« PreviousContinue »
In fulfilling the duties growing out of his official position in connection with this Society, your Vice-President of the Geography of the Air has been so closely occupied with executive and other official duties devolving upon him as to preclude his giving that amount of time and labor to this annual report that the subject merits. Indeed, no report would be submitted this year had it not seemed better to insure a continuity of these annual addresses, even if one of them might not be up to the high standard which should be maintained for them.
It must have impressed every general reader of scientific journals that the past year has been marked by the publication of an unusual number of controversial articles relating entirely or in part to meteorology. Some of the discussions of this subject appear to be in the nature of speculation, which, by good authority, is defined to be “chiefly the work of the imagination, and has little to do with realities.” The status of the meteorological discussion which has been going on for some time seems to be this: A number of men, applying themselves to investigation in separate branches or stages of the same science, are attempting to reconcile their views, which, based as they are upon entirely different processes of investigation, are not entirely accordant. Some, at least, of these writers are still apparently groping in the preliminary, the "natural history” stage of the 7-NAT. GEOG. MAG., VOL. III, 1891.
science of meteorology, while one alone stands as the exponent of the “natural philosophy" of meteorology.
To me it seems that it could not have failed to impress any interested reader who has followed the late publications on the convectional theory that, in order to clear the ground for definite meteorological discussion, it is necessary to determine the exact meaning of the various technical terms employed by the various writers. Whether from looseness of verbiage originally or from the not infrequent habit of disputants when worsted to change their ground by claiming to be misunderstood, we find that some writers are unwilling either to stand by their first criticisms or to openly abandon them; they prefer to explain away their defective statements and gradually shift around to positions almost diametrically opposed to those originally assumed.
The generally accepted theory as to cyclones attributes their initiatory formation to an unequal distribution of temperature with resulting mean diminution of pressure, and the movement of the air from places of high to places of low pressure, the lower air ascending with a gyratory motion, while air particles moving from opposite directions form couples which produce rotation. When energetic motions raise the ascending air to such a height that the temperature, cooled dynamically in ascending, goes below the dew-point, then the great store of latent heat thereby set free becomes, it is assumed, the main source of energy in maintaining the upward convectional movement. The subsidiary causes are attributed to the diminution of pressure on the collapse of the vapor, and also to the direct absorption of the sun's heat at the upper cloud surface.
In anticyclones a slow gyratory descending motion of the air is assumed. Ferrel considers the cyclone and anticyclone one system, and believes that air flowing into the cyclone from a “high ” at the ground passes out into the higher atmospheric strata.
Dr. Hann has put forth the hypothesis that the genesis of cyclones and anticyclones may be sought in the general atmospheric circulation through a difference of temperature of the air from the equator to the poles. He speaks of a congestion in the upper or anti-trade winds, where the air heaps up to a great height, this being the cause of the anticyclones; and he maintains that the low temperature of the “high” is due to ground radiation, and that no part of the high pressure is the result of low temperature.
To this hypothesis of Dr. Hann, ascribing the genesis of storms to the general circulation of the atmosphere, no application of the laws of dynamics has yet been made with a view of developing it into an acceptable " theory.” If it should be established it does not follow that it will in any way affect the truth of the commonly accepted “convectional system,” which, founded as it is on the well-known laws of thermo-dynamics, is not likely to be successfully assailed. There may be an improved nomenclature for the laws of statics and dynamics that will express to the mind more clearly the relation of cause and effect; but until the advance of scientific research modifies the present formulation of these laws the convectional theory will be generally accepted as giving the true interpretation of all the phenomena to which it could be applied.
Professor Russell, in commenting on this subject, expresses the opinion that the low temperature is due to the convective interchange of air at a low temperature in the upper strata with air of a high temperature in lower strata, such convective interchange tending to make the whole body of air of a temperature coinciding throughout with the adiabatic rate of upward diminution, with the consequent result of rendering the air at the surface of the earth cooler than previously and the upper air warmer. When the upward diminution of temperature is less than the adiabatic rate, in the forced circulation of air crossing a mountain ridge, there occurs the dynamie heating which is observed in the case of the foehn winds. The low temperature near the earth he does not believe could ever be entirely produced by nocturnal radiation from the ground. The high pressure, in his opinion, is largely the result of greater density due to low tenperature, as is very clearly indicated by the fact that the temperature is almost inversely proportional to the pressure, and that the places of lower temperature substantially coincide with the places of greatest pressure.
In advancing hypotheses and inviting discussion the real object is, or at least should be, to discover the essential cause or causes which determine the initial formation and subsequent maintenance and progress of the cyclone. Some real progress in charting lines of equal density seems to have been made by M. Nils Ekholm following Professor Abbe's system of " isostaths," one using the term density, the other buoyancy. Professor Abbe also introduces the factor of the orographic gradient, but the
latter is simply the measure of a resistance. The objection to this form of determination is this, that it is a measure of mass only. The density of two masses of air is determined to be the same; but as the density may result from two entirely different causes, their physical relations cannot be fully expressed in units of gravity. The methods of Professor Abbe and of M. Nils Ekholm undoubtedly give good results. partly from the coincidence that humidity usually varies directly as the temperature.
The method proposed by Captain James Allen in 1888, which is briefly described in appendix 24 to the annual report of the Chief Signal Officer for 1890, appears to afford the means of more clearly expressing the relations that exist between the mass of the atmosphere and the forces available for the generation and movement of storms. Its tentative application at the Signal Office has anticipated and explained storm movements not indicated or accounted for by the usual methods.
As pertinent to this matter, there is instanced a study of the progress of thunder-storms made by Berg, who observes that the line of storm front in every case investigated made a decidedly conspicuous bend into the densest part of the lines representing the absolute humidity.
Scientific conditions have so changed that in these later years it becomes more and more difficult for investigators to publish any work which
may be characterized as magnum opus. Under this head, however, must be classed Buchan's important memoir on the distribution of atmospheric pressure, temperature, and wind direction over the whole world; a large quarto volume, which contains much new material. It has been incorporated with the results of observations during the Challenger expedition, in which series this work appears. The isobars and isotherms for each month in the year for the whole earth are charted on Mercator's projection, and for the northern hemisphere on a chart constructed on a polar projection. In connection with an abstruse subject, to which Buchan has paid so much attention, the diurnal variation of pressure, he opines from the Challenger observations that the oscillations are due to the heat taken from the solar rays directly in passing through the air and instantaneously communicated through the whole mass from top to bottom by heating and evaporation of water on innumerable dust particles. The afternoon minimum, he thinks, is caused by upward currents removing a portion of the lower air. Marked