CLOUDS, masses of minute globules of water or crystals of ice and snow suspended in the atmosphere.
The minute globules of water that make the cloud are evidently. condensed from the invisi ble moisture that is always present in the air in greater or less quantities. The studies of Aitken, J. J. Thompson, Wilson and others have thrown considerable light on the processes of condensation. It pure, dry air—that is, air from which all dust and traces of electncity have been removed—be mixed with pure vapor of water and the moisture be cooled below the temperature of saturation, it will be found that condensation of water vapor does not generally occur. If, however, fine dust be injected into the pure mixture without altering its tempera ture or pressure, a fine mist is developed at once. It has also been discovered that if a charge of electricity, however small, be intro duced, condensation takes place at once. From these experiments it is inferred that nuclei of UMW sort, such as dust particles or electric par ticles, are necessary for the formation of water globules, other conditions being favorable.
The original classification of clouds by How ard in 1803 took account of form only. A more recent classification—the one now in gen eral use—takes account of both the form of the cloud and its altitude. In this system three principal forms are recognized, namely, cir rus, cumulus and stratus, with their modifica tions. These three main classes present the most obvious differences in form and shape; they develop at widely different altitudes and move with varying velocities. It is possible tu further subdivide each class into a number of varieties, and in practice this is done when an observer becomes skilled in distinguishing the special forms and subdivisions of the main tYPes• The definitions of the principal types as rec ommended by the International Cloud Commit tee, and generally adopted by meteorologists, are given below.
Cirrus (CO.— Isolated feathery clouds of fine fibrous texture, generally of a white color, frequently arranged in bands, which spread ldre the meridians on a celestial globe over a part of the sky and converge in perspective toward one or two oppoeite points of the horizon. (In the formation of such bands Ci. S. and Ci. Cu. often take part).
Cirrus clouds are the highest of all and move with the greatest velocity. They develop up to a height of about 10 miles, although their mean height is a little over six miles, being higher in sununer than in winter. Therr aver age velocity in winter is about 75 miles per hour, and in stunmer 65 miles per hour. They may be fortned by mixture of air strata of different temperatures or by the cooling of a moist streak tn the upper atmosphere by expan sion and radiation. In the latter case, if the
surrounding air be very dry, the newly-formed cloud is soon dissipated.
Cirro-stratus (Ci. S.).— Pine whitish veil, sometimes quite diffuse, giving a whitish appearance to the sky. and called by many cirrus haze, and sometimes of more or less distinct strecture, exhibiting tangled fibres. The veil often produces halos around the sun and moon.
This cloud, as its name indicates, is one of the composite forms. Its structure is much on the same order as that of pure cirrus; in fact it is generally composed of cirrus fibres tangled and interlaced in a most complex rnanner. The variety of forms, however, is practically unlim ited and it undergoes rapid changes, especially in advance of a cyclonic disturbance. Like cir rus, it is composed of crystals of snow or ice, develops at a great elevation and moves with high velocity.
Cirro-sunislats (Ci. Cu.).— Fleecy cloud. Small white balls and wisps without shadows, or vrith very faint shadows, which are arranged in groups and often in rows.
This is a broken layer of cloud, of which the component masses are not fibrous, like cir rus, but more or less rounded. A second, al though rather rare form of cirro-cumulus, is the rnacicerel sky of northern Europe_ The cloudlets in this case are rounded, but not fleecy. The altitude of cirro-cumulus ranges from four to six miles and its movement, lace cirrus and cirro-stratus, is quite rapid.
Cumulus (Cu.).— Woolpack clouds. Thick clouds whose summits are domes with protuberances. but whose bases are fiat. These clouds appear to form in a diurnal ascensions' movement which is almost always apparent. When the cloud is opposite the sun the surfaces which are usually seen by the observer are more brilliant than the edges of the protuberances. When the illumination comes from the aide this cloud shows a strong actual shadow; on the sunny side of the sky, however, it appears dark with bright edges. The true cumulus shows a sharp border above and below. It is often torn by strong winds, and the detached parts (fracto cumulus) present continual changes.
This cloud is undoubtedly formed by the condensation of water vapor in the summit of an ascending column of air. The flat base of the cloud marks the level when the condensa tion temperature is reached, and the upper dome-shaped summit represents the head of the air column protruding into colder space. In the temperate latitudes it is usually a cloud of the warm season and forms at an altitude of about a mile above the surface of the earth. Its average rate of movement ranges from about 20 miles per hour in summer to 30 in winter. There are many special varieties and forms of cumulus cloud.