Meteorology
From Britannica 11th Edition (1911) Meteorology (Gr. ,uerhopa, and Xl yos, i.e. the science of things in the air), the modern study of all the phenomena of the atmosphere of gases, vapours and dust that surrounds the earth and extends to that unknown outer surface which marks the beginning of the so-called interstellar space. These phenomena may be studied either individually or collectively. The collective study has to do with statistics and general average conditions, sometimes called normal values, and is generally known as Climatology (see Climate, where the whole subject of regional climatology is dealt with). The study of the individual items may be either descriptive, explanatory, physical or theoretical. Physical meteorology is again subdivided according as we consider either the changes that depend upon the motions of masses of air or those that depend upon the motions of the gaseous molecules; the former belong to hydrodynamics, and the latter are mostly comprised under thermodynamics, optics and electricity.
History
The historical development of meteorology from the most ancient times is well presented by the quotations from classic authors compiled by Julius Ludwig Ideler (Meteorologia veterum graecorum et romanorum, Berlin, 1832). We owe to the Arabian philosophers some slight advance on the knowledge of the Greeks and Romans; especially as to the optical phenomena of the atmosphere. The Meteorologia of Aristotle (see Zeller, Phil. der Griechen) accords entirely with the Philosophica of Thomas Aquinas, the poetic songs of the troubadours, and the writings of Dante (see Kuhn's Treatment of Nature in Dante's Divina Commedia; London, 1897). Dante's work completed the passage from the ancient mythological treatment of nature to the more rational recognition of one creator and lawgiver that pervades modern science. The progress of meteorology has been coincident with the progress of physics and chemistry in general, as is shown by considering the works of Alhazen (1050) on twilight, Vitellio (1250) on the rainbow, Galileo (1607) on the thermometer and on the laws of inertia, on attractions and on the weight of the air, Toricelli (1642) on the barometer, Boyle (1659) on the elastic pressure of the air in all directions, Newton (1673) on optics; Cavendish (1760), elastic pressure of aqueous vapour; Black (1752), separation of carbonic acid gas from ordinary air; Rutherford (1772), separation of nitrogen; Priestley and Scheele (1775) and Cavendish (1777), separation of oxygen; Lavoisier (1783), general establishment of the character of the atmosphere as a simple mixture of gases and vapour; De Saussure's measurement of relative humidity by the accurate hair hygrometer (1780), Dalton's measurement of vapour tension at various temperatures (1800), Regnault's and Magnus's revision of Dalton's tension of water vapour (1840), Marvin's and Juhlins's measurements of tension of ice vapour (1891), and the isolation of argon by Rayleigh and Ramsay (1894).
Theoretical meteorology has been, and always must be, wholly dependent on our knowledge of thermodynamics and on mathematical methods of dealing with the forces that produce the motions within the atmosphere. Progress has been due to the most eminent mathematicians at the following approximate dates: Sir Isaac Newton (1670), Leonhard Euler (1736), Pierre Simon Laplace (1780), Jean Baptiste Joseph Fourier (1785), Simon Denis Poisson (1815), Sir George Gabriel Stokes (1851), Hermann von Helmholtz (1857), Lord Kelvin (1860), C. A. Bjerknes (1868), V. Bjerknes (1906), and to their many distinguished followers.
The earliest systematic daily record of local weather phenomena that has survived is that kept by William Merle, rector of Driby, during seven years 13 3 1-1 33 8: the manuscript is preserved in the Digby MS., Merton College, Oxford, and was published in facsimile by George G. Symons in 1891. Doubtless many similar monastic diaries have been lost to us. In 16J3 Ferdinand II. of Tuscany organized a local system of stations and daily records which extended over and beyond northern Italy. This was the first fairly complete meteorological system in Europe. The records kept during the years1655-1670at the Cloister Angelus near Florence were reduced by Libri, professor of mathematics at Pisa, and published in 1830.
The history of meteorology is marked by the production of comprehensive treatises embodying the current state of our knowledge. Such were Louis Cotte's Traite de meteorologie (Paris, 1774) and his Memoires sur la meteorologic, supplement au traite (1788); Ludwig Karatz's Lehrbuch der Meteorologie (Halle, 1831-1836) and his Vorlesungen (1840; French 1842, English 1845); Sir John Herschel's Meteorology (London, 1840); the splendid series of memoirs by H. W. Brandes in Gehler's Physikalisches Worterbuch (Leipzig, 1820-1840); E. E. F. W. Schmid's Grundriss der Meteorologie (Leipzig, 1862); Ferrel's Recent Advances in Meteorology (Washington, 1885); the 'great works of Julius Hann, as summarized in his Handbuch der Klimatologie (1883; 2nd ed., Stuttgart, 1897; vol. i. English 1903) and his Lehrbuch der Meteorologie (Leipzig, 1901, 2nd ed. 1906); the extensive studies of J. E. Woeikoff (Voeikof), as presented in his Klima der Erde (Russian 1883, German 1885) and his Meteorologic (Russian 1904).
The development of this science has been greatly stimulated by the regular publication of special periodicals such as the Zeitschrift of the Austrian Meteorological Society, 1866-1885, vol. 21 appearing with vol. 3 of the Meteorologische Zeitschrift of the German Meteorological Society in 1886, and since that date this journal has been jointly maintained by the two societies. The analogous journals of the Royal Meteorological Society, London, 1850 to date, the Scottish Meteorological Society, 1860 to date, the Meteorological Society of France, 1838 to date, the Italian Meteorological Society, and the American Meteorological Journal, 1885-1895, have all played important parts in the history of meteorology. On the other hand, the Annals of the Central Meteorological Office at Paris, the Archiv of the Deutsche Seewarte at Hamburg, the Annals and the Repertorium of the Central Physical Observatory at St Petersburg, the Annales of the Central Meteorological Office at Rome, Bulletin of International Simultaneous Met. Obs. and the Monthly Weather Review of the Weather Bureau at Washington, the Abhandlungen of the Royal Prussian Meteorological Institute at Berlin, the Meteorological Papers of the Meteorological Office, London, and the transactions of numerous scientific societies, have represented the important official contributions of the respective national governments to technical meteorology.
The recent international union for aerial exploration by kites and balloons has given rise to two important publications, i.e. the Verofentlichungen of the International Commission for Scientific Aerostatics (Strassburg, 1905, et seq.), devoted to records of observations, and the Beitreige zur Physik der freien Atmosphere (Strassburg, 1904, et seq.), devoted to research.
The necessity of studying the atmosphere as a unit and of securing uniform accuracy in the observations has led to the formation of a permanent International Meteorological Committee (of which in 1909 the secretary was Professor Dr G. Hellmann of Berlin, and the president Dr W. N. Shaw of London). Under its directions conferences and general congresses have been held, beginning with that of 1872 at Leipzig. Its International Tables, Atlas of Clouds, Codex of Instructions, and Forms for Climatological Publications illustrate the activity and usefulness of this committee.
Modern meteorology has been developed along two lines of study, based respectively on maps of monthly and annual averages and on daily weather maps. The latter study seems to have been begun by H. W. Brandes in Leipzig, who first, about 1820, compiled maps for 1783 from the data collected in the Ephemerides mannheimensis, and subsequently published maps of the European storms of 1820 and 1821. Simultaneously with Brandes we find William C. Redfield in New York compiling a chart of the hurricane of 1821, which was published in 1831, and was the first of many memoirs by him on hurricanes that completely established their rotary and progressive motion. Soon after this Piddington and Sir William Reid began their great works on the storms of the Orient. About 1825 James Pollard Espy, in Philadelphia, began the publication of his views as to the motive power of thunderstorms and tornadoes, and in 1842 was appointed " meteorologist to the U.S. government " and assigned to work in the office of the surgeon-general of the army, where he prepared daily weather maps that were published in his four successive " Reports." In 1848 the three American leaders united in letters to Professor Joseph Henry, secretary of the Smithsonian Institution, urging that the telegraph be used for collecting data for daily maps and weather predictions. Favourable action was taken in .1849, the Smithsonian maps began to be compiled about 1851 and were displayed in public from 1853 onwards. Meanwhile in England James Glaisher, with the help of the daily press, carried out similar work, publishing his first map in 1851 as soon as daily weather maps of sufficient extent could be promptly prepared by the help of the telegraph. The destructive storm of the 14th of November 1854, in the Crimea gave U. J. J. Le Verrier, at Paris, an opportunity to propose the proper action, and his proposals were immediately adopted by the secretary of war, Marshal Vaillant. On the 17th of February 1855 the emperor ordered the director-general of government telegraph lines to co-operate completely with Le Verrier in the organization of a bureau of telegraphic meteorology. The international daily bulletin of the Paris Observatory began to be printed in regular form on the 1st of January 1858, and the daily map of isobars was added to the text in the autumn of 1863. The further development of this bulletin, the inclusion of British and ocean reports in 1861, the addition of special storm warnings in 1863, the publication of the Atlas des mouvements generaux covering the Atlantic in 1865, the study of local thunderstorms by Hippolyte Marie-Davy, Sonrel, Fron, Peslin, in France, and the work of Fitzroy, Buys-Ballot, Buchan, Glaisher and Thomson in Great Britain, parallel the analogous works of the American students of meteorology and form the beginnings of our modern dynamic meteorology.
The details of the historical development of this subject are well given by Hugo Hildebrand-Hildebrandsson and Leon Teisserenc de Bort in their joint work, Les Bases de la meteorologie dynamique (Paris, 1898-1907). The technical material has been collected by Hann in his Lehrbuch. Many of the original memoirs have been reproduced by Brillouin in his Memoires originaux (Paris, 1900), and in Cleveland Abbe's Mechanics of the Earth's Atmosphere (vol. i., 1891; vol. ii., 1909).
The publication of daily weather charts and forecasts is now carried on by all civilized nations. The list of government bureaux and their publications is given in Bartholomew's Atlas (vol. iii., London, 1899). Special establishments for the exploration of the upper atmospheric conditions are maintained at Paris, Berlin, Copenhagen, St Petersburg, Washington and Strassburg.
The general problems of climatology (1900) are best presented in the Handbook of Dr Julius Hann (2nd ed., Stuttgart, 1897). The general distribution of temperature, winds and pressure over the whole globe was first given by Buchan in charts published by the Royal Society of Edinburgh in 1868, and again greatly revised and improved in the volume of the Challenger reports devoted to meteorology. The most complete atlas of meteorology is Buchan and Herbertson's vol. iii. of Bartholomew's Atlas (London, 1899). Extensive works of a more special character have been published by the London Meteorological Office, and the Deutsche Seewarte for the Atlantic, Pacific and Indian Oceans. Daily charts of atmospheric conditions of the whole northern hemisphere were published by the U.S. Weather Bureau from 1875 to 1883 inclusive, with monthly charts; the latter were continued through 1889. The physical problems of meteorology were discussed in Ferrel's Recent Advances in Meteorology (Washington, 1885). Mathematical papers on this subject will be found in the author's collection known as The Mechanics of the Earth's Atmosphere; the memoirs by Helmholtz and Von Bezold contained in this collection have been made the basis of a most important work by Brillouin (Paris, 1898), entitled Vents contigus et nuages. A general summary of our knowledge of the mechanics and physics of the atmosphere is contained in the Report on the International Cloud Work, by F. H. Bigelow (Washington, 1900). The extensive Lehrbuch (Leipzig, 1901; 2nd ed., 1906) by Dr Julius Hann is an authoritative work. The optical xviii. g a phenomena of the atmosphere are well treated by E. Mascart in his Traite d'optique (Paris, 1891-1898), and by J. M. Penter, Meteorologische Optik (1904-1907). Of minor treatises especially adapted to collegiate courses of study we may mention those by Sprung (Berlin, 1885); W. Ferrel (New York, 1890); Angot (Paris, 1898); W. M. Davis, (Boston, 1893); Waldo (New York, 1898); Van Bebber (Stuttgart, 1890); Moore (London, 1893); T. Russell (New York), 1895. The brilliant volume by Svante Arrhenius, Kosmische Physik (Leipzig, 1900) contains a section by Sandstrom on meteorology, in which the new hydrodynamic methods of Bjerknes are developed.
I. - Fundamental Physical Data There can be no proper study of meteorology without a consideration of the various physical properties of the atmospheric gases and vapours, each of which plays an independent part, and yet also reacts upon its neighbours.
Atmospheric air is a mixture of nitrogen, oxygen, aqueous vapour, carbonic acid gas (carbon dioxide), ammonia, argon, neon, helium, with slight traces of free hydrogen and hydro-carbons. The proportions in which these gases are present are quite constant, except that the percentage of aqueous vapour is subject to large variations. In an atmosphere that is saturated at the temperature of 90° F., as may occur in such a climate as that of Calcutta, the water may be 2-1% of the whole weight of any given volume of air. When this aqueous vapour is entirely abstracted, the remaining dry gas is found to have a very uniform constitution in all regions and at all altitudes where examination has been carried out. In this so-called dry atmosphere the relative weights are about as follows: Oxygen, 23.16; nitrogen and argon, 76.77; carbonic acid, o04; ammonia and all other gases, less than ooi in the lower half of the atmosphere but probably in larger percentages at great altitudes. Of still greater rarity are the highly volatile gases, argon (q.v.), neon, krypton and helium (q.v.).
Outer Limit. - These exceedingly volatile components of the atmosphere cannot apparently be held down to the earth by the attraction of gravitation, but are continually diffusing through the atmosphere outwards into interstellar space, and possibly also from that region back into the atmosphere. There are doubtless other volatile gases filling interstellar space and occasionally entering into the atmosphere of the various planets as well as of the sun itself; possibly the hydrogen and hydro-carbons that escape from the earth into the lower atmosphere ascend to regions inaccessible to man and slowly diffuse into the outer space. The laws of diffusion show that for each gas there is an altitude at which as many molecules diffuse inwards as outwards in a unit of time. This condition defines the outer limit of each particular gaseous atmosphere, so that we must not imagine the atmosphere of the earth to have any general boundary. The only intimation we have as to the presence of gases far above the surface of the globe come from the phenomena of the Aurora, the refraction of light, the morning and evening twilight, and especially from the shooting stars which suddenly become luminous when they pass into what we call our atmosphere. (See C. C. Trowbridge, " On Luminous Meteor Trains " and " On Movements of the Atmosphere at Very Great Heights," Monthly Weather Review, Sept. 1907.) Such observations are supposed to show that there is an appreciable quantity of gas at the height of Too m., where it may have a density of a millionth part of that which prevails at the earth's surface. Such matter is not a gas in the ordinary use of that term, but is a collection of particles moving independently of each other under those influences that emanate from sun and earth, which we call radiant energy. According to Stormer this radiant energy is that of electrons from the sun, and their movements in the magnetic field surrounding the earth give rise to our auroral phenomena.
