Clouds, balloons, and kites naturally supplement one another. While clouds indicate the direction and velocity of the air at different heights, yet the lower clouds often conceal the upper strata, or there may be no clouds at all, in which case balloons or kites will aid us to determine the drift of the currents. When there is little wind at the ground, or to reach heights of several miles, we must employ balloons, but otherwise kites are preferable in most cases. The thermal and hygrometric conditions of the free air can be ascertained only by personal observations in balloons, or by raising self-recording instruments with balloons and kites, and this latter method it is predicted will be the path of greatest progress.
CHAPTER II
CLOUDS—FORMATION AND CLASSIFICATION—MEASUREMENTS AT BLUE HILL—THE INTERNATIONAL OBSERVATIONS
Clouds must have been among the earliest observed natural phenomena, and they were used from time immemorial as weather signs. Yet their every-day occurrence was very likely the reason why their origin was not studied until about a century ago. Father Cotte, in his classic work on meteorology, published in 1774, devotes only a couple of paragraphs to clouds, but Abbé Richard, in his contemporary Histoire Naturelle de l'Air, discusses the appearance and theories of clouds in ten chapters. The cause of evaporation was unknown in the last century, and it was not until its close that Dalton, the English chemist, proved that water-vapour exists independently in the air, and Hutton explained that precipitation was produced by the contact of a current of saturated air with a colder one. Although there remains much to be learned about cloud formation, yet it is now pretty well established that its most effective cause is the ascent, and consequent cooling by expansion of the air, rather than the mixture of masses of air having different temperatures. The ascent of the air may result from its being forced up a mountain slope by its horizontal movement, or from its being drawn up in a vortex, but most commonly the air rises from its lessened specific gravity when warmed. If the temperature of the quiescent air decreases faster than 1° for each 183 feet of height, which is the adiabatic rate of cooling for dry air, as explained in the last chapter, air warmed locally will rise and cool at this rate until the dew-point is reached. Then the vapour in the air will be condensed upon particles of dust, which Aitken found to be more numerous in clouds than outside them. The most conspicuous of the clouds formed by rising currents is the cumulus, or rounded summer cloud, which has been aptly termed "the visible capital of an invisible column of air." Saturated air cools as it rises more slowly than dry air, consequently the upward motion is maintained through the cloud mass, causing the swelling up of the tops of the cumulus clouds, which reach their highest development in the thunder clouds, or cumulo-nimbus, as they are called. The lower limit of the cloud region is determined therefore by the height at which the rising currents reach their dew-point, and the altitude of the cloud formation depends upon the humidity of the ascending current, the drier it is, so much the higher must it rise to have its vapour condensed. In storms the rising current mingles with the stronger horizontal current above, which carries with it the upper portion of the cloud, and covers the whole sky with a uniform sheet. The wave, or ripple cloud, has been explained by von Helmholtz and von Bezold to be due to the undulations in a horizontal current producing alternate rarefaction and condensation of its water-vapour through changes of temperature. Still another cause of low-lying clouds is the cooling of the air to its dew-point by contact with a cold surface, such as the earth when cooled by radiation during a clear night, or the polar currents of the ocean. Fog is often formed in this way, which we call stratus cloud when it rises above us. The highest clouds consist of ice crystals, because the temperature of the air where they are is much below that of freezing water. Although it is possible to cool drops of water considerably below 32° Fahrenheit without congelation, yet it can be told with certainty that the clouds are composed of ice if the sun and moon when seen through them are surrounded by the large rings or halos, which the theory of optics shows can only result from refraction of light by ice crystals, whereas water drops in the clouds produce the smaller coloured rings, which are called coronæ. The old question, why clouds float unless their particles are hollow, is easily answered, for they do not float, and always tend to sink if they are not supported by the currents of air. In sinking into warmer air the particles are vapourized and become invisible, but others rising are condensed and take their places, so that the cloud persists, although its particles change. This is illustrated by the "cloud banners," which frequently stream from mountain peaks, and are caused by the rise of air up the mountain side. Even in a strong wind the cloud remains attached to the peak, showing that its particles are being renewed continually; but if, as is often the case, the wind descends on the leeward side of the mountain, the cloud particles disappear.
