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from a factory chimney, or to the steam blowing off from 3 passing locomotive, and still less to a bank of fog or mist. We admit, of course, that fog or mist has a close analogy to cloud; bat we do not think it correct, except under special circumstances, to say that 'a fog is a cloud viewed from * within, and a cloud is a fog viewed from without.' We conceire a fog to be an aggregate of particles of water formed in the air by actual contact with, or the immediate proximity of, colder earth or sea. These water-particles may be more frequent and the fog more dense by reason of the presence in the air of a vast number of material particles, products of imperfect combustion, such as is not uncommon in London or other large towns. A cloud resting on a mountain side is a fog for any one in it; and so far as it is caused by the chilling effect of the earth in actual or proximate contact, it is really a fog ; but when, as is more frequently the case, it is due to the upward movement of a lower stratum of air pressed horizontally against the slope of the mountain, it is a cloud.
The necessity of strict definition may, however, in this instance be waived. Clouds are things of which we all have familiar cognisance, though we do not all understand either how they are formed or what they signify. So far as meteorology is concerned, all clouds may be said to be caused by the partial condensation of the aqueous vapour in the air. All air, it must be remembered, contains some aqueous vapour, more or less according to its temperature and the locality; air resting over a warm sea contains more than air resting over a cold sea or over the land; and if air which contains as much, or nearly as much, vapour as it can carry is subjected in any way to chilling, the immediate result is the condensation of some of the vapour into visible cloud or rain. Now warm moist air may be chilled in various ways. It may pass over cold water, as it very commonly does near the banks of Newfoundland, when it immediately forms fog; or it may be forced upwards, when it is chilled by expansion. Thanks to the labours of Joule, of Helmholtz, and of Tyndall, the way in which any body is chilled by expansion is now generally understood; and a mass of air, as it moves upwards and is relieved from part of the pressure of the superincumbent air, expands, and is therefore chilled; some of its vapour is condensed and becomes cloud or rain; but the condensation gives off the heat of evaporation, which it is often convenient to speak of by its old name of latent heat,' and some of the cloud again becomes vapour.
One striking instance of this condensation of vapour and the consequent manifestation of heat is the Föhn of mountain countries. The name originally belonged to a peculiar hot dry wind occasionally experienced in the valleys of the north-eastern cantons of Switzerland, but has been applied by meteorologists to similar winds in different parts of the earth: in the Arctic, in New Zealand, in Norway, and in North America. The heat and dryness of the snow-eater, as it was sometimes called in Switzerland, were so remarkable that, in the infancy of meteorology, it was assumed to be an outburst of hot air from the Sahara, a northerly prolongation of the dreaded Scirocco. Careful observation, however, has shown that, so far from being an extension of the Scirocco, it is an extension of the warm south-west wind from the Atlantic, and that the heat which eats up the snow in the north-east of Switzerland is the heat, not of the Sahara, but of the Gulf Stream. When such a current of warm moist air strikes on the westerly and south-westerly faces of the Swiss mountains, it is forced upwards, as on an inclined plane, and being chilled by the consequent expansion, throws down its moisture, often in devastating floods. But by this condensation of vapour, the air is again warmed, so that it passes over the mountain tops deprived of its moisture and baving a temperature very much higher than that due to the elevation; so that when, under certain conditions, it is forced down into the valleys on the other side, and is heated by the compression, it has a temperature far exceeding that which it had as it first struck on the western face of the mountains. And not only is this wind exceedingly hot, but it is very dry. Its vapour having been taken from it at the low temperature of the mountain tops, the want of moisture at the high temperature which it brings into the valleys is excessive; the Föhn thus greedily seizes on any moisture that comes in its way, and blowing over snow-fields, although so hot, does not so much melt the snow as evaporate it.
In winds of this character we have clear illustrations of the condensation by elevation, and the warming by condensation, such as, in the free air, are continually going on under varying conditions. The impact on the land, as a mass of air comes in from the sea, is one great cause of an upward motion; mountains, of course, are an extreme cause; but every inequality in the earth's surface, every house, tree, or hedgerow on shore, every wave at sea, produces a similar effect in different degrees. Air mixed with vapour is, bulk for bulk, lighter than dry air of the same temperature and elastic force, and may thus frequently have a tendency to rise on very slight suggestion. There are thus causes continually at work to force upwards very considerable masses of air, sometimes dry, more commonly moist, which, as they ascend, by expanding and cooling, either part with their own vapour, or condense and render sensible the vapour of the strata of air through which they pass.
Mr. Ley conceives such an upward current passing through successive layers of air already saturated with moisture at their respective temperatures. The current, by its rapid expansion, is cooled considerably below the temperature due to the height above the earth's surface, and thus, by contact, cools the strata through which it is passing, and condenses their vapour. The heat so given out expands a portion of the adjacent air, which joins itself to the ascending current, enlarging its volume, while the vapour, rendered visible, spreads out in the lowest layers in which this condensation began, and forms a bank of cloud, heaped up on a level base. Such a cloud is what Mr. Ley calls 'a cloud of inversion,' and in its simplest form is commonly known as cumulus — a heap-cloud,' or 'wool-pack.' Mr. Ler points out that a low temperature is unfavourable to a great upward extension of a warm current of air; a Inigh temperature is favourable; and thus, that cumulus is a cloud of day rather than of night, and especially of the afternoon, towards the hottest part of the day. It does not prend unsettled weather; it is not the precursor of any kind of storm; and, though frequently spoken of as a
thundercloud,' this fact is merely due to the inadequacy at the study bestowed on the cloud.'
