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358. Clouds are visible vapours floating in the air at a considerable height; thus differing from mists and fogs, which float near the surface. Both arise from the same causes.
359. During the warmest part of the day, when evaporation is greatest, warm moist air currents are constantly ascending from the earth. As they rise in succession, the moist air is pushed high up into the atmosphere, and losing heat by expansion, a point is at length reached when it can no longer retain in solution the moisture with which it is charged; hence condensation takes place, and a cloud is formed which increases in bulk as long as the air continues to ascend. But as the day declines, and evaporation is checked, the ascending current ceases, and the temperature falling from the earth's surface upwards, the lower stratum of air contracts, and consequently the whole mass of air begins to descend, and the clouds are then dissolved by the warmth they acquire in falling to lower levels. The whole of this process is frequently seen on a warm summer day. In the morning the sky is cloudless, or nearly so; as the heat becomes greater, clouds begin to form before noon and increase in numbers and size, often presenting scenes of unparalleled beauty as, lighted up by the sun into dazzling brilliance, they sail slowly and smoothly across the blue sky; but as the heat diminishes, they contract their dimensions, and gather round the setting sun, lit up with the fiery splendours of his beams. In a short time they disappear, and the stars come out, shining in a cloudless sky.
360. The balloon ascents of Mr Glaisher and other aëron auts, as well as observations of the clouds, show us that the whole atmosphere, to a great height, is constantly traversed by many aërial currents superimposed on each other and flowing in different and frequently in opposite directions. Masses of air of different temperatures thus frequently combine together; and since the several portions when mingled cannot hold in suspension the same quantity of vapour that each could retain before they were united, the excess is condensed and appears as cloud.
361. But again, when a dry and heavy wind begins to set in, or take the place of a moist and light wind, it generally does so by edging itself beneath the moist wind and forcing it wedgeways into the upper regions of the atmosphere, where condensation rapidly follows, and dense black clouds, often heavily charged with rain, are formed. This is a frequent cause of cloud and rain in Great Britain, when the cold heavy east wind, or polar current, thrusts high up into the air the rain-bringing south-west wind, thus causing it to darken the sky and pour down its surplus moisture in torrents of rain.
362. Currents of air driven up the sloping sides of hills and mountains by the winds, have been already referred to as a frequent cause of the formation of clouds.
363. A very natural inquiry is, How are clouds suspended in the air? The example of a cloud appearing to rest on the top of a hill though a strong wind be blowing at the time (par. 356) suggests an explanation. The cloud itself may appear stationary or suspended, but the particles of which it is composed are undergoing constant renewal or change. The particles are upheld by the force of the ascending current in which they are formed; but when that current ceases to rise, or when they become separated from it, they begin to fall through the air by their own weight till they melt away and are dissolved in the higher temperature into which they fall. Hence, as Espy has reasoned, every cloud is either a forming cloud or a dissolving cloud. While it is connected with an ascending current, it increases in size, is dense at the top, and well-defined in its outlines; but when the ascending current ceases the cloud diminishes in size and density.
364. When a cloud overspreads the sky, its lower surface is for the most part horizontal, or more generally it seems as if it was an impression taken from the contour of the earth's surface beneath it. This arises from the high temperature of the air below the cloud, which is sufficient to dissolve the particles as they descend below its level.
365. On ascending through this lower stratum of cloud, the temperature is found frequently to rise, and the air to be quite clear of clouds for a considerable thickness. Higher up a second stratum of clouds succeeds, and again another clear space, and so on, cloud and clear sky following each other several times in succession. These phenomena arise from the different currents which are encountered, superimposed over each other and differing in temperature and humidity.