According to Professor E. W. Morley, of Cleveland, Ohio, the relative proportions of oxygen and nitrogen vary slightly at the surface of the earth according as the areas of high pressure and low pressure alternately pass over the point of observation; his remarkably exact work seems to show a possible variation of a small fraction of 1%, and he suggests that the air descending within the areas of high pressure is probably slightly poorer in oxygen. The proportion of carbonic acid gas varies appreciably with the exposure of the region to the wind, increasing in proportion to the amount of the shelter; it is greater over the land than over the sea, and it also slightly increases by night-time as compared with day, and in the summer and winter as compared with the spring and autumn months. During the year 1896 Professor S. Arrhenius in the Phil. Mag., and in 1899 Professor T. C. Chamberlin in the Amer. Geol. Jour., published memoirs in which they argued that a variation of several per cent. in the proportion of carbonic acid gas is quite consistent with the existence of animal and vegetable life and may explain the variations of climate during geological periods. But the specific absorption of this gas for solar radiations is too small (C. G. Abbot, 1903) to support this argument. The question whether free ozone exists in the atmosphere is still debated, but there seems to be no satisfactory evidence of its presence, except possibly for a few minutes in the neighbourhood of, and immediately after, a discharge of lightning. The general proportions of the principal gases up to considerable altitudes can be calculated with close approximation by assuming a quiescent atmosphere and the ordinary laws of diffusion and elastic pressure; on the other hand, actual observations show that the rapid convection going on in the atmosphere changes these proportions and brings about a fairly uniform percentage of oxygen, nitrogen and carbonic acid gas up to a height of Io m.
Aqueous Vapours
The distribution of aqueous vapour is controlled by temperature quite as much as by convection and has very little to do with diffusion; the law of its distribution in altitude has been well expressed by Hann by the simple formula: log e = log eo - h/6517 where h is the height expressed in metres and e and eo are the vapour pressures at the upper station and sea-level respectively. Hann's formula applies especially to observations made on mountains, but R. J. Suring, Wissenschaftliche Luftfahrten, III. (Berlin, 1900) has deduced from balloon observations the following formula for the free air over Europe log e= log eo - h (i + h/20000) /6000.
He has also computed the specific moisture of the atmosphere or the mixing ratio, or the number of grams of moisture mixed with I kilogram of dry air for which he finds the formula log m = log mo - h(I +3h/40)/9000.
Altitude Metres. h. | Relative Vapour Pressure. e/eo. | Relative Specific Moisture. m/mo |
0 | 1000 | 1000 |
1000 | 665 | 759 |
2000 | 431 | 555 |
3000 | 266 | 391 |
4000 | 158 | 264 |
5000 | 91 | 172 |
6000 | 50 | 108 |
7000 | 27 | 65 |
8000 | 14 | 38 |
The relative humidity varies with altitude so irregularly that it cannot be expressed by any simple formula. The computed values of e and m are as given in the following table: - In addition to the gases and vapours in the atmosphere, the motes of dust and the aqueous particles that constitute cloud, fog and haze are also important. As all these float in the air, slowly descending, but resisted by the viscosity of the atmosphere, their whole weight is added to the atmosphere and becomes a part of the barometric record. When the air is cooled to the dew-point and condensation of the vapour begins, it takes place first upon the atoms of dust as nuclei; consequently, air that is free from dust is scarcely to be found except within a mass of cloud or fog.
Mass
According to a calculation published in the U.S. Monthly Weather Review for February 1899, the total mass of the atmosphere is 1/1,125,000 of the mass of the earth itself but, according to Professor R. S. Woodward (see Science for Jan. 1900), celestial dynamics shows that there may possibly be a gaseous envelope whose weight is not felt at the earth's surface, since it is held in dynamic equilibrium above the atmosphere; the mass of this outer atmosphere cannot exceed i n i nth of the mass of the earth, and is probably far less, if indeed it be at all appreciable.
Conductivity
Dry air is a poor conductor of heat, its coefficient of conduction being expressed by the formula: 0.000 0568 (Id-0.00190 t) where the temperature (t) is expressed in centigrade degrees. This formula states the fact that a plate of air I centimetre thick can conduct through its substance for every square centimetre of its area, in one second of time, when the difference of temperature between two faces of the plate is 1 ° C., enough heat to warm gram of water 0.000 0568° C. or I gram of air 0000 239° C., or a cubic centimetre of air 0.1850° C., if that air is at the standard density for 760 millimetres of pressure and 0° C. The figure 0.1850° C. is the thermometric coefficient as distinguished from the first or calorimetric coefficient (o000 0568° C.), and shows what great effect on the air itself its poor conductivity may have.
Diathermancy
Dry air is extremely diathermanous or transparent to the transmission of radiant heat. For the whole moist atmosphere the general coefficient of transmission increases as the waves become longer: and for a zenithal sun it is about o4 at the violet end of the spectrum and about o8 at the red. By specific absorption many specific wave-lengths are entirely cut off by the vapours and gases, so that in general the atmosphere may appear to be more transparent to the short wave-lengths or violet end of the spectrum, but this is not really so. When the zenithal sun's rays fall upon a station whose barometric pressure is 760 mm., then only from 50 to 80% of the total heat reaches the earth's surface, and thus the general coefficient of transmission for the thickness of one atmosphere is usually estimated at about 60%. Of course when the rays are more oblique, or when haze, dust or cloud interfere, the transmission is still further diminished. In general one half of the heat received from the sun by the illuminated terrestrial hemisphere is absorbed by the clearest atmosphere, leaving the other half to reach the surface of the ground, provided there be no intercepting clouds. The thermal conditions actually observed at the immediate surface of the globe during hazy and cloudy weather are therefore of minor importance in the mechanism of the whole atmosphere, as compared with the influence of the heat retained within its mass.
The transmission of solar radiation through the earth's atmosphere is the fundamental problem of meteorology, and has been the subject of many studies, beginning with J. H. Lambert and P. Bouguer. The pyrheliometer of C. S. M. Pouillet gave us our first idea of the thermal equivalent of solar radiation outside of our atmosphere or the so-called " solar constant," the value of which has been variously placed at from 2 to 4 calories per sq. cm. per minute. At present the weight of the argument is in favour of 2.1, with a fair presumption that both the intensity and the quality of the solar radiation as it strikes the upper layers of our atmosphere are slightly variable. It is also likely that this " constant " does not represent the sun proper, but the remaining energy after the sunbeam has sifted through masses of matter between the sun and our upper atmosphere, so that it may thus come to have appreciable variations.
Wave Length. | Coefficient of Atmospheric Transmission (Abbot). | ||
|---|---|---|---|
1901-1902. | 1903. | Mean by Weights. | |
microns. | |||
0.40 violet | - | 0.484 | - |
0 '45 | - | 0.557 | - |
0.50 | 0.765 | 0.627 | 0.700 |
0.60 | 0.769 | 0.692 | 0.730 |
0.70 | 0.857 | 0.753 | 0.808 |
0.80 red | 0.897 0.910 | 0.797 0.825 | 0.847 0.8 56 |
I. 00 | 0.921 | 0.847 | 0.884 |
1.20 | 0'933 | 0.874 | 0.903 |
i60 | 0.930 | 0.909 | 0.920 |
2.00 | 0.950 | 0.912 | 0.919 |
Date. | Abbot. Calories. | Fowle. Calories. |
1902 Oct. 9 | 2.19 | 2.19 |
7/„ 15 | 2.19 | - |
„ „ 22 | 2.16 | - |
1903 Feb. 19 | 2.28 | 2.27 |
3 | 2.26 | -- |
,, „ | ||
2.10 | - | |
26 | 2.07 | 2.09 |
„ April 17 | 1.99 | 2.18 |
„ „ | ||
„ July 7 | - | 2.14 |
„ Oct. 14 | - | I.96 |
„ Dec. 7 | - | 1.94 |
„ ,,23 | -- | 1.99 |
Jan. 27 | - | 2.02 |
„ Feb. II | - | 2.26 |
„ May 28 | - | 2.09 |
„ Oct. 5 | - | 2.32 |
„ Nov. 16 | - | 1.98 |
The coefficients of absorption for specific wave-lengths were first determined by L. E. Jewell, of Johns Hopkins University, for numerous vapour lines in 1892 (see W. B. Bulletin, No. 16). In 1904 C. G. Abbot published a table based on bolograph work at Washington showing the coefficient of atmospheric transmission for solar rays passing through a unit mass of air-namely, from the zenith to the ground. He showed that this coefficient increased with the wavelength; hence any change in the quality of the solar radiation will affect the general coefficient of transmission. The following table gives his averages for the respective wave-lengths, as deduced from ten clear days in1901-1902and nine clear days in 1903: Any variation in the energy that the atmosphere receives from the sun will have a corresponding influence on meteorological phenomena. Such variations were simultaneously announced in 1903 by Charles Dufour in Switzerland and H. H. Kimball in Washington (Monthly Weather Review, May 1903); the latter was then conducting a series of observations with Angstrom's electric compensation pyrheliometer, and his conclusions have been confirmed by the work of L. Gorczynski at Prague (1901-1906) and C. G. Abbot at Washington. Kimball's pyrheliometric work on this problem is still being continued; but meanwhile Abbot and Fowle from their bolometric observations at the Smithsonian Astrophysical Observatory have deduced preliminary values of the observed total energy, or the solar constant, for numerous dates when the sky was very clear, as follows (see Smithsonian Mis. Coll., xlv. 78 and xlvii. 403, 1905) If the relative accuracy of these figures is i %, as estimated by Abbot,. then they demonstrate irregular fluctations of 5%. But different observers and localities vary so much that Abbot estimates the reliability of the mean value, 2.12, to be about io %. The causes of this variation apparently lie above our lower atmosphere and move slowly eastward from day to day, and as the variability is comparable with that of other atmospheric data, therefore conservative meteorologists at present confine their attention to the explanation of terrestrial phenomena under the assumption of a constant solar radiation. The large local changes of weather and. climate are not due to changes in the sun, but to the mechanical. and thermodynamic interactions of earth and ocean and atmosphere. Excellent illustrations of this principle are found in the studies of Blanford, Eliot and Walker on the monsoons of India, of Sieger (1892) on the contrasts of temperature between Europe and North America, of Hann (1904) on the anomalies of weather in Iceland, of Meinardus (1906) on periodical variations of the icedrift near Iceland.
The absorption of solar radiation by the atmosphere is apparently explained by the laws of diffuse reflection, selective diffusion and fluorescence in accordance with which each atom and molecule and particle becomes a new centre for the diffusion in all directions of the energy represented by some specific wave-length. The specific influences of carbon dioxide and water vapour are less than those of the liquid particles (and of cloud and rains) and of the great mass of oxygen and nitrogen that make up the atmosphere.
Specific Heat.-The capacity of dry air for heat varies according as the heat increases the volume of the air expanding under constant pressure, or the pressure of the air confined in constant volume. The specific heat under constant pressure is about 1.4025 times the specific heat under constant volume. The numerical value of the specific heat under constant pressure is about o2375-that is to say, that number of gram-calories, or units of heat, is required to change the temperature of I gram of air by I ° C. This coefficient holds good, strictly speaking, between the temperatures-30° and -?-10° C., and there is a very slight diminution for higher temperatures up to 200°. The specific heat of moist air is larger than that of dry air, and is given by the expression C 5 " = (0.2 375 -10'4805 x)' where x is the number of kilograms of vapour associated with 'I kilogram of dry air. As x does not exceed 0.030 (or 30 grams) the value of C 5 " may increase up to 0.2519. The latent heat evolved in the condensation of this moisture is a matter of great importance in the formation of cloud and rain.
Radiating Power.-The radiating power of clean dry air is so small' that it cannot be measured quantitatively, but the spectroscope and bolometer demonstrate its existence. The coefficient of radiation of the moisture diffused in the atmosphere is combined with that of the particles of dust and cloud, and is nearly equal to that of an equal surface of lamp-black. From the normal diurnal change in temperature at high and low stations, it should be possible to determine the general coefficient of atmospheric radiation for the average condition of the air in so far as this is not obscured by the influence of the winds. This was first done by J. Maurer in 1885, who obtained a result in calories that may be expressed as follows: the total radiation in twenty-four hours of a unit mass of average dusty and moist air towards an enclosure whose temperature is i ° lower is sufficient to lower the temperature of the radiating air by 3.31° C. in twenty-four hours. This very small quantity was confirmed by the studies of Trabert, published in 1892, who found that i gram of air at 278° absolute temperature radiates 0.1655 calories per minute toward a black surface at the absolute zero. The direct: observations of C. C. Hutchins on dry dusty air, as published in 1890, gave a much larger value-evidently too large. Slight changes in water, vapour and carbon dioxide affect the radiation greatly. The investigation of this subject prosecuted by Professor F. W. Very at the Allegheny Observatory, and published as " Bulletin G of the U.S. Weather Bureau, shows the character and amount of the radiation of several gases, and especially the details of the process going on under normal conditions in the atmosphere.
Density.-The absolute density or mass of a cubic centimetre of dry air at the standard pressure, 760 millimetres, and temperature o° C., is 0001 29305 grams; that of a cubic metre is I. 29305 kilograms;. that of a cubic foot is 0.08071 lb avoirdupois. The variations of this density with pressure, temperature, moisture and gravity are given in the Smithsonian meteorological tables, and give rise to all the movements of the atmosphere; they are, therefore, of fundamental importance to dynamic meteorology.
Expansion.-The air expands with heat, and the expansion of aqueous vapour is so nearly the same as that of dry air that the same coefficient may be used for the complex atmosphere itself.. The change of volume may be expressed in centigrade degrees by the formula V = Vo (i +0.000 3665t), or in Fahrenheit degrees, V = Vo (1+0.000 237t).