(a the other hand, when cumulus, formed rather in the asending column than in the strata through which it
s takes the shape of a pillar of cloud, it may give impurtant indications of coming weather. As a general rule,
upper strata of air travel faster-much faster--than the lwer; and, if the air through which the ascending column Pes*** is all moring in the same direction, the pillar of
as it forms leans forward, because its upper part is Hi taster than the lower. But if the upper strata are 149 Hering in the same direction as the lower, then the
recloud leans to one side or the other, or, possibly,
a southerly point in Europe, while the general under7**** b* live the st, we at once know that a more southerly wind
prevails at no very great altitude. Thus, in fine weather in April or May, in England, when this is the case, we can, with some safety, hazard the conclusion that a cyclonic disturbance, having a southerly wind on its advancing margin, is progressing eastwards over the western coasts of the British Isles. Such a disturbance, often preceded by fine weather, is itself accompanied by unsettled weather, and, after a long spell of fine weather, may be the precursor of many areas of unsettled weather.'
When cumulus is forming, the ascending current gradually loses its upward motion, and becomes colder and denser than the air through which it has passed ; air from it flows outwards and downwards, condensing the adjacent vapour which falls down the sides of the heap and is again vapourised. But if the column reaches such a height that the vapour, instead of turning into particles of water, turns into particles of ice, the increase of temperature by the latent heat given off tends to carry the column to a still greater height. If, then, the air surrounding the summit of the cloud is so cold that these ice particles are not revapourised, the cloud becomes cumulo-nimbus, which is the 'true shower-cloud,' and gives forth lightning and thunder, with precipitation of ice, either as hail or melted into rain.
Cumulo-nimbus,' says Mr. Ley, generally exhibits to the spectator appearances that are very beautiful and very grand, and to some spectators, in its accidental postures, phenomena more or less terrible. Massive dimensions impress us; brilliant colour-contrasts please us; it is agreeable to any lover of nature to watch for and to calculate beforehand the first disruptive electrical discharge. . . . It is important to the agriculturist and to the navigator to know the probabilities of the violent rain or hail, and of the direction and force of the squall. . . Stillness before storm, broken now and then by the cry of the green woodpecker, leaflets at rest before a rude carousal, a glassy sea very near the edge of dark and gust-tossed waves, these and many another memory affect the writer while he writes.'
Such indications, however, are sometimes at fault. We have before us notes of such a cloud seen simultaneously at places fifteen miles apart, one July afternoon—a cloud of deep copper-colour, overspreading the sky, in form as of a giant or the destroying angel. The ghostly stillness, the shadowy light seemed to portend some terrific outburst; but nothing came of it. The portentous appearances faded away; the evening was calm and soft, and the next day was fine. It was a season of great drought and heat, which did not break up till some weeks later,
To an important and interesting class of clouds Mr. Ley gives the name clouds of interfret,' which he explains :
If two horizontal or nearly horizontal currents, differing in velocity, in direction, or in both velocity and direction, move, the one over the other, the particles of air will intermingle to a certain extent at the surface of contact, and whirls, ripples, and waves will be produced, whose size and shape depend on the respective velocities and directions of these currents. Now, if the upper current be colder than the lower, as will usually be the case, and if the lower current contain watervapour near its maximum tension, it is evident that condensation will very likely take place over the crests of these waves; for these portions of the lower current are pushed far up into the colder upper current and thus have their temperature reduced. To this process we give the title “ Interfret."
These clouds are of various types, including, in a general way, clouds which form lines or belts approximately parallel to each other, or speckled patches producing what is familiarly known as a 'mackerel sky. The varieties of cirrus are all of this class, and are of very great importance as weather-prognostics. Cirrus forms at a very great height - from twenty-five to thirty or even forty thousand feet above sea level-and consists of ice particles or spicula, which, by the law of ice crystallisation, tend to range themselves end to end—as may often be seen in the ice-flowers on a window in time of frost-as though forming threads; so that cirrus closely examined has more or less the appearance of a wisp of hair. When the cloud is formed in strata moving with different velocities, the upper part, as in the pillar-like cumulus, streams out in front; and when this streaming out is very marked, Mr. Ley distinguishes the cloud as cirro-filum, or gossamer cloud. The distinctionthough it has not as yet been generally adopted by meteorologists—is a happy one, for there is no question that cirrofilum differs from the quieter cirrus both in appearance and in its indications. Simple cirrus which has very little slant may usually be taken as indicating distant local showers, and, in these latitudes, as the relics of distant thunderstorms. But when the streaming forward of the wisps of hair or thread becomes very marked, the great difference in the velocities of the several strata, so pointed ont, is indicative of unsettled conditions which may not unfrequently herald the approach of bad weather. The * more rapid its movement, the more unfavourable the pro'gnosis,' especially if the movement is from the north-west. • When it occurs with, or just after, an increase of barometric