366. The gorgeous appearance clouds occasionally present in balloon ascents, is thus graphically told by Mr Glaisher in describing his ascent from Mill Hill, near Hendon, on 21st August 1862: “ Twenty-seven minutes after leaving the earth, a white mist enveloped the balloon; the temperatures of the air and dew-point were alike, indicating complete saturation. The light rapidly increased, and, gradually emerging from the dense cloud into a basin surrounded by immense black mountains of cloud rising far above us, shortly afterwards there were deep ravines of grand proportion beneath open to the view. The sky immediately overhead was dotted with cirrus clouds. As the balloon ascended, the tops of the mountain-like clouds were tinged with silver and gold. On reaching their level, the sun appeared flooding with light all that could be seen both right and left, tinting with orange and silver all the remaining space. It was a glorious sight. The ascent still continued, but more quickly as the sun's rays fell upon the balloon, each instant opening to view deep ravines and a wonderful sea of clouds. Here arose shining masses of cloud in mountain-ranges, some rising perpendicularly from the plains with summits of dazzling brightness, some pyramidal, others undulatory. Nor was the scene wanting in light and shade; each large mass of cloud cast a shadow, thereby increasing the number of tints and beauty of the scene."
367. Height of Clouds.-Kaemtz has collected the results arrived at by many distinguished observers, and deduced the
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heights between which clouds range as from 1300 to 21,320 feet. This extreme height is, however, much too small, as has been proved by balloon ascents. Thus Gay Lussac, in September 1804, when at the height of 23,000 feet, saw clouds floating apparently at a great height above him; and Glaisher has also inade the same observation. It is probable that the cirrus cloud is often ten miles above the earth.
368. Since clouds are subject to certain distinct modifications from the same causes which produce the other atmospheric phenomena, the face of the sky may be regarded as indicating the operation of these causes, just as the face of man indicates his mental and physical states. The ancient meteorologist was content with discerning the face of the sky in order to predict the coming weather. It is to this chiefly that the weatherwise sailor and the farmer still look in foretelling the weather; and their predictions are frequently more correct than are those made solely from the indications of the barometer and other meteorological instruments. The best system of weather-prediction comprises both methods. Considering, therefore, the importance of clouds, a nomenclature specifying their different modifications becomes necessary, in order that the experience of one observer may be communicated to others. The classification universally adopted is that proposed by Luke Howard, and published by him in 1803.
369. By this nomenclature clouds are divided into seven kinds ; three being simple, the cirrus, the cumulus, and the stratus; and four intermediate or compound, the cirro-cumulus, the cirro-stratus, the cumulo-stratus, and the cumulocirro-stratus or nimbus.
370. Cirrus Cloud.—This cloud consists of parallel, wavy, or diverging fibres which may increase in any or in all directions. Of all clouds it has the least density, the greatest elevation, and the greatest variety of extent and direction, or figure. It is the cloud first seen after serene weather, appearing as slender filaments stretching like white lines pencilled across the blue sky, and thence propagated in one or more directions, laterally, or upward, or downward. Sometimes the thin lines of cloud are arranged parallel to each other, the lines lying in the northern hemisphere from north to south, or from south-west to north-east; sometimes they diverge from each other in the form of the tail of a horse ; whilst at other times they cross each other in different ways like rich delicate lace-work. It is probable that the fine particles of which this cloud is composed are minute crystals of ice or snow-flakes. The duration of the cirrus varies from a few minutes to many hours. It remains for a short time when formed in the lower parts of the atmosphere and near other clouds, and longest when it appears alone in the sky, and at a great height.
371. The cirrus, though apparently motionless, is closely connected with the movements of the great atmospheric currents. It is this intimate connection which has long caused it to be considered as a most valuable prognostic of stormy weather, and as such it deserves more attention than has hitherto been given to it.
372. Small groups of regularly formed and arranged cirrus scattered over the sky often accompany fair weather with light breezes; these do not indicate the approach of a storm for some time at least. Horizontal sheets of this cloud which fall quickly and pass into the cirro-stratus cloud indicate, in an unmistakable manner, continued wet weather. When streaks of cirrus run quite across the sky in the direction in which a light wind happens to blow, the wind will probably soon blow hard, but it will continue in the same direction; in other words, the variable winds and fitful gusts which accompany storms are not likely to be experienced.
373. When the fine threads of the cirrus appear blown or brushed backward at one end as if by a wind prevailing in these elevated regions, the wind on the surface will ultimately veer round to that point. If the direction indicated be from the south-west, whence the storms of Europe come, wind and rain may be expected; and it matters not how fair and settled-like the weather appear at the time, a storm more or less