Elasticity.-The air is compressed nearly in proportion to the pressure that confines it. The pressure, temperature and volume of the ideal gas are connected by the equation pv = RT, where T is the absolute temperature or 273° plus the centigrade temperature p is the barometric pressure in millimetres and v the volume of a unit mass of gas, or the reciprocal of the density of the gas. The constant R is 29.272 for dry atmospheric air when the centimetre, the gram, the second and the centigrade degrees are adopted as units of measure, and differs for each gas. For aqueous vapour in a gaseous state and not near the point of condensation R has the value 47.061. For ordinary air in which x is the mass of the aqueous vapour that is mixed with the unit mass of dry air, the above equation becomes pv=(29.272+47.061x) T. This equation is sometimes known as the equation of condition peculiar to the gaseous state. It may also be properly called the equation of elasticity or the elastic equation for gases, as expressing the fact that the elastic pressure p depends upon the temperature and the volume. The mose exact equations given by Van der Waals, Clausius, Thiesen, are not needed by us for the pressures that occur in meteorology.
Diffusion
In comparison with the convective actions of the winds, it may be said that it is difficult for aqueous vapour to diffuse in the air. In fact, the distribution of moisture is carried on principally by the horizontal convection due to the wind and the vertical convection due to ascending and descending currents. Diffusion proper, however, comes into play in the first moments of the process of evaporation. The coefficient of diffusion for aqueous vapour from a pure water surface into the atmosphere is o18 according to Stefan, or o1980 according to Winkelmann; that is to say, for a unit surface of 1 sq. centimetre, and a unit gradient of vapour pressure of one atmosphere per centimetre, as we proceed from the water surface into the still dry air, at the standard pressure and temperature, and quantity of moisture diffused is 0-1980 grams per second. This coefficient increases with the temperature, and is 0.2827 at 49.5° C. But the gradient of vapour pressure, and therefore rate of diffusion, diminishes very rapidly at a small distance from the free surface of the water, so that the most important condition facilitating evaporation is the action of the wind.
Viscosity
When the atmosphere is in motion each layer is a drag upon the adjacent one that moves a little faster than it does. This drag is the so-called molecular or internal friction or viscosity. The coefficient of viscosity in gases increases with the absolute temperature, and its value is given by an equation like the following; 0.000 248 (1+oo03 665t) 2 / 3, which is the formula given by Carl Barus (Ann. Phys., 1889, xxxvi.). This expression implies that for air whose temperature is the absolute zero there is no viscosity, but that at a temperature (t) of 0° C., or 273° on the absolute scale, a force of 0.000 248 grams is required in order to push or pull a layer of air 1 centimetre square past another layer distant from it by 1 centimetre at a uniform rate of 1 centimetre per second.
Friction
The general motions of the atmosphere are opposed by the viscosity of the air as a resisting force, but this is an exceedingly feeble resistance as compared with the obstacles encountered on the earth's surface and the inertia of the rising and falling masses of warm and cold air. The coefficient of friction used in meteorology is deduced from the observations of the winds and results essentially not from viscosity, but from the resistances of all kinds to which the motion of the atmosphere is subjected. The greater part of these resistances consists essentially in a dissipation of the energy of the moving masses by their division into smaller masses which penetrate the quiet air in all directions. The loss of energy due to this process and the conversion of kinetic into potential energy or pressure, if it must be called friction, should perhaps be called convective friction, or, more properly, convectiveresistance.
The coefficient of resistance for the free air was determined by Mohn and Ferrel by the following considerations. When the winds, temperatures and barometric pressures are steady for a considerable time, as in the trade winds, monsoons and stationary cyclones, it is the barometric gradient that overcomes the resistances, while the resulting wind is deflected to the right (in the northern hemisphere) by the influence of the centrifugal force of the diurnal rotation of the earth. The wind, therefore, makes a constant angle (a) with the direction of the gradient (G). There is also a slight centrifugal force to be considered if the winds are circulating with velocity v and radius (r) about a storm centre, but neglecting this we have approximately for the latitude G sin 'a' = 2wv sin 0, Gcos 'a' = Kv, where is the coefficient connecting the wind-velocity (v) with the component of the gradient pressure in the direction of the wind. These relations give 2w sin 4)/tan a. The values of a and v as read off from the map of winds and isotherms at sea level give us the data for computing the coefficients for oceanic and continental surfaces respectively, expressed in the same units as those used for G and v. The extreme values of this coefficient of friction were found by Guldberg and Mohn to be 0.00002 for the free ocean and 0.00012 for the irregular surface of the land. For Norwegian land stations Mohn found cp = 61° a = 56.5° 0.0000845. For the interior of North America Elias Loomis found4 = 37.5° a = 42.2° = 0.0000803.
Gravity
The weight of the atmosphere depends primarily upon the action of gravity, which gives a downward pressure to every particle. Owing to the elastic compressibility of the air, this downward pressure is converted at once into an elastic pressure in all directions. The force of gravity varies with the latitude and the altitude, and in any exact work its variations must be taken into account. Its value is well represented by the formula due to Helmert, g = 980.6 (1 - o0026 cos 20) X (1 - fh), where ¢ represents the latitude of the station and h the altitude. The coefficient f is small and has a different value according as the station is raised above the earth's surface by a continent, as, for instance, on a mountain top, or by the ocean, as on a ship sailing over the sea, or in the free air, as in a balloon. Its different values are sufficiently well known for meteorological needs, and are utilized most discreetly in the elaborate discussion of the hypsometric formula published by Angot in 1899 in the memoirs of the Central Meteorological Bureau of France.
Temperature at Sea-Level
The temperature of the air at the surfaces of the earth and ocean and throughout the atmosphere is the fundamental element of dynamic meteorology. It is best exhibited by means of isotherms or lines of equal temperature drawn on charts of the globe for a series of level surfaces at or above sea-level. It can also be expressed analytically by spherical harmonic functions, as was first done by Schoch. The normal distribution of atmospheric temperature for each month of the year over the whole globe was first given by Buchan in his charts of 1868 and of 1888 (see also the U.S. Weather Bureau " Bulletin A," of 1893, and Buchan's edition of Bartholomew's Physical Atlas, London, 1899). The temperatures, as thus charted, have been corrected so as to represent a uniform special set of years and the conditions at sea-level, in order to constitute a homogeneous system. The actual temperature near the ground at any altitude on a continent or island may be obtained from these charts by subtracting 0.5°C. for each loo metres of elevation of the ground above sea-level, or I° F. for 350 ft. This reduction, however, applies specifically to temperatures observed near the surface of the ground, and cannot be used with any confidence to determine the temperature of points in the free air at any distance above the land or ocean. On all such charts the reader will notice the high temperatures near the ground in the interior of each of the continents in the summer season and the low temperatures in the winter season. In February the average temperatures in the northern hemisphere are not lowest near the North Pole, but in the interiors of Siberia and North America; in the southern hemisphere they are at the same time highest in Australia, and Africa and South America. In August the average temperatures are unexpectedly high in the interior of Asia and North America, but low in Australia and Africa.
Temperature at Upper Levels
The vertical distribution of temperature and moisture in the free air must be studied in detail in order to understand both the general and the special systems of circulation that characterize the earth's atmosphere. Many observations on mountains and in balloons were made during the 19th century in order to ascertain the facts with regard to the decrease of temperature as we ascend in the atmosphere; but it is now recognized that these observations were largely affected by local influences due to the insufficient ventilation of the thermometers and the nearness of the ground and the balloon. Strenuous efforts are being directed to the elimination of these disturbing elements, and to the continuous recording of the temperature of the free air by means of delicate thermographs carried up to great heights by small free " sounding balloons," and to lesser heights by means of kites. Many international balloon ascents have been made since 1890, and a large amount of information has been secured.
Stations. | 1000 ft. | 1500 ft. | 2000 ft. | 3000 ft. | 4000 ft. | 5000 ft. | 6000 ft. |
° | ° | ° | ° | ° | ° | ° | |
Washington, D.C.. . | 5' 6 | 4'4 | 4.0 | 3.5 | 3' 2 | 3.0 | 3.1 |
Cairo, Ill... . | 9.7 | 6.6 | 6o | 4.9 | 4'7 | 4.3 | - |
Cincinnati, O... . | 13.0 | 6.3 | 6.9 | 5' 8 | 5' 6 | 4.7 | 4'2 |
Fort Smith, Ark.. . | 7.2 | 7.0 | 6.7 | 5.8 | 3.8 | - | - |
Knoxville, Tenn.. . | 8.4 | 6.2 | 6.6 | 5.4 | 5 .o | - | - |
Memphis, Tenn.. . | 7.8 | 6.8 | 5.0 | 3' 8 | 3.7 | 3.5 | - |
Springfield, Ill... . | 7' 6 | 5.7 | 5' 1 | 4.4 | 4.0 | 3.7 | 3'6 |
Cleveland, O. | 5'7 | 4' I | 3' 6 | 3.5 | 4' 1 | 4' I | 4'3 |
Duluth, M inn.. . | 5' 2 | 4' 8 | 4' 6 | 4' 6 | 4.3 | 3' 8 | 4.6 |
Lansing, Mich. | 7.5 | 6o | 4.7 | 4' I | 3.9 | 3' 8 | - |
Sault Ste Marie, Mich. . | 6.6 | 6.2 | 5' 2 | 4'5 | 3.9 | 3 .o | - |
Dodge, Kans. . | 6.3 | 5' 2 | 4' 8 | 3.7 | 3' 1 | 3.2 | 3'2 |
Dubuque, Iowa | 6.9 | 5.9 | 4' 6 | 3.5 | 3' 2 | 3.3 | - |
North Platte, Neb. | 6.8 | 6.5 | 5.9 | 5.2 | 4.4 | 4.7 | 5'4 |
Omaha, Neb. . | - | 5.4 | 4.9 | 3' 6 | 3' 2 | 3.5 | 3'8 |
Pierre, S. Dak. . | 5.9 | 5' 1 | 4' 8 | 4.3 | 3.7 | 4.4 | 4.0 |
Topeka, Kans. . | 7.4 | 6.2 | 4.9 | 4.0 | 3' 8 | 3'9 | 4.5 |
Average. . | 7.4 | 5.8 | 5' 2 | 4.4 | 4' 0 | 3.8 | 4.1 |
Stations | Altitude. | Temperature. | |
|---|---|---|---|
Feet. | Gradient. | Reduction. | |
° F. | ° F. | ||
Washington | 210 | -3.00 | -15.2 |
Cairo | 315 | -4'30 | -25'6 |
Cincinnati | 940 | -5.15 | -27.5 |
Fort Smith | 527 | ? | ? |
Knoxville | 990 | -5.00 | -21.5 |
Memphis | 319 | -3'50 | -17'3 |
Springfield | 684 | -3'85 | -171 |
Cleveland | 705 | -4.10 | -18.8 |
Duluth | 1197 | -4.30 | -17.6 |
Lansing | 869 | -3.85 | -17.0 |
. Sault Ste Marie. .. . | 722 | -3.45 | -15'7 |
Dodge | 2473 | -4.10 | -11.6 |
North Platte | 2811 | -5.40 -13.3 | |
Omaha | 1241 | -3.20 | -12.9 |
Pierre. .. ... . | 1595 | -3'90 | -14'4 |
Topeka | 972 | -3.83 | -16.5 |
The development of kite-work in the United States began in October 1893, at the World's Columbian Congress at Chicago, when Professor M. W. Harrington ordered Professor C. F. Marvin of the Weather Bureau to take up the development of the Hargrave or box kite for meteorological work. At that time W. A. Eddy of Bayonne, New Jersey, was applying his " Malay " kite to raising and displaying heavy objects, and in August 1894 (at the suggestion of Professor Cleveland Abbe) he visited the private observatory of A. L. Rotch at Blue Hill and demonstrated the value of his Malay kite for aerial research. The first work done at this observatory with crude apparatus was rapidly improved upon, while at the same time Professor Marvin at Washington was developing the Hargrave kite and auxiliary apparatus, which he brought up to the point of maximum efficiency and trustworthiness. When he reported his apparatus as ready to be used by the Weather Bureau on a large scale, Professor Willis L. Moore, as the successor of Professor Harrington, ordered its actual use at seventeen kite stations in July 1898. This was the first attempt to prepare isotherms for a special hour over a large area at some high level, such as 1 m., in the free air. Daily meteorological charts were prepared for the region covered by these observations; but it became necessary to discontinue them, and nothing more was done by the Weather Bureau in this line of work until the inauguration of kite work at Mount Weather in 1906. Meanwhile a special method for the reduction and study of such observations was devised by Bjerknes and Sandstrom, and was published in the Trans. American Philosophical Society (Philadelphia, 1906). The general average results as to temperature gradients were compiled by Dr H. C. Frankenfield and published in the United States Weather Bureau " Bulletin F.": from these were deduced the following tables, published in the Monthly Weather Review:- Mean Temperalure Gradients in degrees Fahrenheit per woo ft. from the ground up to the respective altitudes In this table the second column gives the altitude of the ground at the reel on which the kite wire was wound. The third column shows the average gradient in degrees Fahrenheit per moo ft. between the reel at the respective stations, and a uniform altitude .5280 ft. above sea-level. The fourth column shows the total reduction to be applied to the temperature at the reel in order to obtain the temperature at the i m. level above sea. These gradients and reductions are based upon observations made only during the six warm months from May to October 1898.
The kite-work at the Blue Hill Observatory has been published in full in the successive Annals of the Harvard College Observatory, beginning with 1897, vol. xlii. It has been discussed especially by H. H. Clayton with reference to special meteorological phenomena, such as areas of high and low pressure, fair and cloudy weather, the winds and their velocities at different elevations, insolation, radiation, &c., and has served as a stimulus and model for European meteorologists. Kite-work has also been successfully prosecuted at Trappes, Hamburg, Berlin, St Petersburg, and many other European stations. The highest flights that have been attained have been about 8000 metres.
The great work of L. Teisserenc de Bort began with 1897, when he founded his private observatory at Trappes near Paris devoted to the problems of dynamic meteorology. His results are published in full in the Memoirs of the Central Meteorological Bureau of France for 1897 and subsequent years. Beginning with the sounding balloons devised by Hermite, he subsequently added kite work as supplementary to these. In the Comptes rendus (1904), he gives the mean temperatures as they result from five years of work, 1899-1903, at Trappes. Out of 581 ascensions of sounding balloons there were 141 that attained 14 km. or more, and the following table gives the average temperatures recorded in these ascensions. It will be seen that there is a slow decrease in temperate up to 2 km.; a rapid decrease thence up to 10 km., and a slow decrease, almost a stationary temperature, between I 1 and 14 km.; this is the " thermal zone " as discovered and so called by him.
