Cirrus cloud

“ But this last is an accidental feature of the particular example, and not in any way a specific character of the cloud. The reason for regarding it as a distinct variety is the total absence of sharply defined lines, not only the heads of condensation, but even the long streamers attached to them being uniformly hazy and ill-defined. It is a form of cirrus which comes at all seasons, but most frequently in summer; it moves always with great slowness, indicating a quiet atmosphere free from disturbance of any kind. ”

“ The condensation is evidently attended by rapid movements, which draw out the cloud, as fast as it is formed, into long curving lines which mark lines of motion. The variety is always, therefore, an indication of strong winds and rapid eddying movements in the region in which it occurs. Such strong disturbances overhead almost always accompany similar but less intense movements at the ground-level, and when they do not accompany them they precede them. The cloud is well named windy cirrus, which may be converted into a specific name, cirrus ventosus. ”

“ The loftiest variety of cirrus appears in the afternoon in very hot weather, sometimes quite late in the evening; and in autumn it is by no means a rare event for it to suddenly form just when the sunset colours are fading, or even after they have paled into twilight. Under such circumstances it stands out of a shining silvery grey colour against the background of the darkening sky. A specimen of it is shown in Plate 5, which shows its extreme slightness of form and delicacy of texture. ”

“ The opportunity offered in the film-like cirrus clouds, which so frequently lie as the background to the more positive forms of the cumulous, for securing the oppositional feeling, is one frequently adopted by sky painters. Besides strengthening the structure pictorially such arrangement frequently imparts great swing and movement in the lines of a sky, carrying the eye away from the horizon. When positive cloud motion is desired these oppositional masses may become very suggestive of wind, different strata showing a contrasted action of air currents. ”

“ The high clouds, which are called cirrus, under certain conditions drop down to where they begin to melt into ordinary moisture globules, and while this process is going on we have a combined cloud effect which is called cirro-stratus. This form of cloud may be recognized, when looking off toward the horizon, by its being formed into long straight bands. It is sometimes called thread-cloud. As it further descends it takes on a different form called the cirro-cumulus, or curl-heap. ”

“ As cirrus begins to thicken, it will change into cirrostratus. In this form it is popularly known as “mares’ tails.” If it continues to thicken and lower, with the ice crystals melting to form water droplets, the cloud formation is known as altostratus. When this occurs, rain may normally be expected within 24 hours. The more brush-like the cirrus when the sky appears, the stronger the wind at the level of the cloud.
style="width:75%;margin-right:auto; margin-left:auto;line-height:100%;text-align:center;font-style:italic;">Figure 3614g. ”

“ The day may close lurid with the frayed streamers of lofty cirrus clouds streaking across the sky—the vaporous overflow of a coming storm—or a week of the same bright weather may continue with the wind all the while blowing softly out of the south, but sooner or later the inevitable winter storm must close this foretaste of the spring. ”

“ CUMULO-NIMBUS Grandest of all clouds are the huge mountains of vapour which are the parents of summer thunder-storms. They are at once distinguished from ordinary cumulus by their upper parts, which sometimes reach beyond the region of the alto clouds high into the realm of cirrus, and extend outwards as a broad disc, which is occasionally indistinguishable from the cirro-nebula and cirro-stratus which form the van of a cyclone cloud canopy. ”

“ Vapour becomes rarer with increased height and with diminished temperature, so that it must, on the whole, be less frequently present in cloud-producing quantity as the height increases. At great altitudes it will be seldom that the quantity is great enough to produce a stratiform cloud, though it may well be enough for cirro-macula, or the detached forms of cirrus, like cirrus excelsus. ”

“ Plate 14 shows a hazy form of cirro-stratus developed from the nebulous cirrus. Its altitude was great, being about 10,000 metres. The processes of growth and change could be studied easily. First would appear some faint spots and streaks; these quickly fused together into larger patches, which again joined to their neighbours. In a few minutes the cloud so formed would return to the mottled or streaky appearance, and either disappear entirely or become very thin, only to recommence the process. ”

“ The observations of cirrus clouds in various parts of the world show that they always move from a general westerly direction, while below this primary drift toward the east occur the relatively permanent or transient differences of pressure which cause the deviations from the normal circulation of the atmosphere, and give rise to the local circulation in storms. ”

“ All the cumulus clouds, by which we mean to include alto-cumulus and cirro-cumulus, are most frequent when the levels of condensation are rising, while the stratiform clouds are an indication of no vertical movement or of active descent. Pure cirrus is indicative rather of movement in a horizontal direction, and may occur when the condensation levels are stationary, or when they are rapidly changing either way. ”

“ On examining minutely the higher regions of the air, especially after the sky has been clear for some time, the spectator will probably see the cirrus descending from above in the form of threads or locks and feathers, which go on increasing until they fill the sky. ”

“ Suffice it say that we do not now see fanciful forms in cirrus-stripes, but rather the product of threads of vapour being condensed and drawn out by currents of air 20,000 feet above the earth; and that we can often get useful information respecting coming weather by noting the direction and motion of these thin lines of hairy cloud.
Fleecy Structure.
There is another type of structure to which it is impossible to give a better epithet than fleecy. We often see a lovely, bright cloud high in the heavens that looks exactly like a sheep's fleece, and totally unlike any other cloud-form. ”

“ It is comparatively rarely that the motion, say of nimbus and cirrus, is in the same direction. On a lofty mountain, strata of cloud which from below were seen to be cumulus may be passed through as layers of mist, and on emerging from them their upper surface may be seen below one. In many mountains the cloud-belt is as sharply defined as the snow-line, and its variations should be carefully observed. ”

