Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/148

138 METEOROLOGY [AQUEOUS VAPOUR. narrow limits, as regards their temperature. The deflexions of the isothermals near the Baltic, Mediterranean, Black, and Caspian Seas and the freshwater lakes of America all point to the disturbing influence of these sheets of water on the temperature. The height and direction of mountain ranges is an important element in determining climate. If the ranges are perpendicular to the prevailing winds and of a con siderable height, they drain the winds of much of their moisture, thus causing to places to leeward colder winters and hotter summers, by partially removing their protecting screen of vapour, and exposing them more completely to solar and terrestrial radiation. Of this Norway and Sweden and the British Islands form excellent illustrations. It is this that makes the most important distinctions among climates in regions near each other, as respects both animal and vegetable life. With regard to the decrease of temperature with height, very much yet remains to be done before an approximation to the law of decrease can be stated. During the five months observations were made on Ben Nevis in the summer of 1881 the difference between the mean temperature at sea-level adjoining and at the top of the Ben, 4406 feet above the sea, was 15 7, which shows a mean decrease of 1 Fahr. for every 280 feet of elevation. The actual differences from day to day varied from 1 4 to 23 "2. As Ben Nevis forms a peak, and is in the very middle of the strong winds from the Atlantic, it is highly probable that this rate of decrease is a close approximation to the true decrease of the tempera ture of the air during the summer months in that part of the British Islands. When observations are made on elevated plateaus of some extent, the rate of decrease deduced from the observations will be less than the true rate in the free atmosphere in summer and greater in winter. The rate is thus a variable quantity, varying with latitude, situation, dampness or dryness of the air, calm or windy weather, and particularly with the season of the year. One degree Fahrenheit for every 300 is the rate of decrease generally assumed. Amount of Aqueous Vapour. It is scarcely possible to overestimate the importance of a knowledge of the hori zontal and vertical distribution in the atmosphere of its aqueous vapour, for it may be truly said that it forms one of the prime factors in all the larger problems of atmo spheric physics. A first rough approximation to the geo graphical distribution of the vapour of the atmosphere was published by Mohn in 1875 in his Grundziige der Meteor o- logie, p. 84, in which vapour-pressure curves are drawn fur the globe for January and July. These leave much still to be done, not only in a further discussion of observations already made, but also in improvement of the methods of observation and in the tables for their reduction. The chief point of interest in Mohn s vapour curves is their striking resemblance to the isothermals of the same months, and they also suggest that this line of inquiry is yet des tined to make large contributions to our knowledge of the unceasing changes which occur in the pressure, temperature, cloud, rain, and movements of the atmosphere. Still less is known of the vertical distribution of aqueous vapour. It decreases, like temperature, with the height, and if the statement generally made be at all correct, that half of the whole vapour of the atmosphere is contained in the lowest 6000 feet, and that at 20,000 feet high there is only about a tenth of what is at the earth s surface, the rate of decrease with height proceeds at a greatly more rapid rate than is consistent with the supposition that it forms an independent vapour atmosphere existing under its own pressure. The establishment of an increased number of high-level stations, and a more systematic inquiry than has yet been attempted into the upper currents of the atmo sphere, are much needed in the further development of this branch of meteorology. In carrying out the inquiry, invaluable assistance will be obtained from observations of the diurnal range of the barometer and from well-devised methods of observing the effects of solar radiation at the earth s surface. Amount of Cloud. In Scotland, which lies completely within the region swept by the south-westerly winds from the Atlantic, and presents a well-defined mountain range lying across the track of these winds, the clouds have a distinct annual period. In the west, at places quite open to these westerly breezes, the amounts of cloud in spring, summer, autumn, and winter are respectively 67, 69, 71, and 74, and the annual mean 70. 1 In the east, in such districts as East and Mid Lothian, which have extensive ranges of hills between them and the Atlantic, the propor tions are 59, 63, 62, and 60, and the annual mean 61. Thus about a tenth more of the sky is covered with cloud at the western as compared with the eastern situations, and the distribution of cloud differs materially in western and eastern climates. In the west winter is the cloiidiest season, but in the east it is summer, and these are respec tively the months when most rain falls in the several climates. Everywhere spring is the season when the sky is clearest. In England, owing to the protection afforded by Ireland and Wales to the west and the comparative absence of ranges of hills, the amount of cloud is less than in Scotland, and it is more equally distributed over the country. The minimum amount occurs in spring, and tho maximum in winter and autumn. Some of the best illustrations of the seasonal variation in the distribution of cloud are afforded by the Old Continent. These variations are the simple consequence of the systems of wind caused by the high winter and low summer pressures of that conti nent. In eastern Siberia the prevailing winds in winter are N.W. or continental, and in summer S.E. or oceanic; and accordingly at Ajan, Nertchinsk, and Blagoweshtchensk the mean amounts of cloud in these two seasons are 18 and 44. On the other hand, in western Siberia and eastern Europe the prevailing winds in winter are S.W. , or from lower to higher latitudes, and in summer N.W. , or from higher to lower latitudes. Kazan may be taken as fairly representing this extensive region, and there the amounts of cloud for the four seasons beginning with winter are 71, 48, 44, and 62. As the N.W. winds of summer rise over the Ural mountains in their course, condensation of the aqueous vapour is increased, and hence over this region the cloud in winter and summer is nearly the same, the mean amounts at Bogoslpvsk, Ekaterinburg, and Zlatoust being respectively 53 and 52. At Tiflis and Kutais, situated on the high ground which lies between the Black Sea and the south of the Caspian Sea, the means for winter and summer are 53 and 55. On the eastern coast of the Black Sea the westerly winds of summer are accompanied with the annual maximum cloud, the winter and summer amounts at Kedut-Kale being 59 and 69. In Central Siberia, to which the S.W. winds of winter do not extend, and to the north of latitude 55, the amount of cloud is much diminished, and the cloudiness of summer is nearly the same as that of winter. In India, in all regions which lie open to the summer monsoon, the minimum amount of cloud occurs during the winter and the maximum in summer, the mean amounts being 19 and 74 at Calcutta, 16 and 86 at Bombay, 48 and 71 at Colombo, and 25 and 90 at Rangoon. At Trincomalee, on the east coast of Ceylon, and thus exposed to the rains of the N.E. monsoon of winter, and largely protected from the rains of the S.W. monsoon of summer, the amounts of cloud in these seasons are 52 and 59. At Darjiling (6912 feet) and Chakrata (7022 feet high), both on the Himalayas, whither the summer monsoon penetrates, the mean amounts are respectively 53 and 86, and 43 and 73. At Leh, in Kashmir, the amounts are 59 and 51, the excess being thus in winter. In the Punjab and to westwards, or those regions in southern Asia to which the summer monsoon does not extend, the cloud in winter is everywhere greater than in summer. Thus the amounts are 24 and 18 at Mooltan, 38 and 25 at Peshawar, 27 and 19 at Jacobabad, and at Quetta, in Baluchistan, 5500 feet high, 42 and 14. Similar relations as to cloud obtain in Australia and the other continents where high pressures rule in the interior during 1 In this section the amount of cloud is stated in percentages of the

sky covered with cloud.