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 AbbÈ 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 coronÊ. 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 PoÎy, 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 Kˆppen 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.

=Cirro-stratus= form a thin whitish veil, more or less fibrous, which often produces halos around the sun and moon and other optical phenomena.

=Cirro-cumulus= are flocks of small detached fleecy clouds, generally white and without shadows.

=Alto-stratus= is a grey or bluish veil through which the sun and moon are faintly visible, occasionally giving rise to coronÊ. Its altitude is only about half that of Cirro-stratus.

=Alto-cumulus= are flocks of larger, more or less rounded, white or partially shaded masses, often touching one another, and frequently arranged in lines in one or more directions.

=Strato-cumulus= are large globular masses or rolls of dark cloud, frequently covering the whole sky, especially in winter.

=Cumulus= are piled clouds with conical or hemispherical tops and flat bases. They are formed by rising currents of heated air, and are therefore most common in summer and in tropical regions. When broken up by strong winds the detached portions are called Fracto-cumulus.

=Cumulo-nimbus= is the massive thunder shower cloud rising in the form of mountains or turrets, and generally having above a screen of fibrous appearance (False Cirrus), and underneath a mass of cloud similar to Nimbus from which rain falls.

=Nimbus= is a dense, dark sheet of ragged cloud from which continued rain or snow generally falls. Broken clouds underneath, forming the scud of the sailors, are called Fracto-nimbus.

=Stratus= is a thin uniform layer of cloud at a very low level. When the sheet is broken up into irregular shreds it is called Fracto-stratus.

Having described the origin and appearance of the different clouds, an account will now be given of the measurements made at Blue Hill Observatory and the information which they give about the circulation of the atmosphere. The work there was taken up in 1887 in consequence of the interest of the meteorologist, Mr. Clayton, in the study of clouds; his discussion of the cloud observations, published two years ago with the Blue Hill observations, has been termed by far the most thorough study of the kind ever undertaken in America if not in the world. Most of the conclusions which are stated popularly here have their scientific expression in his work.

