CLOUD.—Although the study of the clouds constitutes a very important branch of meteorological science, yet we do find that it occupied so much of the attention of ancient naturalists, as many other much less interesting atmospheric phenomena,—a circumstance which may possibly have arisen from the very great and almost endless variety of their external forms; in consequence of which they seemed, on a superficial view, to be incapable of being so arranged and clasped, as to receive specific names whereby their varieties could be commemorated. Thus, meteorologists having no means either of accurately recording their own observations, or of benefiting by those of contemporary observers, made very little progress in the knowledge of these interesting phenomena; and the various figures of clouds were regarded as being the sport of winds, or of accidental circumstances not demonstrably subject to general laws. In taking a slight survey of the observations of different writers, from the time of Aristotle to the present day, we shall see how very little progress was made in this branch of the science, till Mr Howard of London published his Nomenclature and Observations on Clouds. Different natural philosophers had made observations on certain properties of clouds, but no one had attempted a systematic description of them. Aristotle, in his work on Meteorology, seems to have been the first who attended to clouds. He remarked that they varied much in appearance; that they refracted different colours, at different times; that they possessed properties in their structure, capable, under peculiar circumstances of position with respect to the sun, of producing the phenomena of rainbows, halos, coronae, the parhelia, &c. and he gave very minute descriptions of these phenomena. He also notices the constant evaporation and reappearance of clouds, and gives it as his opinion, that clouds are taken into composition by the air, which, being decomposed again, the aqueous particles reassumed the forms of visible clouds. And this opinion, as we shall have occasion to notice hereafter, coincides with that of several eminent modern meteorologists.

Theophrastus, who had been the pupil of Aristotle, noticed, in a vague manner, some of the different forms of clouds, with respect, principally, to the prognostics of changes of the weather which might be deduced therefrom. He remarked, in particular, the appearance of strait horizontal sheets of clouds lodged on the summits of mountains, as an indication of wind and rain. His description of this kind of cloud corresponds so accurately with that called by the moderns cirrostratus or wanecloud, that it would be readily identified, even if the well known circumstance of its indicating a change to rainy weather did not render the description of this writer more clearly intelligible.

The modifications of clouds are as follows:

**Cirrus or Curlcloud.** Plate LVIII. fig. 1. Def. Nubes tenuis flexuosa et undique crescentes; altissimo acre appareat.

The great resemblance of some varieties of this cloud to a distended lock of hair, has suggested the name of Cirrus, of which Cirrulus and the English Curl are the diminutives. The curling and flexuous forms of this cloud constitute its most obvious external character.

**Cumulus or Stackencloud,** fig. 7. Def. Nubes annulata densa sursum crescentes.

This cloud is well known by its flattened base and cumulated irregularly hemispherical superstructure; hence its name Cumulus. The mode of its formation is by the gathering together of detached clouds into one large and elevated mass, hereafter to be spoken of. This mass, collecting all the fragments of newly formed cumuli in its vicinity, which seem as it were piled up, has received the name Stackencloud.

**Stratus or Fallcloud,** fig. 9. Def. Nubes strata vel aquae modo expansa terraque procumbens.

The term Stratus is applied to fogs, mists, and other extensive sheets of cloud which rest on the earth's surface. There are, however, some clouds which rest on the ground in the same manner as the stratus, which, in other particulars, resemble the cirrostratus, hereafter to be described. The stratus is generally formed by the subsidence, on the approach of evening, of the vapours carried into the atmosphere by the evaporation during the day. Hence the name Fallcloud has been used to distinguish this modification.

**The Cirrocumulus of Sondercloud,** fig. 2. Def. Nubes constans e nubeculis subrotundis et quasi in agmine appositis.

The Cirrocumulus consists of a number of little orbicular masses of clouds, arranged in extensive beds, and it is to be distinguished from those features of the Cirrostratus, which most nearly resemble it, by the dense and compact form of its component nube-cula. From the intermediate nature of this cloud, between the cirrus and cumulus, it has been called Cirrocumulus. It has received in English the name of Sondercloud, a word of Saxon derivation.

**The Cirrostratus or Wanecloud,** fig. 3, 4, 5. Def. Nubes tenuis expansa evanescens, vel constans e nubeculis, hujus generis, in agmine appositis.

The Cirrostratus is a cloud always distinguished by its flatness and great horizontal extension, in proportion to its perpendicular height. Under all its various forms, it preserves this characteristic, and is generally changing its figure, and slowly subsiding,—hence it is called the Wanecloud.—It often results from the fibres of the cirrus subsiding into strata of a more regularly horizontal direction; hence it is called Cirrostratus.

