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.
Fig. 1.—Nephoscope at Blue Hill Observatory.
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 Hagström 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.
Fig. 2.—Cloud Theodolite at Blue Hill Observatory.
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 formulæ 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.