BÀI GIẢNG KHÍ TƯỢNG THỰC HÀNH CHƯƠNG 5

CHAPTER Observation of Cloud CHAPTER Observation of Cloud A cloud is a hydrometeor consisting of water droplets or ice crystal, or a mixture of both, suspended in the free air above the earth's surface including the sea surface It is different from fog only in that the latter is in contact with the earth's surface The results of cloud observation show a state of the atmosphere indirectly and a distribution related to meteorological disturbances including Typhoons or extra-tropical lows Though wide range observations by meteorological satellites have been introduced, the importance of cloud observation by ships over the oceans remains significant as before 5.1 Observation of cloud Observation of cloud on board ship covers; - Amount of all clouds and of each type of cloud; - Identification of cloud types present; and - Height of the base of each cloud Observe the total amount of cloud first, then from lower cloud to higher cloud Among the same height of clouds, the groups with larger amount should be observed earlier 5.1.1 Cloud amount (Cloud cover) "Total cloud amount" is the rate of the apparent area of the sky covered by all clouds over the whole sky area On the other hand, the rate of the amount of a specified type of cloud over the whole sky area is called "cloud amount" for the type of cloud The observer should estimate, by taking into consideration the evolution of the sky, the cloud amounts at different levels as if no other types of clouds were present Total summation of the cloud amounts could then exceed the total cloud amount Cloud amount is measured as if there is no cloud in question in the sky, as 10 if the whole sky is covered by the cloud in question and as 1-9 if the cloud amount is in between If the amount is too small to be measured as I, put 0+ If the amount is almost 10 but at least one break is present, use 10- Care should be taken as follows: 1) On the occasion of fog, haze, or other analogous phenomena which is too thick to tell whether there is a cloud above or not, record as unknown (sky obscured) with the code in the logbook If the cloud can be seen through the fog, etc., the cloud amount should be estimated as well as possible 2) If the sun, moon or stars can be seen through the fog, etc., and there is no evidence of any cloud above the fog, the state of the cloud amount should be estimated as O 3)If you can not estimate the cloud amount at night, not make an ambiguous guess but just record it as unknown with the code x 31 CHAPTER Observation of Cloud 5.1.2 Cloud type (genus) Clouds are classified into the 10 principal types (genera) Clouds are incessantly changing and not necessarily appear in such basic and typical types You need therefore carefully to observe which types are prevailing 5.1.3 Cloud height Cloud height is the height of the cloud base above the sea surface and usually measured in the unit of 100m Above the sea surface, cloud height is up to about 10 km A cloud can be first classified according to its cloud height under the three cases: high level cloud (CH), middle level cloud (CM) and low level cloud (CL) The maximum cloud height observed varies according to the latitude It is larger in the tropics than in the higher latitude When you have difficulty to discern the type of cloud, consideration of its cloud height might help you to find its appropriate type 5.2 Details of the cloud types (genera) Table.5.1 shows 10 types of clouds which belong to levels of cloud height General features, comparison with other types and/ or characteristics of formation and development of each type of cloud are as follows 5.2.1 High level clouds (CH) (1) Cirrus (Ci) - Cirrus consists of detached clouds, with delicate and fibrous appearance, without shading, generally white in colour, often of a silky appearance, and composed of small particles, mostly ice crystals of usually white particles, or of narrow bands Cirrus appears in the most varied forms, such as isolated tufts, lines drawn across a blue sky, branching featherlike plumes and curved lines ending in tufts, lines ending in turfs These lines are often arranged in bands which cross the sky in lines and which, owing to the effect of perspective, appear to converge to a point on the horizon, or to two opposite points (i e polar bands) - Cirrostratus and cirrocumulus often take part in the formation of these bands - Before sunrise and after sunset, cirrus is sometimes coloured bright yellow or red Owing to their great height cirriform clouds are illuminated long before other clouds and fade out much later Observation of cirrus at night is difficult but, if thick and extensive, it may be noted by its dimming effect on stars (2) Cirrocumulus (Cc) - A cirriform layer or patch composed of small white flakes of very small globular masses, without shadows, which are arranged in groups or lines, or more often in ripples resembling those of the sand on the sea-shore - In general, cirrocumulus represents a degraded state of cirrus and cirrostratus, both of which may change into it In this case the changing patches often retain some fibrous structures in places Real cirrocumulus is uncommon It must not be confused with small altocumulus on the edges of altocumulus sheets In the absence of any