Altitude. | Winter. Dec., Jan., Feb. | Spring. Mar., Apl., May. | Summer. June, July, Aug. | Autumn. Sept., Oct., Nov. |
Km. | ° C. | ° C. | ° C. | ° C. |
Ground | + I. 9 | + 5.1 | +13'0 | + 7'5 |
0.5 | + 14 | + 4'7 | +13.6 | + 7'7 |
I. O | - O. 2 | + 2.4 | +11.8 | + 6.I |
I. 5 | - 0.2 | + oI | 9.7 | + 4.0 |
2.0 | - 1.4 | - 2.1 | 7.3 | + 22 |
2 '5 | - 3'7 | - 4.3 | 5'0 | + 0'4 |
3. o | - 6. o | - 6.4 | 2.1 | - 17 |
3. 5 | - 8 '7 | - 9.3 | + 0.2 | - 4.2 |
4.0 | -10.9 | - 12.2 | - 2.7 | - 6.5 |
4.5 | - 14.2 | -15.2 | - 5.3 | -9.3 |
5.0 | -17.0 | - 18.5 | - 8.3 | -12.4 |
6o | -23.7 | -25.2 | -14.8 | - 18.7 |
7o | -31.5 | -32.0 | -2 I .7 | -25.8 |
8o | -39'0 | -39.0 | -29.3 | -33'5 |
9.0 | -46.9 | -46.7 | -38o | -41.4 |
Ioo | -54'6 | -52'7 | 45'3 | -48.3 |
11.0 | -57'9 | - 53'6 | 50.3 | -54'4 |
12.0 | -57'9 | -53.1 | 52'7 | -57'1 |
13.0 | -56.9 | -52.2 | 51.5 | -57.1 |
1 4.0 | -55'5 | -52'5 | -51'3 | -57'1 |
It is evident that the annual average vertical gradient of temperature over Paris is between 4° and ' 6° C. per moo metres of ascent in the free air, agreeing closely with the value 5° per moo metres, which has come into extensive use since the year 1890, on the recommendation and authority of Hann, for the reduction of land observations to sea-level. The winter gradients are less than those for summer, possibly owing to the influence of the condensation into cloud and rain during the winter season in France; the same value may not result from observations in the United States, where the clouds and precipitation of winter do not so greatly exceed those of summer. The work at Trappes is therefore not necessarily representative of the general average of the northern hemisphere, but belongs to a coastal region in which during the summer time, at great heights, the air is cooler than in the winter time, since during the latter season there is an extensive flow of warm south winds from the ocean over the cold east winds from the land. Sounding balloons have also been used elsewhere with great success. The greatest heights attained by them have been 25,989 metres at Uccle, Belgium, on the 5th of September 1907, and 25,800 metres at Strassburg, August 1905.
Annual Temperatures and Wind. | ||||||||
|---|---|---|---|---|---|---|---|---|
Tegel 1903. | Tegel 1904. | Lindenberg, 1905. | Lindenberg, 1905. | |||||
Altitude. | Days. | °C. | Days. | °C. | Days. | °C. | Days. | Metres per sec. |
Ground | 365 | 9.2 | 366 | 9'I | 365 | 8'5 | 3 6 5 | 4'65 |
500 m. | 363 | 6.7 | 364 | 6.5 | 365 | 6.2 | 362 | 8.65 |
1,000 „ | 344 | 4'3 | 3 61 | 4'2 | 352 | 4.0 | 356 | 8.85 |
1,500 „ | 252 | 2.0 | 279 | 2.2 | 294 | 2.6 | 306 | 8.55 |
2,000 „ | 170 | 0.0 | 186 | -0.2 | 242 | 0.5 | 257 | 9.5 |
2,500 „ | 98 | -1.8 | 132 | -I7 | 179 | -1.1 | 195 | 10.0 |
3,000 „ | 55 | -3'9 | 79 | -3'6 | 119 | -2.8 | 127 | Io7 |
The most extensive meteorological explorations of the free atmosphere have been those accomplished in Germany by Richard Assmann and Arthur Berson, beginning (1887) in co-operation with the German Verein for the Promotion of Aeronautics and the Aeronautic Section of the German Army, afterwards under the auspices of the Prussian Meteorological Office, but later as a wholly independent institution at Lindenberg. All the details of the work during1887-1889and the scientific results of seventy balloon voyages were published in three large volumes, Wissenschaftliche Luftschiffahrten (Berlin, 1900). The work done at Tegel at the Aeronautical Observatory of the Berlin Meteorological Office, the 1st of October 1899 to April 1905, was published in three volumes of Ergebnisse. But the location at Tegel had to be given up and a new independent establishment, the " Royal Prussian Aeronautic Observatory," was founded at Lindenberg, under the direction of Dr Assmann, who has published the results of his work in annual volumes of the Ergebnisse of that institution, considering it as a continuation of the work' done at Berlin and Tegel. In addition to these elaborate official publications various summaries have been published, the most instructive of which is the chart embodying daily observations with corresponding isotherms at all attainable altitudes, published monthly since January 1903 in Das Wetter. The growth of this aerial work and the reliability of the results may be inferred from a statement of the number of ascensions made each year: 1899, 6; 1900, 39; 1901, 169; 1902, 261; 1903, 481; 1905, 513. This large number, combined with 581 voyages of Teisserenc de Bort at Trappes and many others made in England, Holland and Russia, amounting in all to over 2000, enabled Assmann to compute the monthly and annual means of temperature and wind velocity for each altitude; the German results are given in table at foot of page 269.
The results of these numerous ascents, during thee six years, have also been grouped into monthly means that have a reliability proportionate to the number of days on which observations were obtained at a given level, and we are now able to speak of the annual and even of the diurnal periodicity of temperature at different altitudes in the free air with considerable confidence.
Some of the most important conclusions to be drawn from the best recent work were published by Hann either in special memoirs or in his Lehrbuch, from which we take the following table. The actual temperatures given in this table have only local importance, but the differences or the vertical gradients doubtless hold good over a large portion of Europe if not of the world.
Altitude. | Over Germany. | Over Trappes. |
I, 2, 3 km. | May, June | May 15 |
3, 4, 5 | March | Feb. 15 |
5, 6, 7 | April | Jan. 27 |
7, 8, 9 | July | July 28 |
9, IO, II | - | Sept. 14 |
The differences of temperature between any layer and those above it and below it, or the vertical gradients at each level go through annual periodical changes quite analogous to those derived from mountain observations; the most rapid falls of temperature, or the largest vertical gradients in the free air occur on the following dates over Europe: The values above given as deduced from 141 high ascensions at Trappes show that between I I and 14 km. there was no appreciable diminution of temperature, in other words, the air is warmer than could be expected and therefore has a higher potential temperature. This fact was first confirmed by the Berlin ascensions, and is now recognized as wellnigh universal. The altitude of the base of this warm stratum is about 12 km. in areas of high pressure and 10 km. in areas of low pressure. It is higher as we approach the tropics and above ordinary balloon work near the equator if indeed it exists there. At first this unexpected warmth was considered as possibly a matter of error in the meteorographs, but this idea is now abandoned. Assmann suggested that the altitude is that of the highest cirrus, from which Cleveland Abbe inferred that it had something to do with the absorption of the solar and terrestrial heat by dissolving cirri. But the most plausible explanation is that published simultaneously in September 1908 by W. J. Humphreys of Washington, and Ernest Gold of London.
The daily diagrams in Das Wetter show that both the irregular and the periodic and the geographic variations of temperature in the upper strata are unexpectedly large, almost as large as at the earth's surface, so that the uniform temperature of space that was formerly supposed to prevail in the upper air must be looked for, if at all, far above the level to which sounding balloons have as yet attained. It is evident that both horizontal and vertical convection currents of great importance really occur at these great altitudes. These upper currents cannot be due to any very local influence at the earth's surface, but only to the interchange of the air over the oceans and continents or between the polar and equatorial regions. They constitute the important feature of the so-called. general circulation of the atmosphere, which we have hitherto. mistakenly thought of as confined to lower levels; their general direction is from west to east over all. parts of the globe as far as yet. known, showing that they are controlled by the rotation of the earth. It is likely that masses of air having special temperature conditions or clouds of vapour dust such as came from Krakatoa, may be carried in these high currents around the globe perhaps several times before being dissipated.
Altitude. | Moving eastward. | Moving westward. |
10.0 km. | 36 m. p.s. | 20 M. p.s. |
7.5 | 35 | 2.0 |
5.0 | 26 | I.5 |
3. O | 20 | I.0 |
I. 0 | 8 | 0.5 |
0 | 4 |
Altitude. | Annual Averages. | International. | All countries combined. | ||||
|---|---|---|---|---|---|---|---|
Berlin. 15 Ascents. | Inter- national. 130 Ascents. | Manned balloons. 36 Ascents. | Trappes. 581 Ascents. | Feb. | Aug. | ||
Km. | ° C. | °C. | °C. | °C. | ° C. | ° C. | ° C. |
o | - | 8.3 | - | - | + 0.3 | +18.2 | - |
I | + 5'4 | 6o | 5.5 | + 5.3 | - 1.4 | +15. I | 5'0 |
2 | + 0.5 | 1.7 | + 0.3 | + 0.7 | - 3.6 | + 102 | 0.5 |
3 | - 5.0 | - 3.3 | - 4.4 | - 4.0 | - 8.7 | + 4.8 | - 4.0 |
4 | -IO.3 | - 9.0 | -10.3 | - 9.4 | -14.7 | - 1.0 | - 9.2 |
5 | -16.6 | - 15.3 | -16.5 | -15.4 | -21.9 | - 7.1 | -15.4 |
6 | -24.2 | -22I | -23.0 | -21.9 | -28.9 | -13.3 | -22.0 |
7 | -30.2 | -29I | -30.2 | -29.0 | -36.1 | -19.5 | -29.0 |
8 | -37.4 | -36.2 | -37.0 | -36.2 | -43'7 | -27.1 | -36.2 |
9 | -46.4 | -43'2 | - | -43'5 | -50.1 | -33'8 | -43'2 |
IO | - | -49'0 | - | -49'3 | -55'4 | -39.5 | -49.2 |
Month. | Average temperature gradient per Ioo metres. | Altitude (metres). | Total Fall of Temperature from Ground upward. | ||||
|---|---|---|---|---|---|---|---|
Altitudes. | October to March. | April to September. | |||||
From o to | From moo to | Cloudiness | Cloudiness | Cloudiness | Cloudiness | ||
moo metres. | 2000 metres. i | 0-7. | 8-10. | 0-7. | 8-10. | ||
°C. | °C. | °C. | °C. | °C. | °C. | ||
January | oI I | 0.58 | 2000 | 8.24 | 7.63 | 15.33 | 14.18 |
February | 0.39 | 0.30 | 1800 | 7.22 | 6.60 | 14.20 | 12.97 |
March | 0.33 | 0.40 | 1600 | 6.28 | 6.04 | 13.01 | II .75 |
April | 0.73 | 0'48 | 1400 | 5'35 | 5'15 | 11.66 | 10.59 |
May | 0.90 | 0.66 | 1200 | 4.48 | 4'35 | 10.32 | 9.32 |
June | 0.99 | 0.72 | 1000 | 3.62 | 3.52 | 9.13 | 7.96 |
July | 0.96 | o67 | Boo | 2.20 | 2.82 | 7.55 | 6'65 |
August | o86 | o62 | 600 | 1.54 | 2.33 | 5'77 | 5'23 |
September | 0.77 | 0.58 | 400 | 0.65 | 1'85 | 3.88 | 3'63 |
October | 0.57 | 0.43 | 200 | 0.35 | 1.05 | 1.88 | 1.76 |
November | o36 | 0.53 | 0 | 0.00 | 0.00 | 0.00 | 0.00 |
December | 0.30 | 0.53 | - | - | - | - | - |
Year | 0.61 | 0'53 | |||||
The average eastward movement or the west wind at 3 km. above Germany is 107 m. per sec. or I° of longitude (at 45° latitude) in 42'4 minutes, or such as to describe the whole circumference of this small circle in 10.5 days. At the equator above the calm belt the velocity westward or the east wind as given by Krakatoa volcanic-dust phenomena was 34'5 m. per sec., on 30 of a great circle daily, or around the equator in 12.5 days, while its poleward movement was only, ° per day or 1.3 metre per second. The average motion of the storm centres moving westward in northern tropical and equatorial regions but eastward in the north temperate zone is at Lhe rate of one circumference or a small circle at latitude 45° in 19 days. Observations of the cloud movements gave Professor Bigelow the following results for the United States: Evidently, therefore, the great west wind (that James H. Coffin deduced from his work on the winds of the northern hemisphere and that William Ferrel deduced from his theoretical studies) represents with its gentle movement poleward a factor of fundamental importance. We must consider all our meteorological phenomena except at the equator as existing beneath and controlled, if not Temperature in Free Air over Europe 1899-1904. caused, by this general deep swift upper current of air that began as an ascending east wind above the calm equatorial air but speedily overflowed as west wind settling down to the sea-level in the temperate and polar regions as great areas of high pressure and dry clear cool weather contaning air on its return passage to the equator. The upper air is thrown easily into great billows, and wherever it rises the warm equatorial wind flows in beneath it, but when it descends we have blizzards and dry clear weather. It is a covering for the lower strata of air, it flows over them in standing waves and sometimes mixes with them at the surface of contact. It receives daily access:ons from below and gives out corresponding accessions to the lower strata, by a process of overturning such as has been studied theoretically by Margules and Bigelow.
At the fifth conference of the International Committee on Scientific Aeronautics (Milan, October 1906) Rykatchef presented the results of kite-work during 1904 and 1905 at Pavlosk, near St Petersburg, from which we select the results for these two years given in table at foot of page 270.
Many inversions occur during January below Iwo metres. The decrease is more rapid in summer than in winter and in clear weather than in cloud y, but of course these observations did not extend above the upper level of the cumulus cloud layer. A general survey of the existing state of knowledge of the upper atmosphere is given in the Report of the British Association for 1910.