**Cumulostratus or Twaincloud,** fig. 6. Nubes densa basin planam undique supercrescens vel constans e pluribus nubibus hujus generis communem basin habentibus.

The Cumulostratus is distinguishable from the simple cumulus by the greater degree of its density, and by its having frequently several large masses of cloud rising from a common base. This base is generally of a flat form, and floats on the surface of the vapour plane, like the base of the cumulus. Hence the name cumulostratus. The base being flat like a stratus, and the superstructure resembling a large cumulus overhanging its base, in large fleecy protuberances, or rising into the forms of mountains and rocks. This modification is called Twaincloud, from its often resulting from the visible coalescence of two other modifications, as for example the cirrus and cumulus. When it goes on increasing in density and size, it acquires the form and character of the nimbus (described below) becoming a raincloud, pouring down rain from its lower parts. (See fig. 8.)

**Nimbus or Raincloud,** fig. 8. Def. Nubis vel nubium copia pluviam effundens.

The Nimbus is the ultimate resolution and fall of clouds in rain, previous to which a change of form is observed in them, sufficiently remarkable to constitute a distinct modification.

Mr Howard began his observations about fifteen years ago, and Mr Forster of London has been for several years past employed in making corresponding observations. From all these observations there has resulted, at length, a theory of the formation and changes of the clouds, which has been deduced entirely from the phenomena they have been observed to exhibit during their formation, changes, and eventual resolution into rain. As we wish, however, to avoid, as much as possible, any particular hypothesis in this article, we shall give merely an accurate account of the phenomena observed, and content ourselves with a description of the distinct modification of form. which aqueous vapour is found to assume, during the progress of the formation, changes, and resolution of clouds.

The first general observation with regard to clouds is, that they have a tendency to assume one or other of seven distinct modifications, and that the peculiar character of these seven modifications may be discovered in all the endless configurations exhibited by clouds under different circumstances. It may be observed, further, that the most indefinite and shapeless masses of cloud, if attentively observed, will sooner or later show a tendency to take on the form of some of the modifications; a circumstance which shows not only their distinct nature, but also proves that there are some general causes, as yet undiscovered, why aqueous vapour, suspended in the air, should assume (though with great varieties of size, extent, and figure) certain definable and constant modifications.

We shall now proceed to a more minute description of the formation and changes of the clouds, and of the prognostics of future weather to be deduced from their peculiar appearances.

Formation, Changes, and Varieties of the Cirrus.

The Cirrus or Curlcloud may be distinguished from all others by the lightness of its nature, its fibrous structure, and the great and perpetually changing variety of figures which it presents to the eye. It is generally the most elevated of clouds, occupying the higher regions of the atmosphere. As this cloud, under different circumstances, presents considerable varieties of appearance, it will be proper to consider these separately, with reference to the particular kind of weather in which they prevail.

After a continuance of clear, fine weather, we often observe a fine whitish line of cloud, at a great elevation, like a white thread stretched across the sky, the ends of which seem lost in each horizon; this is often the first indication of a change to wet weather. To this line of cirrus others are added laterally; and sometimes clouds of the same sort are propagated, as it would seem, from the sides of the line, and are sent off in an oblique or transverse direction, so that the whole phenomena has the appearance of net-work. At other times, the lines of cirrus become denser, by degrees, descend lower down in the atmosphere, and by inoculating with others from below, produce rain without exhibiting the above described appearance of transverse masses of cloud.

To the above form of the curlcloud the name of Linear cirrus has been given, and when the reticulated intersection is added to it, the name of reticulated cirrus, or net-like curlcloud, has been applied to it as a mark of distinction.

The above-mentioned varieties of the cirrus are all composed of straight lines of cloud either parallel or crossing each other in different directions, but they are ranged under the head of cirrus or curlcloud, from the analogy of their texture to this cloud when it appears under curling and contorted forms, which first suggested the name. The comoid cirrus, vulgarly called by the country people of England the mare's-tail cloud, is the proper cirrus. It has somewhat the appearance of a distended lock of white hair, or of a bunch of wool pulled out into fine pointed ends, from whence it has derived its name, comoid. This form of the cirrus is most commonly an accompaniment of a variable state of the atmosphere, and often forebodes wind and rain. The direction of the fine and almost evanescent tails of this kind of cirrus, varies considerably in the course of a few hours, in very changeable weather; but when the tails have a constant direction towards the same point in the compass, for any considerable time, it has been frequently observed, that a gale has sprung up from that quarter to which they had previously pointed. In variable and warm weather in summer, when there are light breezes of wind, long and obliquely descending bands of cirrus are often observed in the air, and seem sometimes to unite distant masses of cloud together. Frequently, by means of the interposition of these cirri between a cumulus and some other cloud (as, for example, cirrostratus), the cumulostatus or twaincloud, and ultimately the nimbus or raincloud, is formed. If we attentively examine the cirrus, we shall find that it is in constant motion, not merely changing its form, but very often exhibiting a sort of internal commotion within the substance of the cloud, particularly in the larger end of it. Every particle seems alive and in motion, while the whole mass scarcely changes its place. This motion will, on a minute examination, often appear to consist of the fibres which compose the cirrus, gently waving backwards and forwards, to and from each other. Often, however, it seems like minute specks all in commotion. This takes place most frequently in those large and lofty cirri, with rounded heads and long pointed tails, which occur in general with dry easterly wind during the summer and autumn. (See fig. 1.)