The first investigation related to the amount of cloud at different hours of the day, and during the various seasons. It is customary to note the degree of cloudiness on a scale of from 0, when there are no clouds, to 10, when the whole sky is covered. For twelve years the amount of cloud at each hour of the day has been recorded at Blue Hill. The personal observations have been supplemented during the day-time by an automatic instrument called a sunshine-recorder, for it has been proved that the cloudiness is very nearly the inverse of the bright sunshine. Consequently, if, as is usual there, the sun shines forty-six per cent. of the time when it is above the horizon, the cloudiness is very nearly fifty-four per cent., which is the average for the year. The instrument generally used for this purpose is a glass sphere which acts as a burning-glass, and chars a strip of cardboard placed concentrically around the lower part of the sphere. As the sun moves, the image on the card moves in the opposite direction over the card, burning a line as long as it shines, but leaving the card untouched when it is cloudy. In a similar way a record may be obtained on sensitized "blue paper" by allowing the sun's rays to enter a dark chamber containing the paper. The maintenance of personal observations at each hour of the night is arduous, and, therefore, during ten years an automatic instrument has been used at Blue Hill which deserves to be better known. It is called the pole-star recorder, and was devised by Professor Pickering, director of the Harvard College Observatory. The instrument is very simple, and consists of a telescopic camera focussed on Polaris. This star is not at the north pole of the heavens but a little more than a degree distant, and, consequently, it describes a small circle in the heavens during twenty-four hours. When the sky is clear around Polaris its trail upon the photographic plate is continuous, but when the sky is partly or entirely covered with clouds the trail is broken or obscured. Of course the plate is not exposed until after dark, and a shutter is closed by a clock before dawn. The only hourly records of cloudiness at night in the United States are obtained by this instrument on Blue Hill and at Cambridge. It will be objected, perhaps, that the cloudiness derived from observations of the sun or the pole-star is not the amount over the whole sky, but only that in the region of the luminary. This is true, but it is found that the average of the records for a month or a year agrees very closely with the average of estimates of cloudiness over the whole sky during these periods. The use of the pole-star is preferable to that of the sun, because in our latitude it gives values at a point about half-way between the horizon and the zenith; while since the sun travels at a variable height across the sky, when its altitude is low the same mass of cloud may intercept more sunlight than when it shines vertically. From ten years' observations the following deductions have been made concerning the variation in the amount of cloud at Blue Hill. For all the months the diurnal amount of cloud is greatest about one o'clock in the afternoon, on account of the frequency of cumulus clouds near the warmest part of the day, while the next greatest amount, due to the frequency of stratus clouds, occurs near sunrise, or at the coldest time of day. All over the world the least cloudiness is in the evening, when the sum of the combined effects of radiation and insolation is least. The annual period in the cloudiness is complex, because the amount of cloud is connected with changes of humidity at many different levels in the atmosphere, but in the northern hemisphere there is most cloud during the first half of the year and least during the latter half, probably because the increasing warmth at the earth's surface produces increased ascending currents until summer, while the chilling of the earth's surface in the autumn becomes unfavourable for ascending currents. The distribution of cloud over the globe is intimately connected with the general atmospheric circulation, being greater where there are rising currents and less where there are downward currents. The reason, naturally, is that as descending air becomes warmer and therefore relatively drier, the clouds in it evaporate and disappear. A cloudy belt encircles the earth at the equator, and on either side are two belts of less cloud, but in higher latitudes the cloudiness increases. If we could see our earth from outside its atmosphere, the light reflected from the upper surfaces of the cloud-belts would probably make them appear bright. From the markings on a planet that are known to be caused by condensation, a French meteorologist, M. Teisserenc de Bort, believes that the circulation of its atmosphere can be inferred, for wherever on the surface of the planet bright spots are seen, there the vapour of rising currents should be condensed. If this be true, there is a resemblance between Jupiter, as we see it, and the earth as it would appear from another planet, the bright bands being cloud surfaces, and the dark patches glimpses of the surface of the planet beneath.

Observations of the direction of motion, and apparent velocity of clouds at different heights, have been made at Blue Hill several times a day since 1886. To measure the motion of clouds the nephoscope (Fig. 1) is used. It consists of a horizontal circular mirror with a concentric circle of azimuths and an eye-piece _C_, movable in a plane _BD_ at right angles to the mirror and also around it, through which the image of the cloud is brought to the centre of the mirror _A_. It can be proved by geometry that the motion of the cloud-image is proportional to the movement of the cloud itself, so by noting in what direction and how far the image is displaced in a given time, we have the true direction of motion of the cloud itself and also its relative velocity, comparable with the velocity of all clouds having the same height. If the height is known, then the relative velocity can be easily converted into absolute velocity, and thus the velocity of currents at different heights in the atmosphere is accurately ascertained.

The height of clouds seems to have been measured trigonometrically from two stations as early as 1644 by Riccioli and Grimaldi, two Jesuits of Bologna, but notwithstanding these measurements and some conclusions derived from observations on mountains, and in balloons, the altitudes of the different clouds were not known with any accuracy until in 1884 Ekholm and Hagstrˆm made a series of trigonometrical measurements upon the different kinds of clouds at Upsala, Sweden. About the same time attempts were made at Kew Observatory to measure clouds by photography, and in 1885 probably the first trigonometrical measurements in America were made at Cambridge, Mass., by Professor W. M. Davis and Mr. A. McAdie. In 1890-91 the Swedish methods were employed at Blue Hill by Messrs. Clayton and Fergusson of the Observatory staff, and until recently the measurements there and at Upsala comprised all that was known accurately about the heights and velocities of the various species of clouds.

