BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 4

Chapter Four CHAPTER FOUR Classification of Clouds INTRODUCTION Cloud identification can be and often is somewhat complicated various types of cloud being present at different levels at the same time: on other occasions it may be quite simple with only one cloud type present It is important that a seafarer should be able to recognise the main cloud types because of their bearing upon existing and future weather; he or she does not need to be a weather forecaster but the arrival of a particular type of cloud may confirm the timing of a particular forecast or may give warning of an unexpected weather change This can be particularly useful when weather forecasts are for any reason unobtainable A seafarer also needs to be able to recognise and name cloud types for inclusion in weather reports which are transmitted to a Meteorological Service The cloud formation in any locality is an important feature of most weather situations and provides the meteorologist with valuable information when preparing a forecast SUMMARY OF CLOUD TYPES The names and descriptions of the various cloud types have been agreed internationally by the World Meteorological Organisation The earliest cloud classification, prepared in 1803, contained five types derived from Latin wordsCirrus (meaning 'thread'), Cumulus (heap), Stratus (layer), Nimbus (grey rain cloud) and Fractus (broken) The present classification is an amplif1cation and combination of these Briefly, Cirrus clouds are of a feathery or fibrous appearance: Cumulus are cauliflower shaped above with a flattish base: Stratus clouds form a more or less shapeless or homogenous layer or sheet: Nimbus clouds have a uniform dark grey and threatening appearance, often with precipitation or "virga" trailing beneath Cloud types are further divided into three groups based upon their height above the observer, high, medium and low CLOUD HEIGHTS The names of the main cloud types, their abbreviations and their approximate heights are shown in the table overleaf: 14 Chapter Four CLOUD AVERAGE HEIGHT RM'J"GES CLOUDS GROUP (For middle latitudes) LOW From near surface level Stratus (St) LEVEL To about Nimbostratus (Ns) CLOUDS 2,000 metres (6,500 ft) Stratocumulus (Sc) Clouds From near surface level Cumulus (Cu) of of marked to about Cumulonimbus (Cb) vertical 14,000 metres (45,000 ft) extent (heap clouds) MEDIUM About 2.000 metres (6.500 ft) LEVEL to about Altocumulus (Ac) CLOUDS 5.500 metres (18,000 ft) Altostratus (As) HIGH About 5.500 metres (18,000 ft) Cirrus (Ci) LEVEL to about Cirrocumulus (Cc) CLOUDS 14.000 metres (45,000 ft) Cirrostratus (Cs) The abbreviations as shown above are customarily used by meteorologists and voluntary observers in describing the clouds DESCRIPTION OF CLOUD TYPES (see Plates I to 16) STRATUS (St) A more or less continuous layer or sheet of cloud, similar in appearance to fog but rarely touching the ground except in the case of high land When it does touch the ground it is indistinguishable from fog When broken up into patches by the wind it is called Fractostratus (Fs) Height of base generally between 150 m (500 ft) and 600 m (2,000 ft) NIMBOSTRATUS (Ns) A low, dark, grey shapeless and somewhat ragged cloud, of rainy appearance often having below it pieces of "scud" termed Fractostratus (Fs) from which precipitation may often fall; when it does it is usually continuous Height of base is usually between 150 m (500 ft) and 600 m (2,000 ft) but may be very close to surface and is sometimes as high as 1,200 m (4,000 ft) STRATOCUMULUS (Sc) An extensive layer or patches or rolls of globular shaped light grey clouds They are often arranged in lines or groups orientated in one or two directions When covering the whole sky they give it a wavy appearance Height of base is usually between about 460 m (1,500 ft) and 1,350 m (4,500 ft) (see also Altocumulus and Cirrocumulus) CUMULUS (Cu) Whitish cauliflower shaped clouds with a more or less flat base and appreciable vertical thickness They vary in size and vertical extent 15 Chapter Four and can develop up to great heights (see Cb) These clouds are a feature of the Trade Wind area-but can occur anywhere In certain conditions of light, parts of these look dark Base height is usually between about 460 m (1,500 ft) and 1,500 m (5,000 ft) CUMULONIMBUS (Cb) The thundercloud, a cumulus type of cloud but of great vertical extent The top instead of being rounded like a cauliflower, looks more mountainous or may grow into the shape of a fibrous looking anvil The base is generally flattish and often has Fractostratus (Fs) clouds below it from which precipitation falls If Cumulonimbus cloud is very extensive it may resemble Nimbostratus (Ns) Height of base is usually between 460 m (1,500 ft) and 1,500 m (5,000 