“ Of the clouds above enumerated, cirrus, cirro-cumulus, and cirro-stratus are the highest, and are always ice clouds. They consist in some cases of separate, minute crystals—a fine dust of ice—producing, according to the forms of the crystals, one or another of the various forms of halo around the sun and moon; while in other cases the crystals are aggregated in small snowflakes, so that the cloud is a real snowstorm in midair. The altitude of these clouds generally ranges from 4 to 8 miles. In the equatorial region their height is often 10 miles or more. ”

“ Like cirrus; more generally, descriptive of clouds composed of small particles, mostly ice crystals, which are fairly widely dispersed, usually resulting in relative transparency and whiteness, and often producing halo phenomena not observed with other cloud forms. Irisation may also be observed. Cirriform clouds are high clouds. As a result, when near the horizon, their reflected light traverses a sufficient thickness of air to cause them often to take on a yellow or orange tint even during the midday period. ”

“ Of more importance than the amount of cloud is its nature, elevation, and movement. Distinct species of cloud have been recognised for a [6] long time, and from more recent studies it would appear that they owe their distinctive appearance to the altitude at which they float in the air. Meteorologists distinguish a number of classes and transitional forms of cloud; it is enough for the traveller to be able to recognise the most definite types, viz., Cirrus, Cumulus, Stratus, and Nimbus. ”

“ When such a fog accumulates to a sufficient depth, the particles in their fall pick up others, and the result is a distinct fine rain. This may occur not only near the ground, but at almost any level below that at which the cloud would pass into the region of cirrus.
Plate 34 shows three layers of stratus, in each case much broken up. The highest layer is a good example of alto-stratus maculosus. Lower down, by half a mile or more, come parts of a grey sheet considerably denser and thicker. ”

“ A mackerel sky, that is, one covered with lines of cirrus clouds, causing halos around the sun and moon, presages rain in summer and thaw in winter. By degrees the light clouds descend and pass into either masses of cumulus, or into dense, horizontal stratus, which form at sunset and disappear at sunrise. ”

“ Aeronauts in ascending from the earth pass through many successive alternations of cloud-strata and clear air which owe their existence to the varying temperature and degrees of humidity of the atmospheric currents so superposed.
Luke Howard in his Askesian Lectures, 1802, divides clouds into three primary modifications: cumulus, stratus, and cirrus, with intermediate forms resulting from combinations of the primaries, viz., cirro-cumulus, cirro-stratus, cumulo-stratus, and cumulo-cirro-stratus or nimbus. ”

“ Beginning with a fairly even sheet, this broke up into granules, and they in turn seemed to be frayed out into short hazy and wavy fibres which slowly melted away.
Clearly we have here to do, not with a distinct type of cloud, but rather with the first step towards the formation of one, or the last stage in the life of one which is drying up. But sometimes the life of the cloud is so short that it never passes beyond this first stage; and it is by no means a universal rule for a growing sheet of cirrus to pass through this stage at all. ”

“ As it passes over hill and valley the up-and-down movements of the lower layers, or even the disturbances caused by passing over a wood or clump of trees, all must be propagated upwards. Each disturbance must slowly [40] spread laterally and diminish vertically, so that it will reach the cirrus zone as a broad and gentle dome-like oscillation. Suppose now a series of such slight upheavals to reach the critical level. The passage of the waves will mean alternate expansion and compression. Expansion means cooling, and therefore cloud-production ”

“ Cirrus clouds always appear to move slowly, because they're so high up. As a matter of fact, they fly along at the rate of from one hundred to two hundred miles an hour, and generally in an easterly direction. This photo that looks as if the clouds were a whole pile of spiders' webs, all mixed up, is the second class of clouds, known as Cirro-Stratus. Did you happen to notice, Ralph, whether there was a halo round the sun when you took this? ”

“ Cumulo-stratus.—The cirro-stratus blended with the cumulus, and either appearing intermixed with the heaps of the latter or superadding a widespread structure to its base.
(7) Cumulo-cirro-stratus, or nimbus.—The rain-cloud: a cloud or system of clouds from which rain is falling. It is a horizontal sheet, above which the cirrus spreads, while the cumulus enters it laterally and from beneath. ”

“ When they appear in wet weather, they quickly pass into the cirro-stratus.
According to Dalton, these clouds are from three to five miles above the earth’s surface. When viewed from the summits of the highest mountains they appear as distant as from the p. 81plains. Another proof of their great height is, their continuing to be tinged by the sun’s rays in the evening twilight with the most vivid colours, while the denser clouds are in the deepest shade.
The cirrus appears to be stationary; but, on comparison with a fixed object, it will sometimes be found to make considerable progress. ”

“ These “clouds of relaxation,” if we may so call them, are usually flaky and horizontal, sometimes tending to the silky cirrus, yet showing no fine forms of drift; but when the rain has passed, and the air is getting warm, forms the true clearing cloud, in wreaths that ascend continually with a slow circling motion, melting as they rise. The woodcut, Fig. ”

“ Wind-storms are preceded by a low barometer, with much briefer warning than in the case of rain and snow storms, often occurring within two or three hours after the first instrumental premonition. Temperature rises and humidity increases; cirrus clouds move from west to northwest. ”

“ Cloud forms are four in number, cirrus, cumulus, nimbus, and stratus, all of which may sometimes be seen at once, in the sky of a summer’s day. At times they float above the loftiest mountains. Gay-Lussac, [15] rising in his balloon to an elevation of 21,600 feet, perceived clouds drifting far above him.
AIR CURRENTS—SHOWING THE PRINCIPLE ON WHICH MINES ARE OFTEN VENTILATED.
The work performed by the atmosphere in supplying water to the soil is worthy of profound attention. ”