Of the Formation and Changes of the Cumulus.

The best time for viewing the progressive formation of the Cumulus is in fine settled weather. If we then observe the sky about the time of sunrise, or soon afterwards, we shall see small specks of cloud here and there in the atmosphere. These often appear to be the result of small gatherings, or concentrated points of the stratus or evening mist, which, rising in the morning, grows into small masses of cloud, while the circumjacent atmosphere becomes clearer.

As the sun rises, these clouds get larger,—two or three perhaps which are near each other coalesce—and at length a large cloud is formed, which, assuming a cumulated and irregularly hemispherical shape, has received the name of cumulus or stackencloud. This may properly be denominated the cloud of day, as it usually subsides in the evening, in a manner which forms the exact counterpart to its formation in the morning. It breaks up into small fragments and evaporates, and is succeeded again by the stratus or fallcloud, which has been called the cloud of night by some writers, on account of the period in which it prevails.

There are some varieties in the forms of the Cumulus which deserve particular notice, as they seem to be connected with electrical phenomena. In some kinds of fine weather, when these clouds form soon after sunrise, increase through the day, and subside in the evening, they are of a more hemispherical form, than when they occur in changeable weather. When these well-formed cumuli prevail during many days together, the weather is settled, and the atmospherical electrometer has been observed not to vary much in its indications. These cumuli are whitish-coloured, and reflect a fine strong silvery light when opposed to the sun. The cumuli which are seen in the intervals of showers are more variable in form, they are more fleecy, and have irregular protuberances. Sometimes they are of a blackish colour, like the clouds which the sailors call scud, and at other times they seem of a tuberculated form. Cumuli may, at any time, increase so as to obscure the sky, and they then generally inoculate and begin to assume that density of appearance which characterizes the twaincloud, or cumulostratus.

Of the Formation of the Stratus.

The Stratus or Fallocloud comprehends fogs, and all those creeping mists which in summer evenings fill the valleys, remain during the night, and disappear in the morning. The best time for observing its formation, is on a fine evening, after a hot summer's day. We shall then observe, that, as the cumuli which have prevailed through the day decrease, a white mist forms by degrees close to the ground, or extends only for a short distance above it. This cloud arrives at its density about midnight, or between that time and morning; and it generally disappears after sunrise. It is, for this reason, called by some writers the cloud of night. Under particular circumstances, it lasts longer in the morning than usual, particularly in autumn, the coming in of which season being generally marked by the greater prevalence of the fallocloud. In winter this cloud puts on a still denser appearance, and often lasts through the day, and even for many days successively; a remarkable instance of which occurred in the neighbourhood of London in January 1814. This fog spread over a large proportion of the south and west of England, and lasted about a fortnight. The stratus has often been found positively electrified, and its component particles do not wet leaves, or other substances connected with the earth. It has been supposed that the earth below it, and probably the air above it, contained a negative countercharge. The stratus should be distinguished from that variety of the cirrostratus or wanecloud, which looks much like it in external appearances, and which has usually a similar state of electricity with the earth. The criterion whereby we may judge to which of the two modifications to refer a mist is, that the stratus does not wet objects it alights on, but the cirrostratus moistens everything.

Of the Formation and Changes of the Cirrocumulus or Sondercloud.