The trigonometrical measurements at Blue Hill were made as follows: at two stations, one at the Observatory, the other at the base of the hill about a mile distant, two observers determined simultaneously the angular altitude and azimuth of some point on the cloud which was agreed upon by telephonic conversation. If, as is generally the case, the lines of sight did not meet, the trigonometrical formulÊ gave the height of a point midway between the crossing of these lines. Such was the accuracy of these measurements that the probable error of the calculated heights of the highest clouds is only a few hundred feet. Successive observations at the two stations of the position of the cloud enabled its velocity to be calculated, or, as already explained, this may be got from the relative velocity measured at one station, if the height of the cloud be known. Fig. 2 shows the theodolite on the tower of the Observatory. Five other methods of measuring clouds have been employed at Blue Hill: (1) The only method of finding the height of lofty and uniform cloud strata is by means of the light thrown on them from below, and on Blue Hill the electrical illumination of the surrounding towns is utilized. The angle which the centre of the illumination makes with the horizon is measured, and knowing the distance of the town, the right-angled triangle may be solved. (2) An accurate method for low and uniform clouds is to send kites into them, as will be explained in the closing chapter. (3) When the clouds are low enough to cast shadows on the ground, the angles of the cloud and sun as seen from the Observatory are measured, and with the distance of the shadow from the hill-top, ascertained by a map, this triangle can be solved. The times of passage of the shadow over known points on the landscape afford another means of calculating its velocity. (4) A method that was suggested by Espy, the pioneer American meteorologist, for getting the altitude of the bases of clouds lying within a mile of the earth, is to find the difference in temperature between the air and the dew-point at the ground, and to compute the height at which this difference should disappear. When the temperature of the rising currents increases, as on warm days, and the level of the dew-point rises higher, the cloud can be seen to ascend, and, in fact, the measurements at Blue Hill show that the clouds of moderate altitude are highest during the warmest part of the day. (5) Finally, very low stratus or nimbus may be measured by noting the heights of their bases on the sides of the hill.

The identity of cloud-forms all over the world has been established, and as a result of the measurements at Blue Hill, the heights and speed of all clouds observed there are known. The averages have been plotted in the five levels into which we separate the clouds in Plate IV., Heights and Velocities of Clouds, where ordinates represent heights and abscissÊ velocities, and, consequently, the distances of the various forms of clouds above the horizontal base indicate their heights, and the distances from the left-hand vertical line their velocities. For comparison, the velocity of the wind on Blue Hill, a few hundred feet above the general level of the country, is represented. The mean height of the cirrus is about 29,000 feet, but this cloud sometimes reaches 49,000 feet. The mean height of the cumulus is about a mile, but the tops of the cumulo-nimbus, or thunder-shower cloud, sometimes penetrate into the cirrus level. Generally the base of the nimbus, or rain cloud, is only 2300 feet above the ground, and it frequently sinks below the top of Blue Hill, which is only 630 feet above the sea. The poetic saying, that "Earth wraps her garment closer about her in winter," has a scientific basis, for the average height of all the clouds is greatest in summer and least in winter. But the reverse is true of their velocity, for the entire atmosphere moves twice as fast in winter as it does in summer, and at the lower levels the seasonal change is even greater. The average velocity of cirriform clouds is ninety miles an hour in winter, and sixty miles in summer, but occasionally in winter cirrus have been found to have the enormous velocity of two hundred and thirty miles an hour. In the average, the velocity of the currents increases, from the lowest to the highest clouds, at the rate of about three miles an hour for each 1000 feet of height, but near the ground the increase with height is faster. It has been found that the velocity of the lower clouds is less than the velocity of the wind on a mountain of the same height, which may, perhaps, be explained on the supposition that the mountain acts like a dam to accelerate the flow of air over it. The measurements in Sweden showed that the middle and upper levels of clouds are higher than in America, but that they move less rapidly. This may be because the surfaces of equal temperature in the air are higher in the United States than in Sweden, on account of the direction of the upper currents, while the greater velocity of our high clouds corresponds with the more rapid movement of areas of low and high barometric pressure over the United States.