Distribution of Aqueous Vapour.-The distribution of aqueous vapour is best shown by lines of equal dew-point or vapour tension, though for some purposes lines of equal relative humidity are convenient. The dew-point lines are not usually shown on charts, partly because the lines of vapour pressure are approximately parallel to the lines of mean temperature of the air, and partly because the observations are of very unequal accuracy in different portions of the globe. In general we may consider any isotherm as agreeing with the dew-point line for dew-points a few degrees lower than the temperature of the air. The distribution of moisture is quite irregular both in a horizontal and in a vertical direction. On charts of the world we may draw lines based on actual observations to represent equal degrees of relative humidity, or equal dewpoints and vapour pressures; but as regards the distribution of moisture in a vertical direction we are, in the absence of specific observations, generally forced to assume that the vapour pressure at any altitude h follows the average law first deduced from a limited number of observations by Hann, and expressed by the logarithmic equation, log e=log eo-h/6517, which is quite analogous to the elementary hypsometric formula, log p=log po-h/18400. Therefore, in general, the ratio between the pressure of the vapour and the pressure of the atmosphere at any altitude is represented by the approximate formula, log !e/p=log eo/po-h/Io091. Of course these relations can only represent average or normal conditions, which may be departed from very widely at any moment; they have, however, been found to agree remarkably with all observations which have as yet been published. The average results are given in the following table, which is abbreviated from one published by Hann, but with the addition of the work done by the U.S. Weather Bureau, as reduced by Dr Frankenfield in 1899. The vapour constituent of the atmosphere is not distributed according to the law of gaseous diffusion, but, like temperature and the ratio between oxygen and nitrogen, is controlled by other laws prescribed by the winds and currents, namely-convection.
Authority. | 1500 ft. | 2000 ft. | 3000 ft. | 4000 ft. | 5000 ft. | 6000 ft. | 7000 ft. | 8000 ft. | No. Obs. |
Kites. | o82 | 0.78 | 0.70 | 0.61 | 0.52 | 0.49 | 0.39 | 0.44 | 1123 |
(U.S.W.B.) | |||||||||
Balloons. | 0.97 | 0.96 | 0.87 | o68 | 0.44 | 0.59 | - | - | 4 |
(Hammon.) | |||||||||
Balloons. | 0.89 | o83 | o80 | o78 | o67 | o46 | 0.44 | - | 2 |
(Hazen.) | |||||||||
Balloons. | o84 | 0.80 | o66 | 061 | 0.50 | 0.54 | 0.4 1 | 0.37 | 15 |
(Hann.) | |||||||||
Mountains | 0.83 | 0.81 | o80 | o66 | 0.61 | 0.58 | 0.55 | 0.47 | 6 |
(Hann.) | |||||||||
Computed by Hann. | 0.85 | 0.81 | 0.72 | 0.65 | o58 | 0.52 | 0.47 | 0.42 | - |
Diminution of the Relative Vapour Pressure with Altitude. Note.-The vapour pressure at any altitude is supposea to be expressed as a fraction of that observed at the ground. When the altitudes are given in ft. Hann's formula becomes log e/eo = h/29539. From 78 high balloon voyages in Germany, 1887-1899, Suring deduced the average vapour pressure in millimetres as found in the first line of the table at foot of this page (see Wissenschaftliche Luftffahrten, Bd.III., and Hann, Lehrbuch, 1906, p. 169). The observations on mountains gave Hann the pressures in the second line. Sifting's figures result from the use of Assmann's ventilated psychrometer and are therefore very reliable.
Alt. Alt | Specific moisture. | Relative Humidity. | |
|---|---|---|---|
U.S.A. | Berlin. | ||
Km. | 0 | 0/ | |
0.0 | 100 | - | 77 |
0.5 | - | 65 | 71 |
10 | 0.76 | 65 | 71 |
1.5 | 0.65 | 59 | 62 |
2o | 0.55 | 59 | 57 |
2.5 | 0.47 | 45 | 58 |
3. o | 0.39 | - | 55 |
3.5 | - | - | 49 |
4.0 | o26 | - | 53 |
4.5 | - | - | 54 |
5. o | 0.17 | - | - |
5.6 | OII | - | - |
5.7 | 0.07 | - | - |
5.8 | 0.04 | - | - |
Altitude. Feet. | Relative Tension = e l eo. | Actual Weight Gr. per Cubic Foot. | Total Vapour in the Columns expressed as inches of Rain. | ||||||
|---|---|---|---|---|---|---|---|---|---|
80° F. | 7 0° F. | 60° F. | 5 o° F. | 80° F. | 70° F. | 60° F. | 5 0° F. | ||
o | 1000 | 10.95 | 7.99 | 5.7 6 | 4.09 | 0.0 | 0.0 | 0.0 | O.0 |
6000 | 0.524 | 5.75 | 4.1 9 | 3.02 | 2.14 | I3 | I. 0 | 0.7 | 0.5 |
12,000 | 0.275 | 3.01 | 2.20 | 1.58 | I. 12 | 2I | 15 | 1. I | o8 |
18,000 | 0.1 44 | 1.58 | 115 | 0.83 | 0.59 | 2.5 | I8 | 13 | 0.9 |
24,000 | 0.075 | o82 | o62 | 0.43 | 0.31 | 2.7 | 2.0 | I. 4 | 1.0 |
30,000 | 0.040 | 0.43 | 0.32 | 0.23 | 0.16 | 2.8 | 2. I | I. 5 | I .1 |
The vapour pressure in mm. in free air over Europe is best given by Suring's formula log i ,= log e 0 -6 (I -+ 2) where the altitude is to be expressed in kilometres. From this formula we derive the " specific moisture " or the mass of vapour contained in a kilogram of moist air as given in the following table whose numbers do not appreciably differ from " the mixing ratio " or quantity of moisture associated with a kilogram of dry air. The relative humidities vary irregularly depending on convection currents, but in clear weather when descending currents prevail they have been observed in America and over Berlin as shown in the third and fourth columns of the following table Observed Specific Moisture and Relative Humidity. The total amount of vapour in the atmosphere, according to Hann's formula, is between one-fourth and one-fifth of the amount required by Dalton's hypothesis, as is illustrated by the following table taken from an article by Cleveland Abbe in the Smithsonian Report for 1888, p. 410: Total Vapour in a Vertical Column that is saturated at its base. A heavy rainfall results from the precipitation of only a small percentage of the water contained in the fresh supplies of air brought by the wind; if all moisture were abstracted from the atmosphere it could only affect the barometer throughout the equatorial regions by 2.8/13.6 inches, or about two-tenths of an inch, while at the polar regions the diminution would be much less than one-tenth. Evidently, therefore, it is idle to argue that the fall of pressure in an extensive storm is to be considered as the simple result of the condensation of the vapour into rain.
km. | km. | km. | km. | km. | km. | km. | km. | km. | km. | km. | km. | km. | |
Alt. . | 0.5 | 1.0 | I. 5 | 2'0 | 2.5 | 3' o | 3.5 | 4.0 | 4.5 | 5.0 | 6o | 7.0 | 8o |
mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | |
Suring. | o83 | o68 | 0.51 | 0.41 | 0.34 | 0.26 | 0.20 | 0.17 | 0.14 | 0I I | 0.0 54 | 0.028 | 0.013 |
Hann . | o83 | 0.70 | 0.58 | 0.48 | 0'40 | 0.34 | o28 | 0.23 | 0.19 | 0.16 | - | - | - |
[PHYSICAL DATA |
Barometric Pressure.-The horizontal distribution of barometric pressure over the earth's surface is shown by the isobars, or lines of equal pressure at sea-level; it can also be expressed by a system of complex spherical harmonics. As the indications of the mercurial barometer must vary with the variation of apparent gravity, whereas those of the aneroid barometer do not, it has been agreed by the International Meteorological Conventions that for scientific purposes all atmospheric pressures, when expressed as barometric readings, must be reduced to one standard value of gravity, namely, its value at sea-level and at 45° of latitude. In this locality its value is such as to give in one second an acceleration of 9808 centimetres, or 32.2 English ft. per second. The effect of the variation of apparent gravity with latitude is therefore to make the mercurial barometer read too high, between 45° and the equator, and too low, between 45° and the pole. The gravity-correction to be applied to any mercurial barometric-reading at or near sea-level, in order to get the atmospheric pressure in Diminution of Pressure of Aqueous Vapour in the Free Air. standard units, should be given on the edge of a meteorological chart, unless the isobars shown thereon already contain this correction. On such charts it will be perceived that the barometric pressure at sea-level is by no means:uniform over the earth's surface, and daily weather charts show very great fluctuations in this respect, the lowest pressures being storm centres and the highest pressures areas of clear cool dry weather. But even the normal average charts show high pressures over the continents in the winter and low pressures over the oceans, these conditions being reversed in the summer time; moreover, Schouff (Pogg. Ann., 1832) first demonstrated that the average pressure in the neighbourhood of the equator is slightly less than under either tropic, and that there is a still more remarkable diminution of pressure from either tropic towards its pole. The exact statement of these variations of pressure with latitude was subsequently worked out very precisely by Ferrel, and forms the basis of his explanation of the general circulation of the earth's atmosphere and its influence on the barometer. The series of monthly charts for the whole globe, compiled by Buchan and published by the Royal Society of Edinburgh in 1868, as well as Buchan's later and more perfect charts in the meteorology of the " Challenger " Expedition, Edinburgh, 1889, and in Bartholomew's Atlas, first revealed clearly the fact that the distinct areas of high and low pressure which are located over the continents and the oceans vary during the year in a fairly regular manner, so that the pressure is higher over the continents in the winters season and lower in the summer season, the amount of the change depending principally upon the size of the continent. A part of this annual variation in pressure is undoubtedly introduced by the methods of reduction to sea-level; indeed, if the data of the lower stations are reduced up to the level of Io,000 or 15,000 ft., we sometimes find the barometric conditions quite reversed. These annual changes are intimately connected as cause and effect with the annual changes of temperature, moisture and wind; it is quite erroneous to say that the observed charted pressures control the winds; there is a reaction going on between the wind and the barometric gradient, the resistance and rotation of the earth's surface, such that the true relation between these factors is a complex but fundamental problem in the mechanics of the atmosphere.
The vertical distribution of pressure as deduced from observation shows a rate of diminution with increasing altitude very closely but not entirely accordant with the laws of static equilibrium, as first elaborated by Laplace in his hypsometric formula. The departures from this law of static equilibrium are sufficient to show that, if our atmosphere is really in a state of equilibrium, it must be a matter of dynamics and not of statics. The general average relation of the density of the air to the altitude and temperature, and the total pressure of the superincumbent atmosphere, are shown in the accompanying diagram (fig. 1), which is taken from a memoir on the equations of motion by Joseph Cottier, published in the U.S. Monthly Weather Review for July 1897. The diminution of pressure with altitude, as shown in this diagram for average conditions, but not for the temporary conditions that continually occur, follows a logarithmic law, and can undoubtedly be extended upwards for the normal atmosphere only to a height of 20 or 30 m., owing to our uncertainty as to the actual conditions in the upper portions of the atmosphere. This diagram is based upon the assumption that the atmosphere is in a state of convective equilibrium such that the ascending and descending masses expand and cool as they ascend, or contract and warm up as they descend, nearly but not quite in accordance with the adiabatic law of the change of temperature in pure gases.
The departure of atmospheric temperatures from the strictly adiabatic law, as shown by Cottier, is undoubtedly due largely to the heat absorbed by and radiated from moist or hazy or dusty air. In 1890, Abbe showed that a very moderate rate of radiation from the atmosphere suffices to explain the coolness of slowly descending air. The absorption by the atmosphere of radiations from the earth and sun, or the balance between warming by absorption and cooling by radiation, is the basis of the arguments of W. J. Humphreys (Astrophysics, Jan. 1909), and E. Gold (Proc. Roy. Soc., 1908, lxxxii., 45 A.), explaining the existence of the " thermal layer." The direct evaluation of this radiation and absorption has been attempted by many. The genuine law a(q - p) is adopted by Gold as closely representing nature, whence it follows that (I) the adiabatic rate of cooling in convection currents must cease at a height corresponding to one-half of the barometric pressure at sea-level; (2) an isothermal layer must exist at the level where the absorption of solar radiation equals that of the terrestrial and atmospheric radiation; (3)within this thermal layer convection is difficult or impossible; (4) above this region the vertical temperature gradient must depend essentially on radiation and is less than that needed for convective equilibrium; (5) below this level the atmospheric radiation exceeds the atmospheric absorption and vertical currents can only be kept up by the convection of heat or aqueous vapour from the earth's surface to the adjacent layer of air.
Limit of the Atmosphere
The limiting height of the atmosphere must be at some unknown elevation above 20 m. where the temperature falls to absolute zero. But the uncertainty of the various hypotheses as to the physical properties of the upper atmosphere forbids us to entertain any positive ideas on tkis subject at the present time. If we define the outer limit of the atmosphere as that point at which the diffusion of gases inwsrds just balances the diffusion outwards, then this limit must be determined not by the hypsometric formula, but by the properties of gases at low temperatures and pressures under conditions as yet uninvestigated by physicists.
Cloudiness
It is evident that the clouds (q.v.) are formed from clear transparent air by the condensation of the invisible moisture therein into numerous minute particles of water, ice or snow. Notwithstanding their transparency, these individual globules and crystals, when collected in large masses, disperse the solar rays by reflection to such an extent that direct light from the sun is unable to penetrate fog or cloud, and partial darkness results. In a general survey of the atmosphere the geographical distribution of the amount of cloudy sky is important. When the solar heat falls upon the surface of the cloud i t is so absorbed and reflected that, on the one hand, scarcely any penetrates to the ground beneath, while on the other hand the upper surface of the cloud becomes unduly heated. Even if this upper surface is completely evaporated, it may continually be renewed from below, and, moreover, the evaporated moisture mixing with the air renders it very much lighter specifically than it would otherwise be. Hence the upper surface of the cloud replaces the surface of the ground and of the ocean; the air in contact with it acquires a higher temperature and greater buoyancy, while the ground and air beneath it remain colder than they would be in sunshine. The average cloudiness over the globe is therefore intimately related to the density and circulation of the atmosphere; it was first charted in general terms by L. Teisserenc de Bort of Paris, about 1886. The manifold modifications of the clouds impress one with the conviction that, when properly understood and interpreted, they will reveal to us the most important features of the processes going on in the atmosphere. If the farmer and sailor can correctly judge of the weather several hours in advance by a casual glance at the clouds, what may not the professional meteorologist hope to do by a more careful study? Acting on this idea, in 1868 Abbe asked from all of his correspondent observers full details as to the quantity, kind and direction of motion of each layer of clouds; these were telegraphed daily for publication in the Weather Bulletin of the Cincinnati Observatory, and for use in the weather predictions made at that time. Since January 1872 similar data have been regularly telegraphed for the use of the U.S. Weather Bureau in preparing forecasts, although the special cloud maps that were compiled thrice daily have not been published, owing to the expense. These data were also published in full in the Bulletin of the International Simultaneous Meteorological Observations for the whole northern hemisphere during the years 1875-1884. Abbe's work on the U.S. Eclipse Expedition to the West Coast of Africa in1889-1890was wholly devoted to the determination of the height and motions of the clouds by the use of his special form of the marine nephoscope. The use of such a nephoscope is to be strongly recommended, as it gives the navigator a means of determining the bearing of a storm centre at sea by studying the lower clouds, better than he can possibly do by the observation of the winds alone. The importance of cloud study has been especially emphasized by the International Meteorological Committee, which arranged for a complete year of systematic cloud-work by national weather bureaus and individual observatories throughout the world from May 1896 to June 1897. In this connexion H. H. Clayton of Blue Hill Observatory published a very comprehensive report on cloud forms in 1906. The complete report by Professor F. H. Bigelow on the work done by the U.S. Weather Bureau forms a part of the annual report for 1899, and constitutes a remarkable addition to our knowledge of the subject. Some preliminary account of this work was published in the American Journal of Science for December 1899.