The Cirrocumulus, as has been said before, consists of a number of well-defined orbicular masses of cloud placed in close horizontal apposition, at the same time being quite asunder, or separate from each other. It is subject, however, to some varieties in the size and figure of these orbicular masses, and in their nearer or more distant approximation to each other. The most striking feature of this cloud is observable before, or about the time of thunder-storms in summer. The component nubeculae, are then very dense in their structure, very round in their form, and in closer apposition than usual. See fig. 2. This kind of Sondercloud is so commonly a forerunner of storms, that it has been frequently spoken of by poets as a tempestuous prognostic. In rainy and variable weather, a variety of this cloud appears, strikingly contrasted with the abovementioned kind, being of a light fleecy texture, and having no very regular form in its nubeculae. Under these circumstances, it is sometimes so light and flimsy in its texture, as to approach very nearly to the nature of the cirrostratus. Sometimes this kind of cirrocumulus consists of nubeculae so small as scarcely to be discernible; the sky seems speckled with innumerable little round white, and almost translucent, spots. The cirrocumulus of fair summer weather is of a middle nature; neither being so dense as the stormy variety, nor so light as the one last described. Its nubeculae vary in size, and in the degree of their proximity. In certain kinds of fine dry weather, with light gales of north and easterly wind, small detachments of cirrocumulus rapidly form and subside again, which do not lie in one plane; but, in general, these clouds are in horizontal arrangement. The formation of cirrocumulus is either spontaneous, that is, unprecedented by any other cloud; or, 2d, it may result from the changes of some other modification. Thus the cirrus or cirrostratus often changes into cirrocumulus, and vice versa. When this cloud prevails, we may, in general, anticipate an increase of temperature in summer; and, in winter, it often precedes the breaking up of a frost, and indicates warm and wet weather. In warm weather, in the summer time, several extensive beds of this cloud, ranged in different altitudes, and viewed by moonlight, have a very beautiful and picturesque appearance, and have been compared by poets to a flock of sheep at rest. The cirrocumulus either subsides slowly, as if by evaporation, or it changes into some other modification of cloud.

Of the Formation, Varieties, and Changes of the Cirrostratus or Wanecloud.

The Cirrostratus is characterized by shallowness or great horizontal extent, in proportion to its vertical depth; so that when the cirrus, or any other cloud, is observed to assume this form, we may generally expect that it will end in a cirrostratus. The cirrus, for example, after having existed some time in the higher regions of the atmosphere, often descends lower; its fibres become more regularly horizontal; and it puts on, by degrees, the character of the wanecloud. The cirrus more frequently changes to the cirrostratus than the cirrocumulus does, and the cirrocumulus more frequently than the cumulus.

The cirrostratus being once formed, sometimes re- assumes the character of the modification from which it originated, but more frequently it evaporates by degrees, or, by insouciating with some other modification, produces the twaincloud, and eventually falls in rain.

The Cirrostratus seldom remains long in the same form, but is observed to be constantly subsiding by degrees; hence it has been called the wanecloud from the old English verb to wane, to decline, or waste away. There are many varieties in the figure of the cirrostratus, some of which are more transitory than others. Sometimes this cloud is disposed in wavy bars or streaks, in close horizontal apposition, and these bars vary almost infinitely in size and shape. A flat and nearly horizontal cloud, composed of such streaks, is very common, particularly in variable weather, in summer. The bars, which compose this variety, are generally confused in the middle, and are most distinct towards the edges. See fig. 3. A variety not unlike this is seen in fine summer evenings, and constitutes what has been denominated the Mackerelback Sky. It is often very high in the atmosphere. We have observed, that, on ascending lofty mountains, the apparent distance of the cloud seems hardly diminished, while the cumulus or stackencloud has been sailing along on a level with the point of observation, or even below it. Another common variety of cirrostratus differs from the last in being one plane and long streak, thickest in the middle, and wasting away at its edges. This, when viewed in the horizon, has the appearance of fig. 5. It often seems to alight on the summits of the cumulostratus, as represented in the plate; and, in these cases, the density of the large twaincloud increases in proportion as these long waneclouds form and evaporate on their summit; a circumstance which looks as if the great density of the cloud depended on the insouculation and subsequent intermixture of the two different modifications with each other. The result of this intermixture, and the consequent density of the cloudy mass, is eventually the formation of the nimbus and the fall of rain. Another principal variety of the cirrostratus, is one which consists of small rows of little clouds curved in a peculiar manner. It is called the Cynoid Cirrostratus, and it is a sure indication of stormy weather. See fig. 4. Immediately below this, in the plate, is the representation of another less perfectly formed, having more of the character of the sondercloud. It is a variety often produced when a large cumulus passes under a long line of cirrostratus, like that in fig. 5, and is also a sign of variable and stormy weather. The last variety of cirrostratus to be mentioned, is that large and shallow veil of cloud which extensively over-spreads the sky, particularly in the evening and during the night, and through which the sun and moon but faintly appear. It is in this cloud that those peculiar refractions of the sun and moon’s light, called Halos, Mocksuns, &c. usually appear, and which is the surest prognostic we are acquainted with, of an impending fall of rain or snow. To these principal varieties of the cirrostratus, others less frequent might be added; but as their forms are almost innumerable, every meteorologist must observe them for himself. The usual termination of the cirrostratus is, by forming an intimate union with some other cloud, to produce rain; but, at other times, it evaporates, or changes into some other modification, as observed above.

