Stratus cloud

“ The first is a cloud of fairly stable conditions, the latter is dependent for its existence upon the near approach [79] to critical conditions at one particular level, and, as we have said in other cases, such a critical state is almost always soon passed, with the result that the cloud either masses into a denser form, or else breaks up and disappears. If the up and down currents are strong enough to persist, the result will be strato-cumulus and not stratus maculosus. ”

“ The cumulus clouds therefore indicate, as shown in Chart 1, that the air is rising vertically in certain layers, while drifting eastward, and the stratus that it is moving horizontally with a different velocity in adjacent strata. The lowest stratus clouds are elevated fog; the strato-cumulus is about two miles high; the altostratus averages four miles and the cirro-stratus six miles above the earth.
There is furthermore a stratification of the vapor contents of the atmosphere within every high cumulus cloud, which is interesting. ”

“ The strato-cumulus clouds that follow an anticyclone also indicate a lid. The high fog that completely covers the sky at heights varying from 7000 to 10,000 feet—practically a stratus or an alto-stratus cloud—is a lid. Cross-winds at a very considerable height likewise may indicate a lid. ”

“ The distinguished theorists are mistaken. The stratus-cloud does not over-lie or cover the storm. It is the storm. The winds beneath, whether surface or superimposed, are but its incidents, due to its static induction and attraction. Their direction depends on the shape of the storm cloud, and its course of progression, and the susceptibility of the surface atmosphere in this direction or that, to its inductive and attractive influence. Their force to its depth, its contiguity to the earth, and the intensity of its action ”

“ The rain-cloud is always a form of either stratus or cumulus, or a combination of the two, sometimes in further combination with clouds of the alto class, or even extending upwards to cirro-stratus and cirro-nebula. Where it consists of a single layer, that layer differs from its rainless representative only in greater thickness from base to summit, or in greater density; and when there are several distinct layers [76] of cloud, so that the lowest is shaded by the higher, rain may fall, even though they differ in no visible way from clouds which would be rainless if alone. ”

“ Stratus clouds are low-lying sheets of condensed moisture, which, being usually seen at a low angle, appear like thin layers parallel to the horizon. The transitional type Cirro-Stratus is usually seen in the form of great feather-like clouds stretching across nearly the whole sky. Nimbus is a rather low-lying cloud from which rain is falling even if the rain is re-evaporated before reaching the ground. The lowest clouds of all, those resting on the surface of the ground and enveloping the observer, are called mist and fog. ”

“ The wave, or ripple cloud, has been explained by von Helmholtz and von Bezold to be due to the undulations in a horizontal current producing alternate rarefaction and condensation of its water-vapour through changes of temperature. Still another cause of low-lying clouds is the cooling of the air to its dew-point by contact with a cold surface, such as the earth when cooled by radiation during a clear night, or the polar currents of the ocean. Fog is often formed in this way, which we call stratus cloud when it rises above us. ”

“ The stratus is a smooth, uniform cloud—the true rain cloud of the storm; often forming without much cirrus above, or connected with it. It may be seen in its partially formed state in the bank in the west, at nightfall, or in the circle around the moon in the night. When it becomes sufficiently condensed, rain always falls from it, but in moderation. If there be [Pg 40] large masses of scud running beneath it for its drops to fall through (especially as is sometimes the case, in two or more currents) , the rain may be very heavy. ”

“ At present it is applied to any cloud from which rain is falling, except when the cloud can be identified as a variety of cumulus which is called cumulo-nimbus. But we have already said that a stratus may be a rain-cloud, and so may other varieties. Moreover, whenever a nimbus breaks sufficiently for us to be able to see its upper surface, we invariably find that, if it were viewed from above, we should, without a moment’s hesitation, place it in one of the other groups. It is only when we are underneath it we can see its rain-producing character, and give it the orthodox name. ”

“ This plane therefore becomes a “lid.” Perhaps it may reach the stratosphere, for the stratosphere Is a planetary lid that envelopes the convectional air; but a lid may form anywhere between the ground and the stratosphere. Wherever a layer of warm air rests on one of still, cold air a lid is formed. Smoke, dust, and other fine foreign matter spread out to form stratus cloud when it reaches such a lid. If the two air layers are not turbulent there is little or no mixing. ”

“ If the cloud is a variety of cirro-stratus, it is not always easy, or even possible, to distinguish between the advance of condensation and the movement of the whole, but it can nearly always be done if the cloud shows any definite features upon which attention can be fastened. Sometimes none sufficiently marked can be seen, and when that happens it is still possible in most cases, by watching the edge of the cloud-mass, to see whether new cloud is being added to that edge. ”

“ 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. ”