Although all the international cloud-work of1896-1897has now been published in full by the individual institutions, as in the case of the International Polar Research Work of 1883, yet a comprehensive study of the results still remains to be made. Some of these have, however, been brought together in Mohn's discussion of the observations by Nansen during the voyage of the " Fram " and also in Hann's Lehrbuch and in Bigelow's Report on Cloud-work. The mean altitudes of cirrus and strato-cumulus clouds resulted as follows.
'10111101= ' 'Il 11 ,???B?
.
, 1 44 1111111111 °^ °? ???? ???
1111
e e ?
FIG. I. |
APPARATUS AND METHODS] |
Place. | Lati- tude. | Cirrus. | St. cu. | Highest Cirrus. | Lowest Cirrus. |
° | kil. | kil. | ki!. | kil. | |
Cape Thordsen.. . | 7 8.5 | 7.3 | 2. 5 | - | - |
Bossekop,1838-1842. | 70 | 8.3 | I.3 | II8 | 5.5 |
Storlien.. .. . | 6 3'5 | 8.3 | 18 | - | - |
Upsala, 1884-1885. . „ 1896-1897. . | 60 60 | 8.9 8.2 | 2.3 I8 | 13'4 | 3'6 |
Pavlosk. ... . | 60 | 8.8 | 1.9 | 11.7 | 4.7 |
Dantzig.. .. . | 54.5 | 100 | 2.2 | - | - |
Irkutsk | 52'3 | 10.9 | 2.3 | -- | |
Blue Hill,1890-1891. | 42.5 | 9'o | 3.2 | - | - |
Potsdam, summer . | 52 | 9.1 | 2.2 | - | - |
„ winter . Blue Hill, summer. . | 52 42.5 | 8., 9.5 | 1.4 I'2 | - | - |
„ „ winter | 42.5 | 8.6 | i6 | 15'0 | 5.4 |
Toronto, winter er | 43'6 | - Io o | 15 | - | |
Washinton, summer g | Io | 2 | |||
winter | 39 | 9.5 | 2.4 | 16'5 | 5.0 |
Allahabad. .. . | 2 5'5 | 12'4 | 3.5 | - | - |
Manilla.. .. . | 15 | 109 | 2.0 | 18o | 4.0 |
6 8 | |||||||
m. | m. | m. | m. | m. | m. | ||
Bossekop. . | 6.5 | 7.3 | 125 | 1 5.4 | 1 9.0 | 2 4.4 | - |
Upsala. . | 9-I | 8.7 | 16.0 | 204 | 26.6 | - | - |
Potsdam. . | 9.3 | 103 | 16. 9 | 20.8 | 2 5.4 | - | - |
Blue Hill. . | 9.8 | 14. 2 | 1 7.1 | 34.3 | 34' 2 | (33) | - |
Toronto. . | 9.4 | 17.1 | 18.4 | 32.0 | 308 | 28.8 | - |
Washington 1 | (8.6) | 14.6 | 17.3 | 203 | 25.8 | (28.9) | 26.8 |
Allahabad . | 3.4 | 6.4 | 13.0 | 17.6 | 22.3 | 20.7 | 34.0 |
Manilla. . | 5.5 | 7.1 | 6.5 | 8o | 13.6 | 13o | 13.4 |
15 4 The annual average velocity of hourly movement in metres per second without regard to direction may be summarized as follows: The movements of the upper clouds are more rapid in winter than in summer at these northern stations, but among the median and lower clouds a retardation takes place apparently due to the ascending currents that form rain and snow. Above 8000 metres at Upsala the average velocity in winter exceeds 30 metres per second, whereas in summer it is 20; at Toronto and Blue Hill the absolute velocities are larger but in the same ratio. In the United States the maximum velocities from the west attain loo metres per second and over 80 or 70 metres per second are not rare, but in Europe the corresponding figures are 70, 60, 50. (See also Cloud.) Ii.-Meteorological Apparatus And Methods The observational basis of meteorology is the frequent and, if possible, continuous record of the temperature, moisture and barometric pressure at different altitudes in the free atmosphere, the direction and velocity of the wind, the rain and snow-fall, and the kind, amount and motion of the clouds. For Europe these data have been furnished with more or less accuracy and continuity by thousands of observers ever since 1653, when Ferdinand II., grand duke of Tuscany, organized a system of daily observations in Italy under the general supervision of Luigi Antinori. During the 19th century great efforts were made to obtain equally full records from all parts of the land and ocean, and thousands of navigators were added to the great corps of observers. Other matters have also been investigated, the most important being the intensity of radiation from the earth at night-time and from the sun by day-time, the optical phenomena of the sky, the amount of dust in the air, the electrical condition and the chemical constitution of the atmosphere. Although all the instruments used belong to the category of physical apparatus, yet certain points must be considered as peculiar to their use in connexion with meteorology.
Thermometer.-In using the thermometer to determine the temperature of the free air it is necessary to consider not merely its intrinsic accuracy as compared with the standard gas thermometer of the International Bureau of Weights and Measures at Paris, but especially its sluggishness, the influence of noxious radiations, the gradual change of its zero point with time, and the influence of atmospheric pressure.
[ 1 We have here inserted the Washington data as interpolated from the figures given by Hann, Lehrbuch, 1906, p. 282.] Sensitiveness.-The thermometer indicates the temperature of the outside surface of its own bulb only when the whole mass of the instrument has a uniform temperature. Assuming that by appropriate convection we can keep the surface of the thermometer at the temperature of the air, we have still to remember that ordinarily this itself is perpetually changing both in rapid oscillations of several degrees and in diurnal periods of many degrees, while the thermometer, on account of its own mass or thermal inertia, always lags behind the changes in the temperature of its own surface. On the other hand, radiant heat passes easily through the air, strikes the thermometer, and raises its temperature quite independently of the influence of the air whose temperature we wish to measure. The internal sluggishness or the sensitiveness of the thermometer is usually different for rising and for falling temperatures, and is measured by a coefficient which must be determined experimentally for each instrument by observing the rate at which its indications change when it is plunged into a well-stirred bath of water whose temperature is either higher or lower than its own. This coefficient indicates the rate per minute at which the readings change when the temperature of the surface of the bulb is one degree warmer or colder than the temperature of the bath. Such coefficients usually vary between '.0th of a degree centigrade for sluggish thermometers, and one or two degrees for very sensitive thermometers. Suppose, for instance, that the coefficient is onehalf degree, then when the rate of change in the temperature of the air is one degree per minute this is exactly the same as the rate of change which the thermometer itself undergoes when its own temperature is two degrees different from that of the air; consequently, the thermometer will lag behind the air temperature to that extent and by the corresponding amount of time, assuming that the air itself flows fast enough to keep the surface of the bulb at the air temperature. When the air temperature ceases to rise or fall, and begins to change at the same rate in the opposite direction, the thermometer will fail to record the true maximum or minimum temperature by an appreciable error depending upon the rapidity of the change, and will follow the new temperature changes with the same lag. For example, in the case just quoted, if a rising temperature suddenly changes to a falling temperature, the error of the thermometer at the maximum temperature will be two degrees, and yet the thermometer may be absolutely correct as compared with the standard when it is allowed five or ten minutes' time to overcome the sluggishness. It is very difficult to obtain the temperature of the free air at any moment within nth of a degree Centigrade, owing to the sluggishness of all ordinary thermometers and the perpetual variations in the temperatures of the atmospheric currents.
Radiation.-When a thermometer bulb is immersed in a bath of liquid all radiant heat is cut off, but when hung in the open air it is subject to a perpetual interchange of radiations between itself and all its surroundings; consequently its own temperature has only an indirect connexion with that of the air adjacent to it. One of the most difficult problems of meteorology is so to expose a thermometer as to 'cut off noxious radiations and get the true temperature of the atmosphere at a specific place and time. The following are a few of the many methods that have been adopted to secure this end: Melloni put the naked glass bulbs within open sheltering caps of perforated silver paper. Flaugergues used a protection consisting of a simple vertical cylinder of two sheets of silver paper enclosing a thin layer of non-conducting substance, like cotton or wool. The influence of radiation upon a thermometer depends upon the radiating and absorbing powers of its own surface; a roughened surface of lamp-black radiates and absorbs perfectly; one of chalk powder does nearly as well; glass much more imperfectly; while a polished silver surface reflects with ease, but radiates and absorbs with the greatest difficulty. Fourier proposed to use two thermometers side by side, one of plain glass and the other of blackened glass; the difference of these would indicate the effect of radiation at any moment; but instead of plain glass he should have used polished silver. His method was quite independently devised and used by Abbe in 1865 and 1866 at Poulkova, where the thermometers were placed within a very light shelter of oiled paper. In order to use this method successfully, both the black and the silvered thermometers should be whirled side by side inside the thermometer shelters (see Bulletin of the Philosophical Society of Washington for 1883). Various forms of open lattice-work and louvre screens have been devised and used by Glaisher, Kupffer, Stevenson, Stowe, Dove, Renou, Joseph Henry and others, in all of which the wind is supposed to blow freely through the screens, while the latter cut off the greater part of the direct sunshine and other obnoxious radiations by day, and also prevent obnoxious radiation from the thermometer to the sky by night. The Italian physicist Belli first proposed a special artificial ventilation drawing the fresh air from the outside and making it flow rapidly over the thermometer. Even before his day de Saussure, Espy, Arago and Bravais whirled the thermometer rapidly either by a small whirling machine, or by attaching it to a string and swivel and whirling it like a sling. When this whirling is done in a shady place excellent results are obtained. Renou and Craig placed the thermometer in a thin metallic enclosure or shelter, and whirled the latter. Wild established the thermometer [[[Apparatus And Methods]] in a fixed louvre shelter, but by means of a ventilating apparatus drew currents of fresh air from below into the shelter, where they circulated rapidly and passed out. In Germany, since 1885, Dr Assmann has developed the apparatus known as the ventilated psychrometer, in which the dry-bulb thermometer is placed within a double shelter of thin metallic tubing, and the air is drawn in rapidly by means of a small ventilating fan. In the observations made by Abbe on the cruise of the " Pensacola " to the West Coast of Africa, the dryand wet-bulb thermometers were enclosed within bamboo tubes and rapidly whirled. The inside of the wet-bulb tube was kept wet, so that its surface, being cooled by evaporation, could not radiate injuriously to the thermometer. In the system of exposure adopted by the U.S. Weather Bureau the dry and wet bulbs are whirled by a special apparatus fixed within the louvred shelter, which is about 31 ft. cube, and is placed far enough above the ground or building to ensure free exposure to the wind. In using the whirling and ventilating methods it is customary to take a reading after whirling one minute, and a second reading at the end of the second minute, and so on until no appreciable changes are shown in the thermometer. Of course in perfectly calm weather these methods can only give the temperature of the air for the exact locality of the thermometer. On the other hand, when a strong wind is blowing the indicated temperature is an average that represents the long narrow stream of air that has blown past the thermometer during the few minutes that are necessary in order that its bulb may obtain approximately the temperature of the air.
Change of Zero
All thermometers having glass bulbs, especially those of cylindrical shape, are sensitive to changes of atmospheric pressure. The freezing-point, determined under a barometric pressure of 30 in., or at sea-level, stands higher on the glass tube than if it had been determined under a lower pressure on a mountain top. Therefore delicate thermometers, when transported to great heights, or even during the very low pressure of a storm centre, read too low and need a correction for pressure. The zeropoint also changes with time and with the method of treatment that the bulb has received as to temperature. Owing to the slow adjustment of the molecules of the glass bulb to the state of stable equilibrium, their relations among themselves are disturbed whenever the bulb is freshly heated. At this time the freezing-point is temporarily depressed to an amount nearly proportional to the heating. The normal method of treatment consists in first determining the boiling-point of the thermometer, and, after a few minutes, the freezing-point. If this method is uniformly followed the two fiducial points will stay in permanent relation to each other. A thermometer that has been used for many years by a faithful meteorological observer has almost inevitably been going through a steady series of changes; in the course of ten years its freezingpoint may have risen by 2° or 3° F., and, moreover, it changes by fully a tenth of a degree between summer and winter. The only way completely to eliminate this source of error from meteorological work is to discard the mercurial thermometer altogether; but instead of adopting that course, the use is generally recommended of thermometers whose bulbs are made of a special glass, upon which heating and cooling have comparatively very little influence. Any argument as to secular changes in the temperature of the atmosphere is likely to be greatly weakened by the unknown influence of this source of error, as well as by changes in the methods of exposure and in the hours of observation.
Barometer
The barometer (q.v.) indicates the elastic pressure prevailing in gas or liquid at the surface of the mercury in the open tube or cistern, provided that the fluid at that point is in a state of quiet relative to the mercury.
