The New International Encyclopædia/Meteorology
ME′TEOROL′OGY (Gk. μετεωρολογία, meteōrologia, treatise on celestial phenomena, from μετεωρολόγος, meteōrologos, discussing celestial phenomena, from μετέωρον, meteōron, meteor + λέγειν, legein, to say). The study of the atmosphere and its phenomena. Efforts are being made by every civilized nation to apply to the benefit of mankind the knowledge we possess of meteorology, especially to foretell the winds and weather from day to day and the general character of the seasons from season to season. About fifty official governmental weather bureaus receive reports from their stations by telegraph daily, compile weather maps, issue forecasts, and publish weekly, monthly, or annual climatological summaries, together with frequent special meteorological memoirs. Among the most prominent of these, on account of the extent of their territory and the value of their publications, are those of Austria-Hungary, Great Britain, France, Germany, Italy, Russia, India, Argentine Republic, Canada, and the United States. The total annual expenditure by all Government services on meteorological work is not less than three million dollars, to which should be added an equal sum to represent the great amount of work that is done without pay by voluntary observers. Several private meteorological establishments are maintained by wealthy lovers of science, most prominent among which are those of Vallot, on Mont Blanc; A. Lawrence Rotch at Blue Hill, near Boston; L. Teisserenc de Bort at Trappes, near Paris. There are also numerous municipal observatories, prominent among which are that of the New York City Central Park. Dr. Daniel Draper, director, and those of Montsouris and the Tour Saint Jacques in Paris, of which Dr. J. Joubert is director. Observatories are also maintained by special associations, such as those on the Santis, Austria, the Jesuit observatories of Saint Hélier, Havana, Zikawei, Manila, and the one recently destroyed at Antananarivo, in Madagascar. Special mention should be made of Symons's British Rainfall System, to the development of which his life was devoted and the perpetuity of which is now assured by the terms of his will. Over three thousand stations are maintained in the British Isles. Organized systems of rainfall stations have also been maintained in Mauritius, Jamaica, Barbados, Antigua, and Saint Kitts.
In addition to its material progress in observers and apparatus, theoretical meteorology has especially profited by the labors of eminent physicists. Those who have, since 1850, contributed most to our knowledge of the mechanics and physics of the atmosphere may be enumerated as follows: Adolph Erman, who published in 1868 a memoir on the distribution of winds and pressure over the globe; J. C. Redfield, who showed the mechanism of extended hurricanes; James P. Espy, who published several reports and a volume on the philosophy of storms, explaining in general how atmospheric moisture, by its condensation into cloud and rain, disturbs the equilibrium of the atmosphere and produces both local and general storms; William Ferrel, who published numerous papers developing the laws of the motions of the earth's atmosphere and its general and local phenomena as resulting from the rotation of the earth on its axis, the evaporation and condensation of aquaeous vapor, and the general influence of the solar heat; Lord Kelvin, who first gave the laws of thermal convective equilibrium for dry air; Peslin, who gave the laws of thermal equilibrium for moist air; Von Helmholtz, Willy Wien, Oberbeck, Guldberg and Mohn, Margules, Diro Kitao, Rayleigh, Pockels, Sprung, and F. H. Bigelow have made important contributions to the hydrodynamic problems of the atmosphere; Prof. H. Hertz, W. von Bezold, and Marcel Brillouin have contributed greatly to the perfection of our knowledge of the thermodynamic problems. The most recent contributions in this field include that of Pockels, on the Theory of the Formation of Rain in slowly ascending currents of moist air (see Wiedemann's Annalen, January, 1901); Prof. F. H. Bigelow's tables in his reports on International Cloud Work (Washington, 1900); his report on Barometry (Washington, 1902); Neuhoff's memoir on Adiabatic Changes in the Atmosphere (Berlin, 1900); Berson and Assmann's Scientific Balloon Ascensions, 3 vols., quarto (Berlin, 1900); all which respectively contain highly important investigations.
Our knowledge of meteorological conditions has been obtained for the most part by observation of the clouds or by stations on mountain tops. More recently it has been found desirable to study conditions at considerable altitudes above stations and places. In order to obtain better data for the lower atmosphere, at least partially to meet the needs of the case, Americans have developed the art of obtaining meteorographic records by sending up meteorographs on kites to heights of one or two miles; on the other hand, Europeans have given attention to the development of the balloon and especially the small sounding balloon which can carry a meteorograph to an elevation of six or eight miles above sea level, where man cannot live. The exposure of meteorological apparatus so that the records from different stations on the earth's surface and from vessels on the ocean and from kites or balloons in the atmosphere shall be comparable with each other offers many difficult problems, but the progress toward uniformity throughout the world has been appreciable during the past twenty-five years. Every first-class weather service now keeps close watch of the condition of its apparatus and the correctness of the methods in vogue at its stations. Although much remains to be done, yet the contrast between the condition of affairs in 1850 and that in 1900 is very great, and the present outlook is very encouraging.