These results are suggestive. For instance, the energy of the upper half of the mass of the atmosphere, or that portion which lies above 18,000 feet, has been calculated to possess six times the energy of the lower half in which we live, and as yet, none of this enormous store of energy is applied to the use of man. While it appears certain that no navigable balloon or flying machine will ever be able to stem the enormous velocity of the upper atmosphere, rarified though it is, perhaps in the future aÎrial machines will take advantage of the prevailing currents of the high atmosphere, as our sailing ships do of the trade winds. The observations of cirrus clouds in various parts of the world show that they always move from a general westerly direction, while below this primary drift toward the east occur the relatively permanent or transient differences of pressure which cause the deviations from the normal circulation of the atmosphere, and give rise to the local circulation in storms. In the familiar daily weather map it will be noticed that there is usually some portion marked "low," and another portion marked "high." The former is an area of low barometric pressure, into which the winds at the ground blow spirally inward in the opposite direction that the clock hands turn; the latter is an area of high barometer, out of which the winds at the ground blow in the contrary way. The former when well developed are called "cyclones," and are usually accompanied by stormy weather, and the latter, called "anti-cyclones," bring fair weather. From the observations of the directions from which the clouds move in cyclones and anti-cyclones, we have found that above the cumulus level (at the height of about a mile) the inward inclination of the wind in a cyclone, and the outward inclination in an anti-cyclone, both disappear, and the general drift from the west prevails. The results of the observations are shown in Plate V., Atmospheric Circulation in Cyclones and Anti-cyclones, representing sections of the atmosphere, concentric to the earth's surface, in the five cloud-levels seen from above. The arrows fly with the wind and are proportional in length to its velocity, the dotted arrows indicating the probable flow of the air through the cyclones and anti-cyclones that are indicated by the circles, their axes being assumed to be nearly perpendicular to the earth's surface. Above the cumulus it will be observed that the wind in the cyclone tends to come from the south-west in front and from the north-west in the rear, while in the anti-cyclone the contrary is the case, indicating a deflection of the westerly upper current to the right in cyclones and to the left in anti-cyclones. This sustains the theory that the cyclonic circulation is struggling against a general atmospheric drift from the west which increases with altitude, and above the height of a mile becomes greater than the cyclonic influence. Higher than this, the atmospheric circulation is controlled primarily by the permanent temperature gradient between equator and pole, by the seasonal temperature gradient between ocean and continent, and, in the United States, by the passage of "warm and cold waves." Mr. Clayton's investigations indicate that the motion of the upper clouds is nearly parallel to the lines of equal temperature at the earth's surface. A high temperature, by expanding the air upward, causes in the upper air a high pressure; and a low temperature, by contracting the air towards the ground, causes in the upper air a low pressure, so that the lines of equal pressure in the upper air are parallel to the lower lines of equal temperature, and since there is little friction in the upper air the motion of the wind is nearly parallel to the lines of equal pressure. Below the cumulus level the winds follow the normal cyclonic and anti-cyclonic circulation. There are two theories of the origin of these areas of high and low pressure, the "driven theory" which supposes that they derive their energy and drift from the general atmospheric movement from west to east, and the "convectional theory" which attributes their formation and progression to the difference of temperature between them and the adjacent air. While the observations on mountains have favoured the driven theory, yet the inward spiral motion of the cirrus clouds above the anti-cyclone, indicating a lower pressure than in the surrounding air, contradicts the hypothesis, and the recent observations with kites at Blue Hill strongly support the convectional theory of cyclones.