ft) ALTOCUMULUS (Ac) A layer or patches of flattish, globular shaped, fairly small clouds, white or grey in colour, often arranged in lines, very similar in appearance to Stratocumulus but at a greater height and the individual cloudlets look smaller Base height above 2.000 m (6.500 ft) and not easy to estimate (See also Stratocumulus and Cirrocumulus) ALTOSTRATUS (As) A thin and apparently formless veil or sheet of cloud, usually grey in colour When present it often covers the whole sky, giving it a watery appearance; sun or moon shows through it with blurred outline Sometimes it is thick enough to obscure sun or moon in which case it is dark in colour Base height above 2,000 m (6,500 ft) and not easy to estimate (See also Cirrostratus) CIRRUS (Ci) Nicknamed Mares' tails, feathery fibrous or hairy clouds of delicate texture high in the sky usually coloured white and in various formations Height of base above about 5,500 m (1,800ft) CIRROCUMULUS (Cc) Layers or patches of very small globular cloudlets similar in appearance to diminutive Altocumulus but derived from Cirrus clouds Popularly known as Mackerel sky, the cloudlets are arranged usually in lines and somewhat resemble nocks of sheep Base height above about 5,500 m (18,000 ft) (See also Altocumulus and Stratocumulus) CIRROSTRATUS (Cs) A diffuse and thin veil of cloud whitish in colour somewhat similar to Altostratus but more diffuse Only slightly blurs the outline of the sun and moon: it often produces halo effect which Altostratus does not Sometimes gives the sky a greyish or milky effect Base height above about 5,500 m (18,000 ft) (See Altostratus) WEATHER ASSOCIATED WITH CLOUD TYPES? With a few obvious exceptions a particular cloud-scape does not indicate very much by itself about impending weather changes; it needs to be considered against 16 Chapter Four a background of the process of formation or dissipation of those particular clouds, also the wind and pressure changes that have occurred recently Ideally, a synoptic map is needed to get an overall picture of what is happening to the weather some distance away It can be said, in very general terms, that at sea level the possible indications usually associated with certain cloud types are as follows, but complications may arise if high land is involved Stratus No special significance but may affect visibility if very low: Indicates a stable air mass, i.e., not much convection Drizzle often falls from this cloud Nimbostratus Can be termed the rain cloud Considerable rain probable and prospects of bad weather generally In some cases the rain does not reach the ground Stratocumulus No special significance, not generally a bad weather cloud As with stratus it indicates stability Occasionally light rain or drizzle falls from this cloud Cumulus Generally a fair weather cloud when small When large they indicate unstable air with possibility of showers due to strong convection, and of sudden squally wind when nearby If it thickens to windward it may indicate rain Altocumulus No special significance not generally a bad weather cloud Rain is likely when these clouds thicken to windward Altostratus Is not infrequently a herald of rain (and wind) and may give warning of an approaching depression if it derives from Cirrostratus and is accompanied by a falling barometer Cirrus If it grows in extent it may well indicate the approach of windy and bad weather generally Gives timely warning of a tropical storm especially if followed by a falling barometer If it turns to Altostratus it probably indicates a coming depression or confirms the advent of a tropical storm If it disperses it probably has no significance Cirrocumulus “Not long wet not long dry" seems a fair summary of its message, associated with fair weather and little wind Cirrostratus Gives a fair indication of the approach of rain If it follows Cirrus it may well indicate approach of a depression, or a tropical storm 17 Chapter Four QUESTIONS: 1.(a) Name the ten principal cloud types and write their abbreviations (b) Between what limiting heights are "low", "medium" (or "middle") and "high" clouds found? Give a description of each of the following cloud types: Ci Cs As Cb Ns and Fs ("Scud") (a) What are Mares' tails? (b) What might they indicate if they increase in density? Describe the appearance of altostratus clouds What does this type often portend? Name the clouds associated with thunderstorms 18 Chapter Four Cirrus These long thread like streaks of cloud are composed of tiny ice crystals "Mares' tails" is a common name because they look like the hairs of a horse's tail When these clouds move quickly across the sky, becoming more dense, they may herald the approach of a depression Watch how your barometer behaves from now on 19 Chapter Four Cirrostratus This cloud layer, visible