An y motion of the air will have an influence upon the reading quite independently of the prevailing elastic pressure. The pressure within a mass of gas at any point is the summation of the effects due to the motions of the myriad molecules of the gas at that point; it is the kinetic energy of the molecules striking against each other and the sides of the enclosure, which in this case is the surface of the mercury in the cistern of the instrument. If the barometer moves with respect to the general mass of the gas there is a change in the pressure on the mercurial surface, although there may be none in the general mass of the free gas, and a barometer giving correctly the pressure of the air at rest within a room will give a different indication if the instrument or the air is set in rapid motion so that the air strikes violently against it. If the barometer moves with the air it will indicate the elastic pressure within the air. When the wind blows against an obstacle the air pressure is increased slightly on the windward side and diminished on the leeward side. It is thus obvious that in determining the pressure within the free atmosphere the exposure of the barometer must be carefully considered. The influence of a gale of wind is to raise the elastic pressure within a room whose window faces to the windward, but to lower the pressure if the window faces to the leeward. The influence of the draught up chimney, produced by the wind blowing over its summit, is to lower the pressure within the room. The maximum effect of the wind in raising the pressure is given by the formula, P - Po = 0.000 038 3 X V 2, where the pressure is given in inches and the velocity in miles per hour. This amounts to about one-tenth of an inch in a 50-m. wind, and to nearly fourtenths in a 100-m. wind. The diminution by a leeward window or a draught up chimney is usually less than this amount. This alteration in pressure, due to the local effect of wind, does not belong to the free atmosphere but to the method of exposure of the barometer, and can be eliminated only by methods first described by Abbe in 1882: it is a very different matter from the general diminution of pressure in the atmosphere produced by the movement of the wind over a rotating earth and by the centrifugal force within a vortex. The latter is an atmospheric phenomenon, independent of instruments and locality, which: in hurricanes and tornadoes may amount to several inches of the mercurial column. It is, however, quite common to find in the continuous records of pressure during a hurricane evidence of the fact that the low pressure due to the hurricane and the special diminution due to the exposure of the barometer are combined together, so that when the calm centre of a hurricane passes over a station the pressure temporarily rises by the amount due to the sudden stoppage of the wind and the local exposure effect.
The other sources of error that give rise to discrepancies in meteorological work relate to the temperature of the instrument, the sluggishness of the movement of the mercury, and the inevitable large secular changes in the correction for capillarity, due principally to the changes in the condition of the surfaces of the glass and the mercury, especially those that are exposed to the open air. The international comparisons of barometers show that discrepancies exist between the best normals or standards, and that ordinary barometers must always be compared with such standards at the temperatures and pressures for which they are to be used.
Anemometer
The wind is measured either by means of its pressure against any obstacle or by revolving apparatus that gives some idea of the velocity of its movement. The pressure is supposed to interest the engineer and navigator, but the velocity is the fundamental meteorological datum; in fact, the pressure of the wind varies with the nature of the obstacle, the method of exposure, the density of the air, and even the mass of rain carried along with it.
Pressure anemometers date from the pendulous tablet devised by Sir Christopher Wren about 1667, and such pressure plates continue to be used in an improved form by Russian observers. Normal pressure plates are used at a few English and Continental stations. The windmill anemometers devised by Schober and Woltmann were modified by Combes and Casella so as to make an exceedingly delicate instrument for laboratory use; another modification by Richard is extensively used by French observers. In the early part of the 19th century Edgeworth devised and Robinson perfected a windmill system in which hemispherical cups revolved around a vertical axis, and these have come into general use in both Europe and America. Many studies have been made of the exact ratio between the velocity of the wind and the rotations of the Robinson anemometer. The factor 3 is usually adopted and incorporated into the mechanism of the apparatus, but in ordinary circumstances this factor is entirely too large, and the recorded velocities are therefore too large. The whirling cups do not revolve with any simple relation to the velocity of the wind, even when this is perfectly steady. The relation varies with the dimensions of the cups and arms and the speed of the wind, but especially with the steadiness or gustiness of the wind. The exact ratio must always be determined experimentally for each specific type of instrument; in most instruments in actual use the factor for steady wind varies between 2.4 and 2.6. When the wind is gusty the moment of inertia of the moving parts of the instrument necessitates an appreciable correction; thus, when the gust is at its height the revolving parts receive an impetus that lasts after the gust has gone down, so that the actual velocity of the cups is too high. For this reason, also, comparisons and studies of anemometers made in the irregular natural winds of a free air are unsatisfactory. For the average natural and gusty winds at Washington, D.C., and on Mount Washington, N.H., and the small type of Robinson's anemometer used in the U.S. Weather Bureau Service, Professor C. F. Marvin deduced the table (see p. 275) for reduction from recorded to true velocity. This table involves the moment of inertia of the revolving parts of the instrument and the gustiness of the winds at Washington, and will therefore, of course, not apply strictly to other types of instruments or winds, for which special studies must be made.
Indicated Velocity. | True Velocity. | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
Miles. | 0 | I | 2 | 3 | 4 | s | 6 | 7 | 8 | q |
0 | 5.1 | 6o | 6.9 | 7.8 | 8.7 | |||||
10 | 9.6 | io4 | II3 | 12.1 | 12.9 | 13.8 | 14.6 | 15.4 | 16.2 | 17.0 |
20 | 17.8 | 18.6 | 19.4 | 20.22 | 21.0 | 21.8 | 22.6 | 23.4 | 24.2 | 24.9 |
30 | 25.7 | 20.5 | 27.5 | 280 | 28.8 | 29.6 | 30.3 | 31.1 | 31.8 | 32.6 |
4 0 | 33.3 | 34.1 | 34' 8 | 35' 6 | 3 6 '3 | 37' 1 | 37' 8 | 3 8 '5 | 39'3 | 40.0 |
5 o | 4 o.8 | 4 1.5 | 4 2.2 | 43' 0 | 43'7 | 44'4 | 45' 1 | 45'9 | 4 6 ' 6 | 47'3 |
60 | 48o | 4 8.7 | 49'4 | 5 0.2 | 5 0 '9 | 5 1 ' 6 | 5 2.3 | 53.0 | 53.8 | 54'5 |
7 0 | 55.2 | 55.9 | 5 6 ' 6 | 57'3 | 5 8 ' 0 | 5 8 '7 | 59.4 | 60I | 608 | 61.5 |
80 | 62.2 | 62.9 | 63.6 | 64.3 | 65o | 65.8 | 66.4 | 67.1 | 67.8 | 68.5 |
90 | 69.2 | - | - | - | ||||||
About 1842 a committee of the American Academy of Arts and Sciences experimentally determined, for a large variety of chimney caps, or cowls, or hoods, the amount of suction that produces the draught up a chimney, and shortly afterwards a similar committee made a similar investigation at Philadelphia (see Proc. Amer. Acad. i. 307, and Journal of Franklin Institute, iv. 101). These investigations showed that the open end of the chimney, acting as an obstacle in the wind, is covered by a layer of air moving more rapidly than the free air at a little distance, and that therefore between this layer and the aperture of the chimney there is a space ] within which barometric pressure is less than in the neighbouring free air. The draught up the chimney is due to the pressure of the air at the lower ezd or fireplace pushing up the flue into this region of low pressure, quite as much as it is due to the buoyancy of the heated air within the flue. From such experiments as these there has been developed the vertical suction-tube anemometer, as devised by Fletcher in 1867, re-invented by Hagemann in 1876, and introduced into England by Dines. In his Meteorological Apparatus Marvin's Table for the Reduction of Velocities, given by the smallsized Rcbinson's Anemometer in gusty winds. and Methods (Washington, 1887) Abbe gives the theory of this class of anemometers and develops the following additional forms: Two vertical tubes, whose apertures are respectively directed to the windward and the leeward, and within which are two independent barometers, give the means of determining the barometric pressure plus the wind pressure and minus the wind pressure respectively, so that both the velocity of the wind and the true barometric pressure can be determined. If instead of a simple opening at the top of the tube we place there horizontally the contracted Venturi's tube, we obtain a maximum wind effect, which gives an accurate measure of the wind velocity, and is the form recommended by Bourdon as an improvement on that of Arson. In all anemometers of this class the inertia of the moving parts is reduced to a minimum, and the measurement of rapid changes in velocity and of the maximum intensity of gusts becomes feasible. On the other hand, these researches have shown how to expose a barometer so that it shall be tree from the dynamic or wind effect even in a gale. It has only to be placed within a room or box that is connected with the free air by a tube that ends in a pair of parallel plane plates. When the wind blows past the end of this tube it flows between these plates in steady linear motion, and can produce no disturbance of pressure at the mouth of the tube if the plates are at a suitable distance apart. This condition of stable flow, as contrasted with permanent flow, was first defined by Sir William Thomson (Lord Kelvin) (see Phil. Mag., Sept. 1887). Such a pair of small circular plates can easily be applied to a tube screwed into the air-hole at the back of any aneroid barometer, and thus render it independent of the influence of the wind.
As to the exposure of the anemometer, no uniform rules have as yet been adopted. Since the wind is subject to exceedingly great disturbances by the obstacles near the ground, an observer who estimates the force of the wind by noticing all that goes on over a large region about him has some advantage over an instrument that can only record the wind prevailing at one spot. The practice of the U.S. Weather Bureau has been to insist upon the perfectly free exposure of all anemometers as high as can possibly be attained above buildings, trees and hills; but, of course, in such cases they give records for an elevated point and not for the ground. These are therefore not precisely appropriate for use in local climatological studies, but are those needed for general dynamic meteorology, and proper for comparison with the isobars and the movements of the clouds shown on the daily weather map.
Hygrometer.-Moisture floats in the atmosphere either as invisible vapour or as visible haze, mist and cloud. The presence of the latter generally assures us that the air is fully saturated. The total amount of both visible and invisible vapour contained in a unit volume of cloud or mist is directly determined by the Schwackhofer or Svenson hygrometer, or it may be ascertained by warming a definite portion of the air and fog and measuring the tension of the vapour by Edelmann's apparatus. Both these methods, however, are in practice open to many sources of error. If only invisible aqueous vapour is present we may determine its amount by several methods: (a) the chemical method, by absorbing and weighing it; (b) the dewpoint method, by cooling the air down to the temperature where condensation begins; (c) Edelmann's method, by absorbing the moisture chemically and measuring the change in vapour tension; (d) by adding vapour until the air is saturated, and measuring either the increased tension or the quantity of evaporation; (e) the psychrometric method, by determining the temperature of evaporation.
The wet-bulb thermometer, which is the essential feature of the last method, was used by Baume in 1758 and de Saussure in 1787, but merely as giving an index of the dryness of the air. The correct theory of its action was elaborated by many early investigators: Ivory, 1822; August, 1825; Apjohn, 1834; Belli, 1838; Regnault, 1845. From the last date until recent years no important progress was made in our knowledge of the subject, and it was supposed that the psychrometer was necessarily crude and unsatisfactory; but in its modern form it has become an instrument of much greater p recision, probably quite as trustworthy as the dew-point apparatus or other method of determining atmospheric moisture. In order to secure this accuracy the two bulbs must be of the same size, style and sensitiveness; the wet bulb must be covered with thin muslin saturated with pure water; both thermometers must be whirled or ventilated rapidly, but at the definite prearranged rate for which the tables of reduction have been computed; and, finally, both thermometers must be carefully sheltered against obnoxious radiations. In order to attain these conditions European observers tend to adopt Assmann's ventilated psychrometer, but American observers adopt Arago's whirled psychrometer, set up within an ordinary thermometer shelter. By either method the dew-point should be determined with an accuracy of one-tenth degree C. or two-tenths F. As a crude approximation, we may assume that the temperature of the dew-point is below the temperature of the wet bulb as far as that is below the dry bulb. A greater accuracy can be attained by the use of Ferrel's or Marvin's psychrometric tables or Grossman's formula. But the vapour tension over ice and over water as measured by Marvin and by Juhlin must be carefully distinguished and allowed for. The Smithsonian Meteorological Tables (ed. of 1908) and the new psychrometer tables by Bjerkeland for temperatures below freezing (Christiania, 1907) represent the present condition of our knowledge of this subject. Glaisher deduced empirically from a large mass of observations certain factors for computing the dew-point, but these do not represent the accuracy that can be attained with the whirled psychrometer, nor are they thoroughly satisfactory when used with Regnault's tables and the stationary psychrometer. Especially should their use be discarded when the wet bulb is greatly depressed below the dry bulb and the atmosphere correspondingly dry. For occasional use at stations, and especially for daily use by travellers and explorers, nothing can exceed the convenience and accuracy of the sling psychrometer, especially if the bulbs are protected from radiation by a slight covering of non-conducting material, or even metal, as was done by Craig in1866-1869for the stations of the U.S. Army Surgeon-General. The hair hygrometer gives directly the relative humidity or the ratio between the moisture in the air and that which it would contain if saturated. The very best forms perform very well for a time, and are strongly recommended by Pernter, and must be used in self-recording apparatus for balloons and kites; they are standardized by comparison with the ventilated psychrometer, which itself must be dependent on the standard dew-point apparatus.
Rain and Snow Gauge.-The simple instrument for catching and measuring the quantity of rain, snow or hail that falls upon a definite horizontal area consists essentially of a vertical cylinder and the measuring apparatus. The receiving mouth of the cylinder is usually terminated by a cone or funnel, so that the water running down through the funnel and stored in the cylinder is protected from evaporation or other loss. The cylinder is firmly attached to the ground or building, so that the mouth is held permanently at a definite altitude.