The relation of the clouds to weather forecasting has been investigated at Blue Hill. For instance, it is found, in this region at least, and contrary to the general opinion, that cirrus clouds do not indicate rain, but do foretell a change of temperature that is proportional to the rapidity of motion of the clouds. Alto-cumulus is followed by rain within twenty-four hours three times in four. Rain follows the appearance of all high and intermediate clouds most frequently when the cloud banks are densest toward some westerly point and when they come from the west. Mr. Sweetland, an assistant, has studied special forms of cloud in their relation to the succeeding weather. He concludes that cirrus plumes precede fair weather, while dense clots of cirro-cumulus are followed by rain. Rounded pendants, or mammillated clouds, in the lower levels indicate rain, but in the upper levels fair weather. Of all the forms, the dark sheet of stratus, and clouds of lenticular shape, are most frequently followed by rain. Of clouds presaging changes in temperature, the turreted cumulus, which is connected with thunder-storms, precedes the greatest fall in temperature, and next in order come lenticular clouds, flaky cirrus, and alto-cumulus. In general, flat and flaky clouds, clouds forming and disappearing rapidly, and clouds changing to forms at a higher level precede dry and cooler weather.

It will be seen that this modern study of clouds as prognostics simply adds to the weather proverbs that have come down to us from the time of Theophrastus. It does not appear, however, that cloud forms alone can usually serve to predict rain for more than twenty-four hours, but for a few hours in advance the appearance of certain cloud forms frequently furnishes the observer more trustworthy signs of coming rain than does the synoptic weather map. To a forecaster in possession of telegraphic data, the prevalence of rapidly-moving cirrus over a wide area indicates a rapid storm movement, with sudden and marked changes of weather and of temperature, while slowly-moving cirrus indicate slight changes of temperature and dry weather. The direction of the cirrus movements in front and around a storm centre will usually point out the future movement of the storm, which tends to advance in the same general direction.

The work done at Blue Hill shows the importance of cloud observations to elucidate the general movements of the atmosphere, as well as the circulation of the air above barometric maxima and minima, which can result practically in making accurate weather forecasts possible a day or two in advance. The systematic observation of the upper currents was brought to the attention of the International Meteorological Committee by Dr. Hildebrandsson in 1885, and at the meeting of the International Cloud Committee in 1894, besides the adoption of the nomenclature of clouds and instructions for observing them, it was decided that observations of their motion, as well as measurements of their height, should be made in various parts of the world. Accordingly, the year commencing May 1, 1896, was designated as the "International Cloud-Year," and observations with nephoscopes of the direction of motion and relative velocity of clouds were begun at many stations in Europe and Asia, and at fifteen stations in the United States. Trigonometrical measures of the heights of clouds were undertaken at stations in Norway and Sweden, Russia, Finland, Prussia, and France, as well as at Toronto, Manila, and Batavia; in the United States the measurements already described were recommenced at Blue Hill, and the Weather Bureau equipped a similar station in Washington. In Europe it is thought that the determination of heights by photogrammeters, as the theodolites with attached photographic cameras are called, possesses advantages over the visual theodolites, and it is true that not only is the kind of cloud recorded on the plates, but there are available for calculation as many points on the cloud as can be identified on the two plates exposed simultaneously at both stations. On the other hand, in the case of nearly uniform or dark cloud-strata, it is easier to see points for measurement on the cloud than to fix them on the photographic plates. For this reason, and from the difficulty of manipulating the photogrammeter, visual instruments were adopted both at Blue Hill and at Washington. The work was successfully carried on until May 1, 1897, and the observations and measurements were reduced at Blue Hill according to the plan prescribed by the Committee. Already the observations and measurements made at Upsala, Manila, and Blue Hill are published, and the others will follow. The discussion of the correlated data from the various countries will probably increase our knowledge of the circulation of the atmosphere, which is certainly one of the most interesting and important questions in the physics of the globe. The result will have been reached by international co-operation, of which the benefits to science are everywhere manifest to-day. But for the whole problem to be solved, it is necessary, not only to know the movement of the air, but, as far as possible, to ascertain its conditions of heat and moisture. This may be accomplished by the use of balloons and kites, to be described in the remaining chapters.