in the upper part of the picture, is composed of ice crystals and sometimes appears in patches associated with other types of cirrus When cirrostratus covers the sun or moon a ring of light called a "halo" is often visible If cirrostratus increases to cover the whole sky it may foretell the approach of a depression Watch for the appearance of altostratus with lowering base and falling barometer 20 Chapter Four Cirrocumulus This variety of cloud is not unlike altocumulus but the cloudlets are much higher, smaller and gleam whiter The cloudlets often lie in ripples or lines and are then commonly described as "Mackerel Sky" Cirrocumulus is usually seen in association with patches of cirrus from which it often develops The associated weather is usually fair and quiet and the cloud movement very slow 21 Chapter Four Altostratus (thin) Here the sky is covered with a featureless layer of cloud at medium levels The cloud is thin enough for a watery sun or moon to be visible When altostratus thickens to windward it may indicate the approach of a depression and, if so, a period of continuous rain soon begins Watch your barometer 22 Chapter Four Altocumulus and cirrocumulus in bands There are two types of cloud in this picture Very high altocumulus is present together with cirrocumulus in distinct bands 23 Chapter Four Altocumulus Here the cloud elements are at medium level Sometimes several layers of this cloud type can be seen at different levels, one above the other When altocumulus thickens to windward and loses its rounded shapes a spell of dull rainy weather is likely 24 Chapter Four Stratus (layer) When this grey featureless layer of low cloud covers the sky it is difficult to estimate the height of its base unless there are hills or cliffs nearby as in this case Drizzle often falls from this cloud 25 Chapter Four Stratocumulus This soft-looking layer of cloud shows rolls or rounded masses beneath These rolls or patches are often arranged in a fairly regular pattern of wavy lines or groups Breaks are often visible in the cloud layer Occasional light rain or drizzle sometimes falls from this cloud 26 Chapter Four Nimbostratus with stratus fractus This low level rainbearing cloud is thick enough to obscure the sun The stratus fractus appears as ragged pieces of cloud below the main cloud layer Nimbostratus is frequently associated with fronts 27 Chapter Four Nimbostratus This view against the background of the Rhone Valley allows the stratus fractus at the base of the cloud to be clearly seen 28 Chapter Four Developing Cumulus These small, white, low clouds are developing in rising currents of air If the clouds remain small fine weather is likely to continue 29 Chapter Four Cumulus Small cumulus clouds sometimes continue to grow and develop into mud larger masses with many towers or turrets Their flat bases look dark b) comparison with their gleaming white tops which are usually well rounded and often show a hard outline against a clear blue sky When the tops begin to lose their rounded cauliflower-like appearance and become fibrous showers can be expected Cumulus clouds often tend to flatten out or disperse at sunset when convection ceases 30 Chapter Four Cumulonimbus Large cumulus clouds may develop further, extending from the base upwards to a height above the freezing level, where their tops are fibrous in appearance With still further development their fibrous tops spread out like an anvil Heavy showers of rain, snow or hail fall from these clouds, often accompanied by thunderstorms Sudden hard squalls and a large change or reversal in wind direction may occur in the vicinity of well-developed cumulonimbus clouds Such clouds often cover a wide area, especially when formed along a cold front 31 Chapter Four Sea Fog In this picture of Gibraltar, sea fog can be seen drifting in from the east 32 Chapter Four 33 Chapter Four Orographic Cloud These clouds are produced by the airflow over hills or mountains and are sometimes referred to as wave clouds The clouds in this picture resemble a pile of plates Orographic cloud is continuously forming at the windward edge and evaporating downwind although it appears to be stationary 34 [...]... Sometimes several layers of this cloud type can be seen at different levels, one above the other When altocumulus thickens to windward and loses its rounded shapes a spell of dull rainy weather is likely 24 Chapter Four Stratus (layer) When this grey featureless layer of low cloud covers the sky it is difficult to estimate the height of its base unless there are hills or cliffs nearby as in this case Drizzle... referred to as wave clouds The clouds in this picture resemble a pile of plates Orographic cloud is continuously forming at the windward edge and evaporating downwind although it appears to be stationary 34