In some cases the larger portion of the funds and forces of a weather service is spent upon observations and climatological work, but in most cases the daily forecast work takes precedence, since that promises immediate results in saving life and property. In order to carry on this work properly, numerous stations must be connected by telegraph with the central bureau, at which several simultaneous observations must be received daily from the observers, and weather charts must be promptly made out showing the isobars, isotherms, state of the wind and weather, moisture and clouds over a large region of country. The accompanying charts, for April 3d, 8 A.M., and 4th, 8 A.M., 1892, show the general character of such daily weather maps; they will easily be understood by studying the respective legends. On these charts the reader will see the development of a storm that began with an area of low pressure in Colorado and rapidly developed into the great storm centre shown on Chart 2; the latter then passed northeastward over the Lake Region and the Gulf of Saint Lawrence and was followed by an extensive area of clear cool weather on April 15th. The movements and changes of storms and weather will undoubtedly be fully understood only in proportion as we have better knowledge of the facts and of the mechanical and physical laws that govern the atmosphere, but their approximate prediction from day to day is expected and demanded by reason of the many interests that depend upon the wind, temperature, and weather. At present such forecasts are generally based on the evident trend of events, as shown by comparing together the two or three latest weather maps, and in part also on empirical rules or generalizations, based on the study of similar types of maps in preceding years; but in some cases also one may be guided in part by general physical principles that must apply to the case in hand. The generalizations relative to storm movements for the United States, that is to say, the statistics of storms, have been presented in three memoirs by Prof. Elias Loomis, and printed in the Memoirs of the National Academy of Sciences. Similar data for the Northern Hemisphere as a whole were published in 1893 in Bulletin A of the United States Weather Bureau; this compilation is mostly the work of Prof. E. B. Garriott and is based upon ten years of daily maps (1878 to 1887), originally published in the Bulletin of International Simultaneous Observations. In this volume the paths of the storm centres are classified by different types and displayed on charts that show the frequency with which storm centres pass over each square of latitude and longitude.
Charts of storm paths for Europe, Asia, and Japan have been published by Germany, Russia, and Japan respectively, and monthly charts for the United States have been published regularly since January, 1873. By means of these charts one may, in a general way, anticipate the path and velocity of a storm centre when once it has appeared in any part of the Northern Hemisphere. In the Northern Hemisphere such centres move westward when they lie between the equator and the parallels of 25° or 30° N.; they then curve poleward and move northeastward with increasing rapidity toward the parallel of 60° or 70°. The variations from this general rule can best be understood by studying the charts of storm frequency. A similar rule holds good for the Southern Hemisphere, substituting only south for north. But little is known about the tracks of storms within the Arctic Circle. The region of greatest storm frequency extends in a narrow belt east and west from Lake Superior to Newfoundland and its prolongation eastward ends in the interior of Northern Russia. The region of next greatest storm frequency covers the islands of Japan. The north polar region of cold air, whose tendency is to flow outward toward the equator, is inclosed within an oval curve extending from Luzon over Japan, Southern Alaska, British Columbia, the region of the Great Lakes, Newfoundland, the Hebrides, Northern Norway and Sweden, and ending in Siberia at latitude 60° and longitude 90° east of Greenwich. South of this oval the prevailing winds are west and southwest; north of it they are north and east in the stormy season of the year.
The great whirls that we call general storms occur in connection with these polar and equatorial currents, but not necessarily between them. The whirls are explained as partially due to mechanical reactions between the northern and southern currents, but they are not merely hydrodynamic phenomena, since they have also an additional thermodynamic relationship which is quite as important. The warm, moist southerly winds are underrun by the colder and drier northerly winds. This enforced elevation of the southerly winds is accompanied by a corresponding expansion and cooling of the air that is thus elevated, and generally it is soon cooled to its dew point or below. This is followed by condensation of aqueous vapor and the formation of cloud, rain, hail, or snow with a great liberation of latent heat. Consequently the cloudy region will be warmer, but especially will it have a much smaller specific gravity than before.