“ As the Cumulus belongs to the day, so does the Stratus to the night. It is the lowest of all the clouds, and actually rests upon the earth, or the surface of water. It is of variable extent and thickness, and is called Stratus, a bed or covering. It is generally formed by the sinking of vapour in the atmosphere, and on this account has been p. 85called Fall-cloud. It comprehends all those level, creeping mists, which, in calm evenings, spread like an inundation from the valleys, lakes, and rivers, to the higher ground. ”

“ The other main types of cloud are composed wholly or chiefly of water. Alto-cumulus and alto-stratus are clouds of medium altitude; strato-cumulus and nimbus are low clouds (generally not more than a mile high) ; while stratus, the lowest cloud of all, grades into fog, which commonly rests on the earth. Since cumulus and cumulo-nimbus are produced by the condensation of moisture from rising air currents, the height of their bases varies widely104 with the temperature and humidity of the lower air; the average height is rather less than a mile. ”

“ Group D—cumulus and cumulo-nimbus—do show important differences from the other groups, though it is often difficult to say whether the sky should be described as covered with strato-cumulus or as covered with numerous small cumulus. It is the separation of stratus—placing it in a group by itself, and making that the lowest—which is the worst point. As a matter of fact, stratus may exist at any altitude from sea-level up to such heights that we should not hesitate to call it alto-stratus. ”

“ A good many years ago the writer made a series of measurements of the thickness of detached clouds of the stratus and cumulus types, such as those which may produce a shower. The conclusions reached in consequence of those determinations have since been amply confirmed by subsequent [82] observations. In winter no rain will fall from a cloud unless it reaches a minimum thickness of at least 100 metres, while in summer it must have rather greater thickness. ”

“ 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. ”

“ The cirro-stratus most frequently forms the solar and lunar halo. Hence the reason of the prognostics of bad weather commonly drawn from the appearance of halos.
THE CUMULO-STRATUS, OR TWAIN-CLOUD.
This is a blending of two kinds of cloud (hence the name of twain-cloud,) and it often presents a grand and beautiful appearance, being a collection of large fleecy clouds overhanging a flat stratum or base. When a cumulus increases rapidly a cumulo-stratus frequently forms around its summit, resting thereon as on a mountain, while the former cloud continues to be seen, in some degree, through it. ”

“ It is a common cloud, especially in sultry summer weather with still air. Under those circumstances, after a hazy morning, it may be seen slowly forming during the afternoon, growing in density as the hours go by, until it reaches a maximum about five or six o’clock, after which it melts away, or settles down into small patches of high stratus. Most frequent in summer, it is by no means rare in autumn and winter, but still air is essential. From its hazy appearance it may be called alto-cumulus nebulosus. ”

“ 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. ”

“ The change into cirro-stratus is also attended by a marked fall in altitude, but whether this is due to an actual descent of the cloud particles, or to a downward spread of the conditions which give rise to them, cannot at present be definitely settled. The balance of probability points very strongly towards the downward spread of the conditions. It is likely that the clouds, particularly the cyclonic specimens, are wedge-shaped, and that as they pass overhead we see first the thin edge, and later on the thicker parts, which project much lower down. ”

“ We may also add the shading of the underlying ground by the cloud itself, and the absorption of heat in the partial evaporation of some of the rain during the lower part of its fall. In a desert district the arising currents are so dry that even a very great ascent does not often result in visible cloud; and when it does, the cloud is produced at so great a height that the air is too rarefied to produce anything much denser than thin alto-stratus, from which no falling droplets could reach the earth. ”

“ 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. ”

“ This form goes through all the changes like Cirro-stratus, but according to measurements made at Upsala, its altitude is one-half as great. See fig. 5.
6. Strato-cumulus (S.-Cu.) .—Large globular masses or rolls of dark cloud, frequently covering the whole sky, especially in winter, and occasionally giving it a wavy appearance. The layer is not, as a rule, very thick, and patches of blue sky are often seen through intervening spaces. All sorts of transitions between this form and Alto-cumulus are seen. ”

“ 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. ”

“ Purity of colour in [64] these rings is an indication of uniformity in the size of the particles. When the moon is shining through a sheet of alto-stratus, which thins off to one edge, very beautiful effects may often be noticed, and the change from the colourless blur, when a thicker part of the cloud is interposed, to the brilliant colours of the corona formed by the thinner edges is very striking. Similar phenomena are shown almost equally well by any of the alto clouds, but cirrus thin enough to produce a coloured corona will generally produce a halo. ”

“ But there often follows an immediate and great addition to the quantity of cloud. At the same time the darkness becomes less, because the arrangement, which now returns, gives free passage to the rays of light; the lower broken clouds rise into cumuli, and the upper sheets put on the various forms of the cumulo-stratus, sometimes passing to the cirro-cumulus.
The various phenomena of the rain-cloud are best seen in a distant shower. If the cumulus be the only cloud at first visible, its upper part is seen to become tufted with cirri. Several adjacent clouds also approach and unite at its side. ”