other criterion the term 33 CHAPTER Observation of Cloud cirrocumulus should only be used when; a) there is evident connection with cirrus or cirrostratus, or b) the cloud observed results from a change in cirrus or cirrostratus (3) Cirrostratus (Cs) - A thin whitish veil, which does not blur the outlines of the sun or moon, but gives rise to haloes Sometimes it is quite diffuse and merely gives the sky a milky look; sometimes it more or less distinctly shows a fibrous structure with disordered filaments Cirrostratus may be observed at night by noting the slight diffusion of light around each star, whose brilliance is at the same time dimmed - It is almost impossible to differentiate between thick cirrus and cirrostratus at night in the absence of moonlight 5.2.2 Middle level clouds (CM) (1) Altocumulus (Ac) - A layer or patches, composed of laminae or rather flattened globular masses, the smallest elements of the regularly arranged layers being fairly small and thin, with or without shading These elements are arranged in groups in lines, or waves, following one or two directions and are sometimes so close together that their edges join - When the edge of a thin translucent patch of altocumulus passes in front of the sun or moon a corona appears This phenomenon may also occur with cirrocumulus and with the higher forms of stratocumulus Irisation or iridescence is another possibility with altocumulus - The limits within which altocumulus is met are very wide At the greatest heights, when it is made up of small elements, it resembles cirrocumulus; altocirrus or cirrostratus or evolved from one of these types It is often associated with altostratus and either form may change into the other - Two important varieties of altocumulus are" altocumulus castellanus" and" altocumulus lenticularis" Altocumulus castellanus is a variety peculiar to a thundery state of the atmosphere, and is sure evidence of high-level instability In this form individual cloudlets are extended vertically upwards in heads or towers, like small cumuli The lenticular variety of altocumulus has clouds of an ovoid or lens shape, with clear-cut edges and sometimes showing irisations It occurs frequently over mountainous country and in "foehn", "scirocco" and "mistral" winds It may also often be seen after the passage of weak cold fronts (2) Altostratus (As) - Striated or fibrous veil, more or less grey or bluish in colour This cloud is like thick cirrostratus, but does not show halo phenomena; Sometimes the sheet is thin with forms intermediate with cirrostratus Sometimes it is very thick and dark, perhaps even completely obscuring the sun or moon In this case differences of thickness may cause relatively light patches between very dark parts; but the surface never shows real relief, and the striated or fibrous structure is always seen in places in the body of the cloud Every gradation is observed between high altostratus and cirrostratus on the one hand and low altostratus and 34 CHAPTER Observation of Cloud nimbostratus on the other - In practice it is important to distinguish between altostratus (thin) through which the sun or moon is visible and altostratus (thick) which completely obscures the sun or moon (3) Nimbostratus (Ns) - A low, amorphous (i.e without form), and rainy layer, of a dark grey colour; feebly illuminated seemingly from inside Precipitation from nimbostratus is nearly always "continuous"; but precipitation is not a sufficient criterion Cloud may be described as nimbostratus before precipitation has started There is often precipitation which does not reach the ground; in this case the base of the cloud is always diffuse and looks "wet" on account of the general trailing precipitation, "virga", so that it is not possible to determine precisely the limit of its lower surface - Nimbostratus is usually the result of a progressive lowering and thickening of a layer of altostratus Beneath nimbostratus there is generally a progressive development of very low ragged clouds (scud) These clouds are usually referred to as stratus fractus (St fra) 5.2.3 Low level clouds (Cd (1) Stratocumulus (Sc) - A layer or patches composed of rounded masses or rolls; the smallest of the regularly arranged elements are fairly large; they are soft and grey, with darker parts These elements are arranged in groups, in lines, or in waves, aligned in one or two directions Very often the rolles are so close that their edges join together; when they cover the whole sky as on the continent, especially in winter, they have a wavy appearance The difference between stratocumulus and altocumulus is essentially one of height A cloud sheet called altocumulus by an observer at a lower height may appear as stratocumulus to an observer at a considerably greater - Stratocumulus may form by the spreading out of cumulus This happens over land in the evening when the day-time cumulus clouds begin to spread out prior to dissolving Another example is when developing cumulus meets a pronounced inversion layer If unable to penetrate this layer, the cloud spreads out horizontally in the form of stratocumulus (2) Stratus (St) - A uniform layer of cloud, resembling fog but not resting on the ground When this very low layer is broken up into irregular shreds it is designated