In very small storms, such as tornadoes, waterspouts, etc., this process gives rise to very rapid uprising currents, a very rapid whirl around the central axis and a very low barometric pressure at the centre, but in extensive storms the vertical current is not so conspicuous, although the buoyancy of the central air tends very strongly to maintain the disturbance. The storm centre undoubtedly has a tendency to move toward the region in which the temperature and buoyancy are most disturbed; but as this region is always moving in advance, the storm centre will remain in the rear and its path will advance somewhat to the left of the direction of the greatest disturbance. But the uplifting of the lower moist air may be greatly intensified if the southerly winds on the eastern half of the storm area are being pushed up over high lands, or it may be almost wholly annulled if these winds must necessarily descend from the high lands to the ocean level. Therefore the relation of the storm's motion to the continents must be carefully worked out.
As regards weather prediction, it is evident at once that the descending winds and those that are coming from the north southward are being warmed up, and therefore in their presence the storm disappears and the weather clears away. For the Atlantic coast of North America rain is to be forecasted only when a south and east wind prevails, and especially when it is blowing on the coast. The actual effect of mountains, plateaus, continents, and the underflow of cold air varies so much on every occasion that the best one can do in forecasting is to familiarize himself thoroughly with the illustrations and exceptions that appear on every daily weather map.
The atmosphere would be at rest on the earth's surface and whirl about with the globe were it not for the sun's heat. All the important meteorological phenomena may be considered as resulting from the interaction of the solar heat, the moisture in the air, the varying temperature, and the centrifugal reaction due to the rapid diurnal rotation of the earth on its axis. The solar radiation maintains the temperature of the equatorial regions. The cold air of the polar region is both by gravity and by centrifugal force driven toward the equator. Thus the general currents are maintained moving from the poles toward the tropics and return. They are most intense in the Northern Hemisphere in January, when the sun is farthest south or over the Tropic of Capricorn, because at that time and subsequently the difference of temperature between the equator and the North Pole is greatest, and the reverse holds good in June, when the sun is north of the equator. The general circulation is greatly modified by the difference in temperature and moisture of the air over the land and the ocean, so that in summertime the tendency of the air to flow inward toward a continent or mountain is very decided. The general circulation is also greatly modified by the presence of snow, ice, mountains, plateaus, clouds, forest, etc. The winds, when once formed by differences of temperature and moisture, are themselves affected by the rotation of the earth. No matter in what direction they may be moving they are at once deflected from their polar path; in the Northern Hemisphere they turn to the right; in the Southern Hemisphere to the left. Therefore those flowing toward the equator become the northeast and southeast trade winds and those flowing toward the poles, or the upper return trade winds, become the westerly winds of the north and south temperate zones.
The differences in temperature between the continents and the ocean give rise to the so-called monsoon winds. The general centrifugal action of the winds produces a low pressure in the regions about which the winds rotate, namely, a low pressure in the Arctic and Antarctic regions; a low pressure on the left of the winds blowing around a storm centre, and on the right hand side of these same winds considered as blowing around an adjacent region of high pressure; a low pressure at the equator between the northeast and southeast trades. The reaction of the easterly winds near the equator and the westerly winds farther north also produces a similar area of high pressure between these two systems of wind corresponding to the high pressure under the tropics of Cancer and Capricorn.
A full exposition on these and other theorems by Prof. William Ferrel will be found in his Treatise on the Winds (New York, 1893). The results of later researches are presented in Prof. F. H. Bigelow's report on international cloud observations (Washington, 1900), and his Report on Barometry (Washington, 1902), but these are written for purely technical and mathematical readers. A general résumé of the laws of atmospheric motion is given in the appendix to Hann, Lehrbuch der Meteorologie (Leipzig, 1901). An elementary presentation of the subject, especially adapted to those who are beginning the study of meteorology, will be found in Davis, Elementary Meteorology (Boston, 1894); and in Ward, Practical Exercises in Elementary Meteorology (Boston, 1899). For the history of practical meteorology in the United States, see Weather Bureau.
Some details as to the instruments used in meteorology will be found under the topics: Actinometer; Anemometer; Barometer; Pyrheliometer; Nephoscope; Rain Guage; and Thermometer. Some of the results of observation will be found treated under the topics: Atmosphere; Atmospheric Electricity; Aurora Borealis; Blizzard: Climate; Clouds; Dark Day; Dew; Doldrums; Dust; Equinoctial Storm; Fog; Frost; Hail; Halo; Humidity; Indian Summer; Isobarometric Lines; Isothermal Lines; Lightning; Monsoon; Polarization of Skylight; Scintillation; Simoom; Snow; Snow Line; Storms; Heat; Typhoons; Weather; Whirlwinds; Wind.