Of the Formation of the Cumulostratus or Twain-cloud.

The Cumulostratus might be always regarded as a stage towards the production of rain, and it frequently forms in the following manner: The cumulus, which in common passes along in the current of wind, seems retarded in its progress,—increases in density,—spreads out laterally,—and at length overhangs the base in dark and irregular protuberances. The change to the cumulostratus often takes place at once in all the cumuli which are near to each other; and their bases uniting, while the superstructures remain asunder, rising up with mountain-like or rocky summits, the whole phenomenon has a fanciful appearance. The change from cumulus to cumulostratus is often preceded by the cirrostratus, or some other of the lighter modifications, coming over in an upper current and alighting on the summit of the cumulus. Long lines of wanecloud often appear for a length of time attached transversely to the summits of the twaincloud, and give them the appearance of being transfixed by shafts. See fig. 6. Cumuli sometimes meet together and begin to be arranged along with joined bases, without acquiring the dense black colour of the cumulostratus, and, as the change is gradual, we may view the cloud in an intermediate state. Twain-clouds vary somewhat in appearance. Those in which hard hail showers and thunder storms form, look extremely black before the change to rain, and have a most picturesque but menacing aspect, as they are seen slowly coming up with the wind. The cumulostratus sometimes evaporates, or changes again to cumulus, and sometimes it forms itself spontaneously, without the precurgence of any other cloud, and disappears again. But, in general, it ends at last in the nimbus, and falls in rain. Frequently, in a long range of these clouds, one part changes into nimbus, and rains, while the other remains a cumulostratus. See fig. 8. But this is not frequently the case. Having given this sketch of the modifications, it must be observed, that masses of cloud sometimes appear hardly referable to any of them; but even then, if watched long enough, they will be found to put on sufficient of the character of some of the modifications to be registered under its name.

Of the Formation and Fall of the Nimbus, or Rain-Cloud.

Hitherto we have considered only those modifications by which aqueous vapour is suspended, and kept as it were buoyant in the atmosphere. It remains now to describe a cloud which always precedes the fall of rain, snow, or hail, and which has been denominated Nimbus, agreeably to the notion of the ancients, who distinguish between the imber or a shower of rain, and the nimbus or cloud from which the rain came. Any of the six above described modifications may increase so much as to obscure the sky, without ending in rain, before which the peculiar characteristic of the raincloud may always be distinguished. The best manner of getting a clear idea of the formation of the *nimbus*, is by observing a distant shower, in profile, from its first formation to its fall in rain. We may then observe the *cumulus* first arrested, as it were, in its progress, then a *cirrostratus* or *cirrus* may appear to alight on the top of it. The change to *cumulostratus* then goes on rapidly, and this cloud, increasing in density, assumes that black and threatening appearance which is a known indication of rain. Shortly afterwards the very intense blackness is changed for a more grey obscurity, and this is the criterion of the actual formation of water, which now begins to fall, while a *cirriform* crown of fibres extends from the upper parts of the clouds, and small *cumuli* enter into the under part. After the shower has spent itself, the different modifications appear again in their several stations; the *cirrus*, *cirrostratus*, or perhaps the *cirrocumulus*, appears in the higher regions of the air; while the remaining part of the broken *nimbus* assumes the form of flooky *cumulus*, and sails along in the current of wind which is next the earth. When large *cumulostrati* begin to appear again, they indicate a return of the rain; and these processes are constantly going on in showery weather, when the rapid formation and destruction of rain clouds goes on, and is attended by the other modifications in succession, in the manner above described. In continued rainy days, we cannot observe the upper parts of the *nimbus*, which extends for miles over large tracts of country; but we have no doubt that the same processes go on slower, and on a larger scale, in continued rainy weather, which are more conspicuous in the rapid and partial formation of showers.

From these descriptions, several practical conclusions follow:

1st, The modifications under different circumstances of position, with respect to the sun, refract different colours, which ought to be noted down in meteorological journals, as they appear to indicate different changes of weather; and future observations may either refute or confirm those from which, at present, doubtful indications are deduced.

2d, The prognostics of the coming weather must always be deduced from those clouds which ultimately prevail, and in the course of a day, almost all the modifications may be seen in variable weather.

3d, The prevalence of the *Wanecloud* is always a sign of a fall of rain or snow.

Future observations may possibly add new subdivisions to these seven distinct genera of clouds.

(o. o.)