CODES CL, CM AND CH 1739

1740

There are strict rules and priorities to be followed in order to code the sky view correctly. 1741

The highest priority is at the top. If the first point (1.) is not applicable, continue with the 1742

next point (2.). Continue down until you reach the cloud you are observing. If you identify 1743

more than one cloud, report the cloud with the priority. 1744

1745

Remark: 1746

When applying the coding instructions, the word “present” means, that if this cloud type 1747

occurs simultaneously with others, even predominate clouds, the stated priority leads the 1748

observer to choose the correct code figure for the observed sky. 1749

1750

1751

II.8.3.1 Coding instructions CL 1752

1753

1. If at least a part of one Cumulonimbus present is of the species capillatus, the coding 1754

is CL = 9. 1755

2. If a Cumulonimbus has not yet become clearly fibrous or striated, the coding is CL = 1756

3. 1757

3. If Stratocumulus formed by spreading out of Cumulus is present, the coding is CL = 4. 1758

4. If simultaneous occurrence of Cumulus and Stratocumulus with bases at different 1759

levels are present, the coding is CL = 8. 1760

5. If there are Cumulus mediocris or congestus, all having their bases at the same level, 1761

the coding is CL = 2. 1762

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6. If none of the above are applicable, then choose the cloud that predominates from 1763

the following: 1764

- if Stratus fractus or Cumulus fractus of wet weather predominates, use code 1765

figure CL = 7 1766

- if Stratus nebulosus / fractus of dry weather predominates, use code figure CL = 1767

6 1768

- if Stratocumulus non-cumulogenitus predominates, use code figure CL=5 1769

- if Cumulus humilis / fractus of dry weather predominates, use code figure CL = 1770

1 1771

1772

1773

II.8.3.2 Coding instructions CM 1774

1775

1. If Altocumulus of a chaotic sky is present, the coding is CM = 9. 1776

2. If Altocumulus castellanus or floccus is present, the coding is CM = 8. 1777

3. If Altocumulus is coexisting with Altostratus or Nimbostratus, the coding is CM = 7. 1778

4. If Altocumulus cumulogenitus or cumulonimbogenitus is present, the coding is CM = 1779

6. 1780

5. If Altocumulus is progressively invading the sky, the coding is CM= 5. 1781

6. If Altocumulus patches are continuously changing, the coding is CM = 4. 1782

7. If Altocumulus (of the variety translucidus and/or opacus) are at two or more levels, 1783

the coding is CM = 7. 1784

8. If Altocumulus at a single level is predominantly translucidus, the coding is CM = 3. 1785

9. If Altocumulus at a single level is predominantly opacus, the coding is CM = 7. 1786

10. If the greater part of Altostratus is semi-transparent, the coding is CM = 1 1787

11. If the greater part of Altostratus is dense enough to hide the sun or moon 1788

completely or if there is Nimbostratus, the coding is CM = 2. 1789

1790

1791

II.8.3.3 Coding instructions CH 1792

1793

1. If Cirrocumulus is alone or with other cirriform cloud but predominates the sky, the 1794

coding is CH = 9. 1795

2. If Cirrostratus is covering the whole sky, the coding is CH = 7. 1796

3. If Cirrostratus is not invading the sky and not entirely covering it, the coding is CH = 1797

8. 1798

4. If Cirrostratus is progressively invading the sky and extends more than 45° above the 1799

horizon but is not covering the whole sky, the coding is CH = 6. 1800

5. If Cirrostratus is progressively invading the sky and extends less than 45° above the 1801

horizon, the coding is CH = 5. 1802

6. If Cirrus uncinus and/or fibratus is progressively invading the sky, the coding is CH = 1803

of 53

4. 1804

7. If one of the Cirrus spissatus clouds originates from a Cumulonimbus, the coding is 1805

CH = 3. 1806

8. If the sky cover of Cirrus spissatus non-cumulonimbogenitus and Cirrus castellanus 1807

and/or floccus predominates, the coding is CH = 2. 1808

9. Cirrus fibratus and/or uncinus are coded as CH = 1. 1809

1810

1811

II.8.3.4 Flow charts CL, CM and CH 1812

1813

The graphical method of coding represents the same order/priorities to be respected as the 1814

described coding instructions. Start at the top left corner and answer each question until 1815

you reach a cartoon of a cloud or a sky view. If this situation corresponds to the reality, then 1816

report the code figure on the right hand bottom of the cartoon. If not, start again at the 1817

beginning to find your observed sky, respecting the priorities. 1818 1819

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1820

1821

1822

1823

1824

1825

1826

1827

1828

1829

1830

1831

1832

1833

1834

1835

1836

1837

1838

1839

1840

1841

1842

1843

1844

1845

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1846

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1847

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II.8.4 CLOUD SYMBOLS CORRESPONDING TO THE CL, CM and CH CODES 1848

1849

The clouds corresponding to the different figures of the CL CM and CH codes may be represented by 1850

means of symbols. These symbols are as follows. 1851

1852

CL CM CH

0

1

2

3

4

5

6

7

8

9 1853

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