« EelmineJätka »
tion of the ordinary clouds of the atmosphere; but however beautiful the analogy between the appearances may be, yet the physical explanations are not likely to be so similar.
In the annual reports for 1876 and 1877 of the Chief of Engineers, Mr. O. B. Wheeler gives a valuable reduction of all the observations of changes in level of the great lakes.
Dr. E. Purkyne contributes a very interesting paper on the rainfall at Santa Cruz. The author gives some account of the early condition of the forests, and of the droughts to which the island is subject. There is no evidence of any great change of climate or of the dependence of rainfall upon forests, but rather upon the presence of hills and the position of the water and land relative to the winds.
Dr. Hellmann has carefully studied the summer rainfall in Germany, using ten-day means for 22 years. He finds a double maximum in frequency and quantity - the first in June, the second in August. The cold period of June ends the first rain period, and is due to the irruption of cold air from the northwest.
Dr. Von Bebber (Munich, 1877) has collected a large quantity of data relative to the rainfall of Germany. He finds the average in all Germany to be 71 centimeters. The influence of altitude is to increase the rainfall upon such mountain-tops (up to 1200 meters) as were available. A similar distribution prevails in America on Mt. Washington, and in India on the windward side of the mountain-ranges, all which, by causing the winds to push the air upward, determine the resulting condensation.
Rubensen gives (K. Svensk Vet. p. 13, 1876) the geographical distribution of rain in Sweden. The rainfall is heaviest (700-800 mm.) in Southwest Sweden, and least on the east side of the Scandinavian Mountains.
As to the manner in which raindrops and hailstones are formed, Professor Osborne Reynolds maintains that there are in a cloud large and small particles of water and ice, and that of these the larger ones have demonstrably the greater velocity of fall; they will, therefore, overtake the smaller ones and add them to their own mass. These larger particles are most numerous at the upper surface of a cloud, where the cooling due to radiation takes place most rapidly. Beyond a certain limit raindrops cannot grow, as they will break up
as they rush through the air. The structure of hailstones, especially conical ones, shows that they have been formed by accretions on the lower side or base.
Professor Fritz, of Zurich, has published a characteristically thorough memoir on the geographical distribution of hail.
The classification of hailstones according to their external characteristics has been attempted in considerable detail by Prestel.
The formation of hailstones is considerably elucidated in a short article by Flögel, of Bramstadt, who, in some remarks upon a memoir by Reynolds, explains that the observations made by himself, and in 1791 by Wilke, and in 1844 by Schumacher, all point to the conclusion that a crystal of snow or ice, having once been formed at a considerable altitude, and descending rapidly, grows in size only by additions to its lower side; if, therefore, its original shape allows of it, it will keep the same end always uppermost, and will grow into a conical mass of ice, which will on its exterior be marked by ridges or striæ corresponding to the angles of the original crystal. In this connection we call attention to a fall of remarkably well-developed conical hailstones that is described in the Weather Review for April, of the Army Signal Service.
The hailstorm of April 4, 1877, is described by Godefroy in the Comptes Rendus, with numerous illustrations. Conical stones similar to those above described fell abundantly. The question as to whether hailstones are to be considered as built up from the sphere or the cone as the nucleus is settled by K. Fritsch, whose great experience entitles him to say that both are equally common.
The result of all the recent investigations into the diurnal change of temperature and moisture with altitude is thus summed up by Rubensen, of Upsala: The air at the earth's surface is, by the fall of its temperature, quickly brought to the point of saturation. From this instant on, a deposit of dew and a diminishing absolute humidity closely follows every lowering of the temperature. This diminution appears soon to reach a constant maximum, at which it probably remains for some time. Meanwhile, either through diffusion or by the descending current of air, new aqueous vapor is conducted from the upper strata of air downward to the earth. Therefore every stratum communicates to the next lower a certain quantity of vapor and receives a new share from above; but the new share is not fully equivalent to the loss, as is seen from the fact that an incessant diminution takes place in all the strata, although the precipitation occurs only at the earth's surface itself. The diminution of the humidity which corresponds to the difference between the quantity of moisture in the descending and ascending air strata begins subsequently in the upper air strata, which is a natural consequence of the fact that the diminution has its special cause at the surface of the earth. For the same reason it is also less the greater the altitude is, provided that the comparison between the different air strata is made at the same hour. Moreover, the diminution of humidity tends towards a limit or maximum value which, as the observations seem to show, is greatest at the earth's surface, where it occurs at the time of greatest diminution of temperature, and diminishes as we ascend.