Lamarck, the celebrated naturalist, in the opening year of the present century, first proposed a classification of cloud forms. Two years later Luke Howard, a London merchant, published his epoch-making essay on The Modifications of Clouds. The theories there advanced and the nomenclature proposed have been accepted generally to our day, notwithstanding the more complete classifications devised by Poëy, Ley, and others. Howard believed that clouds are formed by the aqueous vapour which rises from the earth, and that the globules which compose them are solid, and are not filled with hydrogen gas as had been maintained by Deluc and De Saussure. Howard classified the clouds as we do to-day, according to their appearance, into three principal types, viz. stratus, cumulus, and cirrus, which represented also low, middle, and high clouds. Stratus is the sheet of low-lying cloud which forms at night, and commonly rests on the earth; cumulus is the heaped-up cloud of the day-time; and cirrus is the curl cloud of the high atmosphere. These three types were further divided into four intermediate types, viz. nimbus, cumulo-stratus, cirro-stratus, and cirro-cumulus. Howard's nomenclature was used almost exclusively, until in 1889 the International Meteorological Conference that met at Paris recommended the adoption of another classification, based on Howard's, but modified by two experts, Abercromby of England and Hildebrandsson of Sweden. This classification also disregarded the origin of clouds, and was based only on their appearance. The next year an atlas, with coloured pictures of the clouds, separated according to the new nomenclature, with descriptive text, was prepared by Dr. Hildebrandsson, assisted by Drs. Neumayer and Köppen of the Deutsche Seewarte, or German National Meteorological Observatory. This atlas was adopted by the principal meteorological institutions on the continent of Europe for their observers. The preface contained the following statement: "The study of the forms of clouds is daily increasing in importance, both from the standpoints of theory and of weather prediction. Observations taken at the bottom of the atmospheric ocean are plainly insufficient to determine its circulation. The clouds, however, furnish information about the condition and motion of the air at various levels. But, a comparison of the observations of different observers is only possible when the same ideas are connected with the same expressions. It is hardly possible to give a sufficient verbal description of such indeterminate and changeable forms as those of the clouds; graphical representations are therefore necessary, with the help of a short description, in order to enable an observer to connect what he sees in the sky with what he finds in the instructions. In order that a cloud picture may be intelligible to non-specialists, the clouds and the blue sky must, at least, be plainly distinguishable from each other."
The meeting of the directors of the meteorological institutions in different parts of the world, which was held at Munich in 1891, decided to adopt the classification of Abercromby and Hildebrandsson, and a committee was appointed to prepare an Atlas of Clouds, which should be cheaper than the preceding one. This committee, of which the writer has the honour to be the American member, met at Upsala in 1894. It defined the various forms of clouds, selected typical pictures to illustrate them, and drew up instructions for observing. This atlas, which was published in 1896, is the recognized authority on cloud forms.
Meanwhile the United States Weather Bureau had issued a plate of clouds, printed in one colour, to familiarize its observers with the new system. The Navy Department has also an interest in clouds, for several thousand seamen in various parts of the world send their special logs to the United States Hydrographic Office. The Hydrographer, a few years ago, was Captain Sigsbee, who, long before he became known to the public as commander of the ill-fated Maine, had achieved scientific reputation from his investigations upon the depths and the currents of the ocean. Captain Sigsbee desired to render comparable the observations of clouds which were being made all over the world, and to this end he resolved to publish a coloured atlas of the international cloud types which should be intelligible to seamen, and yet not too costly for his office to supply. After two years of experimenting, during which the writer and his assistant, Mr. Clayton, were frequently consulted, the Illustrative Cloud Forms, with and without descriptive text, were issued in 1897 by the Hydrographic Office, and in several respects this atlas is the best. Still, it is impossible for anything but a photograph from the cloud itself to show the extreme delicacy of certain forms. Perhaps it should be explained, however, that as the blue sky and the white clouds act with almost equal actinic effect upon the sensitized plate, in order to obtain the proper contrast between sky and cloud it is necessary either to polarize the light from the sky, or, as is most commonly done, to separate the coloured rays by allowing them to pass through a yellow screen, and to fall upon autochromatic plates.
Before defining the ten principal types of cloud it should be explained that two general classes of clouds are distinguished, separate or globular masses, which are most frequently seen in dry weather, and forms which are widely extended or completely cover the sky, which are typical of wet weather. Both these classes of clouds are found at all heights.
Cirrus are thin, fibrous, detached, and feather-like clouds formed of ice-crystals. They are the highest of all the clouds, and move with the greatest velocity.