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The photographic news
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- 29.1885
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- 1885
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- Bandzählung
- No. 1418, November 6, 1885
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The photographic news
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Band 29.1885
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- Register Index III
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710 THE PHOTOGRAPHIC NEWS. LNovEMBER 6, 1885. fogs are often produced io valleys and over damp meadows in the evening. In this case the stratum of air next to the ground deposits its superfluous moisture as dew, and re mains saturated, and at a lower temperature than the strata of air immediately above it. If, now, cold air from higher levels flow down, and mix with the saturated stratum below, fog will be produced, extending to a certain height, and often having a perfectly level surface. Her- schell calls these “radiation fogs,” as their formation depends upon the air close to the earth being brought to the limit of saturation by the cooling of the earth’s sur face in the evening. The sea-fog which often extends inland along the coast for a few miles, in summer, is pro duced by the mixture of a cold wind with the warmer air, saturated with moisture, lying over the sea. In short, whenever there is from any cause a mixture of cold air with a warm and damp atmosphere, fog or mist will generally be formed. The disappearance of fog after sun rise is owing to the increase of temperature, enabling the air to take up again the vapour which it was before com pelled to part with. The frequent fogs of our great towns are owing partly to the large amount of moisture supplied to the atmosphere by combustion and other domestic pro cesses prevailing in densely crowded localities; but their dark colour and persistent nature is due almost entirely to the excessive quantity of smoky and tarry matter present in the air. The particles of water become coated with a thin film of tar, which not only prevents the evaporation of the fog when the air becomes warmer, but Isc renders extremely difficult the aggregation of the particles into drops of rain. Mist differs from fog chiefly in consisting of larger particles, and in being distinctly wetter. Clouds, when they consist of water, are nothing but masses of mist or fog suspended at a considerable elevation in the air ; but it is probable that many of the higher clouds consist of needles of ice. The lowest level at which clouds float varies in different localities. In our own islands one-third of the total observations of cloud-levels were below 2,500 feet. The causes of the formation of clouds are similar to those which produce fog and mist. Whenever the temperature of a warm and moist mass of air is reduced below the limit of saturation, clouds are the result. Now, since warm, moist air is specifically lighter than cold, dry air, it will ascend, and as the upper part of the ascending column reaches the colder regions above, the vapour will begin to condense, forming what Tyndall calls a “ capital ” of cloud supported upon an invisible column of warm air below. Hence ascending clouds grow continually larger, and on a fine day in summer one may often watch the small white clouds growing into huge masses as the heated air-columns rise higher and higher into the atmosphere. In the evening, when these ascending currents cease, the clouds slowly descend, and gradually melt away into a clear sky. A similar clearing up of the sky may be pro duced by the influx of warm air, causing evaporation of the clouds. A most instructive case of cloud formation is the cloud-banner which is sometimes seen persistently clinging to the summit of a hill, even although a wind is blowing of sufficient strength to drag out the cloud into a long streamer, and even to detach pieces from it and carry them away. Now, although, in this case, the cloud seems stationary, the particles of which it consists are continually changing. As the wind blevs over the mountain top its vapour is condensed, but on passing it the cloud evaporates again. Thus the cloud melts away on the leeward side of the mountain as fast as it is formed on the windward side. Clouds near the horizon are often very deceptive in their greater apparent density than those overhead. In many cases it would appear to a person setting out upon a photographic expedition, that the region to which he is travelling is packed so closely with dark clouds that sun light will be totally absent. Even although the sky over head may be of such a nature as to encourage the best hopes of a good light, the dark, cloudy horizon often gives quite a despondent prospect, especially if the locality is such that a large extent of sky is visible. Often, however, this is quite an illusion ; owing to foreshortening, and perspective effects, a sky with only a few cloud-bands, and wide spaces between them, may appear completely cloud-covered when viewed near the horizon. Even when there are no clouds at all, and the air overhead appears quite clear, the horizon may appear enveloped in a thick haze, although, in fact, it is as clear there as it isoverhead. The reason for this is easily explained. When we look towards the horizon the eye traverses a thicker layer of atmosphere than when we look vertically upwards; and, however thin and imperceptible the haze may seem over head, when the thickness of the atmosphere is many times increased, as is the case when looking horizontally, the obstructive effect of the haze becomes very considerable. With respect to the different forms of clouds, Luke Howard’s classification, now nearly a century old, has become universally adopted. There are three primary types of cloud : the cirrus (Lat. cirrus, a curl), or mare’s- tail cloud ; stratus, in which the streaks of the cirrus become amalgamated into a continuous stratum; and cumulus, or the wool-pack cloud. The cirrus cloud always forms ata great height in the atmosphere. Kmitz never saw it as low as the summit of the Finisteraarhorn, 14,000 feet high. It generally moves in the opposite direction to the wind at the surface of the earth, and probably it always consists of ice or snow in extremely delicate needles. The stratus cloud, according to Howard, is a “ widely- extended continuous horizontal sheet, increasing from below upward,” but the name should not be applied to all thin horizontal layers. It belongs to the lower regions of the atmosphere, and was called by Howard “ ground fog," since it resembles apiece of lifted fog, without any definite form. The cumulus is perhaps the most common of all forms, and is the cloud of summer. By combinations of these primary types many other varieties of cloud are produced. Thus the cirro-stratus or thread-cloud consists of thin layers thinning away towards the edges, sometimes assuming the grained appearance of polished wood, and occasionally looking in the distance like shoals of fish. The ribs of cirro-stratus clouds sometimes stretch right across the sky, converging at each end towards the horizon in the shape of a boat, when it is often called “ Noah’s Ark.” Cirro-cumulus is a high cloud, although lower than the cirrus. It gives a mottled appearance to the sky; the stratification of the cirro-stratus being broken up into detached masses, causing the appearance known as the “ mackerel sky.” Cumulo-stratus represents an intermediate stage between the cumulus and the dark storm cloud or nimbus. It is crossed by horizontal lines of dark cloud, and often covers the whole sky. The base, when visible, is usually flat, denoting the line of separation of a lower stratum of air at a higher temperature, so that the base is dissolved away to a certain level by evaporation into the warmer layer. The cumulo-cirro stratus, or rain cloud, is described as a confused intermingling of heap-cloud, streak-cloud, and sheet-cloud, and, unless its front is visible as it advances from the horizon, its form is usually unable to be dis tinguished. This cloud is also called nimbus. Professor Poey has suggested the term pallium, or cloud cloak, for the uniformly grey veil which often covers the whole sky during rain. He has also proposed the term fracto-cumubUi or wind cloud, for the torn fragments, broken away by the wind from a disintegrating rain-cloud. They are easily distinguished from cumulus by their torn, shred-like appearance. Respecting the motion of clouds, those belonging to th® lower strata of the atmosphere—viz., stratus, cumulus, cumulo-stratus, and nimbus—usually move in the same direction as the wind at the surface of the earth, excep'
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