STORMS. Among the general treatises on meteorology lately published, we notice Scott's “Weather Charts and Storm Warnings,” and especially Rosser's“ Law of Storms,” which latter is an impartial summary of the views of prominent meteorologists as to the rotation of winds about storm-centres.
Professor Loomis's seventh contribution to meteorology treats of rain areas, and shows that they have an elliptical form: they exist under the influence of (or within the limits of) areas of high pressure as well as of low; that rain is not a necessary attendant of an area of low pressure; that the depression at a storm-centre is, as first demonstrated by Ferrel, and now widely accepted, a result of the centrifugal force due to the wind. The heat liberated by condensation into rain does, however, exert a decided influence
the development and progress of low areas. The eighth paper by the same author was read before the National Academy in October, but was not printed until after the 31st of December
During a portion of December, 1876, and January, 1877, both Great Britain and the eastern portion of the United States were visited by a succession of storms, in which high winds, heavy rains or snows, and very low barometric pressures were remarkably frequent. In the United States the tracks of the storm-centres, or areas of lowest pressure, as they moved eastward covered a region apparently far to the south of that which they ordinarily occupy, while their progress was generally very rapid. In Great Britain, on the other hand, the progress of storm-centres was unusually slow, frequently even stationary or retrograde, while the general path of the storm-centres was, as in America, far to the south of its usual position. In British America, on the other hand, and in Russia, low temperatures and high barometers were experienced. The minimum temperature recorded at St. Petersburg was on December 22, –43.4° Fahr., being the lowest observed during the last 124 years. Farther eastward, namely, in Siberia, an unusual prevalence of warm weather was reported; and in the extreme west, on the Pacific coast of North America, unusually little rain and high temperatures prevailed. In fact, a general review of the movements of the atmosphere during these two months shows that there was an excess of cold dry air in northern latitudes and in the interiors of both continents, while over the Atlantic Ocean pressure was low and temperature and moisture were high. Both these conditions, therefore, caused a special development of the tendency to a cyclonic motion around the Atlantic basin.
These oceanic cyclones, as distinguished from smaller storms, are central over the North Atlantic and Pacific oceans and over the Antarctic continent, and must, according to the author's present knowledge of meteorology, vary in their intensity with any change in the solar radiation; the phenomena of the past winter harmonize entirely with the conclusion that during the present period of few sunspots the northern hemisphere has received slightly less heat than when the spots were large and numerous. A similar agreement between meteorological phenomena and this theory was noted by us about two years ago, but the satisfactory pursuit of these investigations can hardly be undertaken until we have a daily weather map of the whole world, or at least of the northern hemisphere. Dr. Blasius, of Philadelphia, has contributed to the Vienna
Zeitschrift a review of the principal points of his work on Storms, their Nature and Classification.”
A welcome contribution to the theory of cyclones is given by Guldberg and Mohn in the third of their papers in the Vienna Zeitschrift elucidating the results given in their "Études." Assuming that descending and ascending currents of air exist, they develop the consequences, and give some of the laws controlling cyclonic and anticyclonic movements. In the interior of a cyclone the path of a particle of air is a logarithmic spiral with a deviation from the radius vector greater than that normal to the latitude and gradient. The formula connecting velocity and gradient as given by them agrees closely with observation.
The lamented J. Elliott, whose death occurred in February, 1877, had completed, shortly before his decease, a memoir on the Backergunde and Vizagapatam hurricanes of October, 1876, which is a model of thoroughness, and to be classed with the admirable monographs of Blanford and Wilson. Elliott inclines to the opinion that an extended region of calm bounded by opposing winds (trades and monsoons) preceded the initiation of these whirlwinds, and is the principal determining feature in all the Indian cyclones.
The report of 1877 of the London Meteorological Office states that a very large number of logs of vessels have been collected relative to the great hurricane of August, 1873, which passed near the coast of Nova Scotia. Daily weather charts and isobars for the whole month have been compiled for the North Atlantic Ocean, and the whole investigation, which will soon be published, is probably the most thorough that has yet been bestowed upon any Atlantic hurricane. It is said to be clearly shown that this hurricane did not reach Great Britain, as was suggested by the present writer in his brief preliminary report to the Chief Signal Officer in September, 1873.
A preliminary communication to the Royal Service Institution by Captain Toynbee, with its invaluable charts, shows that the burricane was to a great extent broken up on the south coast of Newfoundland, and amounted to only a storm when it reached Norway.
Wijkander concludes with reference to the storms of the North Sea near Spitzbergen that they pass either on the west