stratus fractus (St fra) A veil of true stratus generally gives the sky a hazy appearance which is very characteristic, but which in certain cases may cause confusion with nimbostratus When there is precipitation the difference is manifest, stratus cannot give the continuous precipitation usually associated with nimbostratus When there is no precipitation, a dark and uniform layer of stratus can easily be mistaken for nimbostratus The lower surface of nimbostratus, however, always has a wet 'appearance (widespread trailing precipitation or virga); it is quite uniform and it is not possible to make out definite details Stratus on the other hand has a drier' appearance and, however uniform it may be, it shows some contrast and some lighter transparent parts Stratus is often a local cloud and, when it breaks up, the blue sky is often seen I 35 CHAPTER Observation of Cloud - A common mode of stratus formation is the slow lifting of a fog layer due to increase in wind speed or warming of the surface (3) Cumulus (Cu) - Thick clouds with vertical development; the upper surface is dome-shaped and exhibits rounded protuberances, while the base is nearly horizontal When the cloud is opposite to the sun, the surfaces normal to the observer are brighter than the edges of the protuberances When the light comes from the side, the clouds exhibits strong contrasts of light and shade; against the sun, on the other hand, they look dark with a bright edge True cumulus is definitely limited above and below, and its surface often appears hard and clear-cut; but one may also observe a cloud resembling ragged cumulus in which the different parts show constant change This cloud is called cumulus fractus (Cu fra) - Cumulus, whose base is horizontal, clear-cut and generally of a grey colour, has a uniform structure, that is to say it is composed of rounded parts right up to its summit, with no fibrous structure One of the species of cumulus, cumulus congestus, can produce abundant precipitation in the tropics As cumulonimbus generally results from development and transformation of cumulus, it is not possible to decide on the basis of other criteria, the cloud should, by convention, be called cumulus if it is not accompanied by thunder, lightning or hail - Cumulus having but small vertical development and little individual extent is known as "fair-weather cumulus" to distinguish it from the ordinary "large cumulus" (4) Cumulonimbus (Cb) - Heavy masses of cloud, with great vertical development, whose cumuliform summits rise in the form of mountains or towers, the upper parts having a fibrous texture and often spreading out in the shape of an anvil The base of the cloud resembles nimbostratus, and one generally notices "virgo" (trailing precipitation) This base has often a layer of very low ragged clouds below it - Cumulonimbus cloud generally produces showers of rain or snow, and sometimes of hail or soft hail, and often thunderstorms as well If the whole of the cloud cannot be seen, the fall of a moderate or heavy shower is enough to characterize the cloud as a cumulonimbus A cumulonimbus cloud may cover the whole sky, in which case the base alone is visible and resembles nimbostratus from which it is difficult to distinguish it If the cloud mass does not cover all the sky and if even small portions of the upper parts of the cumulonimbus appear, the difference is evident In other cases the distinction can only be made if the preceding evolution of the clouds has been followed or if precipitation occurs Cumulonimbus gives showers whereas nimbostratus is associated with continuous precipitation When there is doubt as to the choice between cumulonimbus and cumulus, cumulonimbus should be reported if it is accompanied by lightning, thunder, or hail - The lower surface of cumulonimbus sometimes has an udder-like or mammillated appearance which is referred to as "mamma" When a layer of menacing cloud covers the sky and mammatus structure and trailing precipitation are both seen, it is a sure sign that the cloud is the base of a cumulonimbus, even in the absence of all other signs - The clouds which develop in the rear of depressions are often cumulonimbus However, by the spreading out of the upper parts of this cloud and the dissolving of the lower parts, 36 CHAPTER Observation of Cloud altocumulus or stratocumulus can form Dense cirrus will develop when the cirriform upper parts of cumulonimbus spread out 5.3 Observation of cloud states and their coding For forecasters who eventually receive and use observers' reports, reports on what clouds are present in the sky are not so important but reports on distributions or arrangements of clouds, namely the states of clouds are much more valuable The state of clouds is coded not only depending on 10 cloud types but also on detail characteristics of clouds under the major specifications of CL (low level cloud), CM (middle level cloud) and CH (high level cloud), such as amount, thickness, variation and combination of clouds The actual procedures to define the code are shown in Figs.5.1-5.3 Typical photos for each code are shown in succeeding pages 37 CHAPTER Observation of Cloud 40 CHAPTER Observation of Cloud 50 [...]...CHAPTER 5 Observation of Cloud 40 CHAPTER 5 Observation of Cloud 50