The sources of error in its use are the spattering into it from the ground or neighbouring objects, and the loss due to the fact that when the wind blows against the side of the cylinder it produces eddies and currents that carry away drops that would otherwise fall into the mouth, and even carries out of the cylinder drops that have fallen into it. As a consequence all the ordinary raingauges catch and measure too little rainfall. The deficit increases with the strength of the wind and the smallness or lightness of the raindrops and snowflakes. If we assume that the correct rainfall is given by a gauge whose mouth is flush with the level of the ground and is surrounded by a trench wide enough to prevent any spatter, then, on the average of many years and numerous observations with ordinary rain-gauges in western Europe, and for the average character of the rain in that region and the average strength of the attending winds, the deficit of rain caught by a rain-gauge whose mouth is i metre above the ground is 6% of the proper amount; if its elevation is 1 ft. above ground, the deficit will be 31%. This deficit increases as the gauges are higher above the ground in proportion approximately to the square root of the altitude, provided that they are fully exposed to the increase of wind that prevails at those altitudes. It is evident that even for [[[Apparatus And Methods]] altitudes of 5 or in ft. the records become appreciably discrepant from those obtained at the surface of the ground. The following table shows in the last column the observed ratio between the catches of gauges at various altitudes and those of the respective standards at the level of the ground. Unfortunately, there are no records of the force of the wind to go with these measurements; but we know that in general, and on the average of many years, corresponding with those here tabulated, the velocity of the wind increases very nearly as the square root of the altitude. Although this deficit with increasing altitude has been fully recognized for a century, yet no effort has been made until recent years to make a proper correction or to eliminate this influence of the wind at the mouth of the gauge. Professor Joseph Henry, about 1850, recommended to the observers of the Smithsonian Institution the use of the " pit-gauge." About 1858 he recommended a so-called shielded gauge, namely - a simple cylindrical gauge 2 in. in diameter, having a wide horizontal sheet of metal like the rim of an inverted hat soldered to it. This would undoubtedly diminish the obnoxious currents of air around the mouth of the gauge, but the suggestion seems to have been overlooked by meteorologists. In 1878 Prof. F. E. Nipher of St Louis, Missouri, constructed a much more efficient shield, consisting of an umbelliform screen of wire-cloth having about sixty-four meshes to the square inch. This shield seems to have completely annulled the splashing, and to have broken up the eddies and currents of wind. With Nipher's shielded gauges at different altitudes, or in different situations at the same altitude, the rain catch becomes very nearly uniform; but the shield is not especially good for snow, which piles up on the wire screen. Since 1885 numerous comparative observations have been made in Europe with the Nipher gauge, and with the " protected gauge " devised by Boernstein, who sought to prevent injurious eddies about the mouth of the gauge by erecting around it at a distance of 2 or 3 ft. an open board fence with its top a little higher than the mouth of the gauge. The boards or slats are not close together, but apparently afford as good a protection as the shield of Professor Nipher, and give good results with both snow and rain.
Situation and Size of Gauge. | Years of Record. | Altitude. | Relative Catch. | |
|---|---|---|---|---|
Metres. | °/ 0 10 | |||
Caine, 5-in. and 8-in.. . Castleton, 5-in. and 8-in. . | 3 | 1 0 2 , | 90 88 | |
Rotherham, 5-in.. . | 8 | 3 | 86 | |
St Petersburg: Central Physical | 4 | 85 | ||
Observatory, 10-in... . | io | 5 | 85 | |
6 | 84 | |||
London: Westminster Abbey | I | 9.1 | 77 | |
Emden. .. ... . | 2 | II | 72 | |
St Petersburg: Central Physi- cal Observatory. .. . | I | 13 | 68 | |
York: Museum. ... . | 3 | 13 | 80 | |
Calcutta: Alipore Observatory | 7 | 15 | 87 | |
Woodside: Walton-on-Thames Philadelphia: F r a n k f o r d | 1 | 15 | 73 | |
Arsenal | 3 | 16 | 95 | |
Sheerness: Waterworks . | 3 | 21 | 52 | |
Whitehaven: St James's | ||||
Church | to | 24 | 66 | |
St Petersburg: Central Physical | ||||
Observatory. ... . Paris: Astronomical Observa- | pp | 25 | 59 | |
tory . | 40 | 27 | 81 | |
Dublin: Monkstown.. . | 6 | 27 | 64 | |
Oxford: Radcliffe Observatory | 8 | 34 | 59 | |
Copenhagen: Observatory . | 4 | 36 | 67 | |
London: Westminster Abbey . | 1 | 46 | 52 | |
Chester: Leadworks . | 2 | 49 | 61 | |
Wolverhampton: Waterworks | 3 | 55 | 69 | |
York Minster. . | 3 | 65 | 60 | |
Boston: St Botolph's Church | 2 | 79 | 47 | |
Altitude and Relative Catch of Rain. In general it is now conceded by several high authorities that the measured rainfall must be corrected for the influence of the wind at the gauge, if the latter is not annulled by Nipher's or Boernstein's methods. A practicable method of measuring and allowing for the influence of the wind, without introducing any very hazardous hypothesis, was explained by Abbe in 1888 (see Symons's Meteorological Magazine for 1889, or the U.S. Monthly Weather Review for 1899). This method consists simply in establishing near each other several similar gauges at different heights above the ground, but in otherwise similar circumstances. On the assumption that for small elevations the diminution of the wind, like that of the rainfall, is very nearly in proportion to the square root of the altitude, the difference between the records for two different altitudes may be made the basis of a calculation which gives the correction to be applied to the record of the lower gauge, in order to obtain the rainfall that would have been caught if there were no wind. It is only when the catch of the gauge has been properly corrected for the effect of the wind on the gauge that we obtain numbers that are proper to serve for the purpose of determining the variation of the rainfall with altitude and locality, the influence of forests and the periodical changes of climate. Methods of measuring dew, frost, hail, sleet, glatteis and other forms of precipitation still remain to be devised; each of these has its thermodynamic importance and must eventually enter into our calculations.
It has been common to consider that the rain-gauge cannot be properly used on ships at sea, owing to the rolling and pitching of the vessel and the interference of masts and rigging; but if gauges are mounted on gimbals, so as to be as steady as the ordinary mariner's compass, their records will be of great importance. Experimental work of this sort was done by Mohn, and afterwards in 1882 by Professor Frank Waldo; but the most extensive inquiry has been that of Mr W. G. Black (see Journal Manchester Geographical Society, 1898, vol. xiv.), which satisfactorily demonstrates the practicability and importance of the marine rain-gauge.
Evaporometer
The moisture in the atmosphere comes from the surface of the earth or ocean by evaporation, a process which goes on continually, replacing the moisture that is precipitated as rain, hail, snow and dew, and maintaining the total quantity of the moisture in the atmosphere at a very uniform figure. The rate of evaporation depends on the temperature, the dryness, and the velocity of the wind. It is not so important to meteorologists to know where the moisture comes from as to know its amount in the atmosphere, and in fact no method has yet been devised for determining how much moisture is given up by any specific portion of the earth, or ocean, or forest. Our evaporometers measure the quantity of moisture given off by a specific surface of water, but it is so difficult to maintain this water under conditions the same as obtain in nature that no conclusions can be safely deduced as to the actual evaporation from natural surfaces. The proper meteorological use of these evaporometers is, as integrating hygrometers, to give the average humidity of the air, the psychrometer giving the conditions prevailing at any moment.
Among the many forms of evaporometer the most convenient is that devised by Piche, which may be so constructed as to be exceedingly accurate. The Piche evaporometer consists essentially of a glass tube, whose upper end is closed hermetically, whereas the lower end is covered by a horizontal disk of bibulous paper, which is kept wet by absorption from the water in the tube. As the water evaporates its descent in the tube is observed, whence the volume evaporated in a unit of time becomes known. So long as the paper remains clean, and the water is pure, the records of the instrument depend entirely upon the evaporating surface, the dryness of the air, and the velocity of the wind. Careful comparisons between the Piche and the various forms of absolute evaporometers were made by Professor Thomas Russell, and the results were published in the U.S. Monthly Weather Review for September 1888, pp. 235-239. By placing the Piche apparatus upon a large whirling machine he was able to show the effect of the wind upon the amount of evaporation. This important datum enabled him to explain the great differences recorded by the apparatus established at eighteen Weather Bureau stations; based upon these results, he prepared a table of relative evaporation within thermometer shelters at all stations. The actual evaporations from ground and water in the sunshine may run parallel to these, but cannot be accurately computed. It is probable that Professor Russell's computations are smaller than the evaporations from shallow bodies of water in the sunshine, but larger than for deep bodies, like the great lakes, and for running rivers. Recent elaborate studies of evaporation have been undertaken in Egypt and in South Africa - but perhaps the most interesting case occurs in southern California. Here the Colorado river, having broken through its bounds, emptied itself into a great natural depression and formed the so-called " Salton Sea," about 80 m. long, 20 wide and loo ft. deep, before it could be brought under control. This sea is now isolated, and will, it is hoped, dry up in eight or ten years. Meanwhile the U.S. Weather Bureau has established a large number of evaporation stations in and around it, and has begun the study not only of the relation between evaporation, wind and temperature, but of the eventual disposition of this evaporation throughout the atmosphere in the neighbourhood of the sea (see the Reports of Professor F. H. Bigelow in U.S. Monthly Weather Review, 1907-1909, as also the elaborate bibliography of evaporation in the same volumes). Although the influence of the evaporation on local climate is scarcely appreciable to our hygrometric apparatus, yet it is said to be so in the development and ripening and drying of the dates raised on the U. S. government experimental date farm " a few miles north-east of the Salton Sea.
APPARATUS AND METHODS] |
Nephoscope
The direction and apparent velocity of the motion of a cloud are best observed by means of the nephoscope, which has now become a necessary item in the outfit of any first-class meteorological station. Among the various forms of this instrument are the nephodoscope of Fornioni, the marine nephoscope of Fineman, the simple mirror with attachments used by Clayton, the cloud camera of Vettin, and the altazimuths of Mohn and Lettry. The most perfect form for use on land is that devised by Professor Marvin in 1896 for the U.S. Weather Bureau stations (see fig. 2); while the most convenient for use at sea is that devised and used FIG. 2. - Marvin's Nephoscope.
in 1889 by Professor Abbe on the cruise of the U.S. ship " Pensacola " to the west coast of Africa, but first described in the report of the International Meteorological Congress held at Chicago in August 1893.
The construction of this instrument is shown in figs. 3, 4, 5. In using it the observer looks down upon a horizontal mirror and observes the reflection of the cloud. By moving his eye he brings any cloudy point into coincidence with the reflection of a small fixed spherical knob K above the mirror, and keeps the images of the knob and the cloud coincident as they pass from the centre of the mirror to its edge. This line of motion shows the azimuth of the horizontal component of the cloud's motion. The course of the vessel is shown by the compass card and lubber line AF seen below the mirror. The apparent angular velocity of the cloud, as it would be if the cloud started from the zenith, is obtained by counting the seconds that elapse between its passage from the centre to the edge, or to a small circle inscribed within the edge. With Marvin's nephoscope two observers a short distance apart may easily determine the apparent altitude, and azimuth, and motion of any cloud, whence its true altitude and velocity may be computed. But when the observer uses Abbe's marine nephoscope on a vessel which is itself in motion he observes the resultant of his own motion and that of the cloud. If his vessel is under his control, so that he may change its velocity or direction at will, he easily determines this resultant for two different courses, and obtains data by which he is enabled to calculate the real altitude and velocity of the cloud in terms of his own velocity. As the marine nephoscope can be used on a wagon moving rapidly over a smooth road, or in a small boat on a smooth pond, almost as well as on a larger sea-going vessel, it becomes an instrument of universal application for cloud study. It is also equally convenient for observing the positions of auroras, halos, meteors, and other special phenomena. For the international work undertaken during the year 1898 the photographic camera established upon an alt-azimuth mounting, or the so-called photogram-meter, was especially developed. In this apparatus photographs of the clouds are taken simultaneously at two or more stations, and in each case the centre of the photographic plate has its altitude and azimuth determined. From this centre one can measure on the plate the additional angles required in order to fix the altitude and azimuth of any point that is photographed, and thus the dimensions of the whole visible cloud and its internal or differential motions can be determined, as well as its general motion. During the years1896-1898about twenty stations were occupied throughout the world for the purpose of determining accurately the altitudes and motions of every layer of cloud.
Sunshine Recorder
The ordinary meteorological record specifies the proportion of sky that appears to be covered with cloud, or the so-called cloudiness, usually expressed in tenths. The observer generally confines his attention to that portion of the sky within sixty degrees of the zenith, and ignores the lower zone, since the clouds that are found therein are often at so great a distance from him that their record is not supposed to belong to his locality. As the cloudiness - or its reciprocal, the sunshine - is supposed to be the most important item in agricultural climatology, and is certainly very important for dynamic meteorology, it is usually considered desirable to obtain more Lcomplete records than are given by only one or two specified hours of observation. To this end apparatus for recording sunshine, or, rather, the effect of cloudiness, is widely adopted. At least three forms are worth describing as being extensively used.
The Jordan photographic sunshine recorder consists of a cylinder enclosing a sheet of sensitive paper; the sun's rays penetrate through a small aperture, and describe a path from sunrise to sunset, which appears on this sheet after it has been properly washed with the fixing solution. Any interruption in this path, due to cloudiness or haze, is of course clearly shown, and gives at once the means of estimating what percentage of the day was clear and what cloudy. The modified form of the instrument devised by Professor Marvin has been used for many years at about forty Weather Bureau stations, but the original construction is still employed by other observers throughout the world. The Stokes-Campbell recorder consists of a globe of glass acting as a burning-glass. A sheet of pasteboard or a block oftwood at the rear receives the record, and the extent of the charring gives a crude measure of the percentage of full or strong sunshine. Many of these instruments are used at stations in Great Britain and the British colonies. The Marvin thermometric sunshine recorder consists of a thermometer tube, having a black bulb at the lower end and a bright bulb at the other. The excess of temperature in the black bulb causes a thread of mercury to move upwards, and for a certain standard difference of temperature of about 5° F., such as would be produced by the sun shining through a very thin cloud or haze, a record is made by an electric current on a revolving drum, and simply shows when during the day sunshine of a certain intensity prevailed, or was prevented by cloudiness. D. T. Maring, in the U.S. Monthly Weather Review for 1897, described an ingenious combination of the thermometer and the photographic register of cloudiness which is worthy of further development. It gives both the quantity of cloudiness and intensity of the sunshine on some arbitrary relative scale.
The intensity of the sunshine, as sometimes employed in general agricultural studies, is crudely shown by Violle's conjugate bulbs, which are thin copper balls about 3 in. in diameter, one of them being blackened on the outside and the other gilded. When exposed to the sunshine the difference in temperature of the two bulbs increases with the intensity of the sunshine, but as the difference is dependent to a considerable extent on the wind, the Violle bulbs have not found wide application. The Arago-Davy actinometer, or bright and black bulbs in vacuo, constitutes a decided improvement upon the Violle bulbs, in that the vacuous space surrounding the thermometers diminishes the effect of the wind. The physical theory involved in the use of the Arago-Davy actinometer was fully developed by Ferrel, and he was able to determine the coefficient of absorption of the earth's atmosphere and other data, thereby showing that this apparatus has considerable pretensions to accuracy. In using it as contemplated by Arago and Davy and by Professor Ferrel, we read simply the stationary temperature attained by the bright and black thermometers at any moment, whereas the best method in actinometry consists in alternately shading and exposing any appropriate apparatus so as to determine the total effect of the solar radiation in one minute, or some shorter unit of time; this method of using the Arago-Davy actinometer was earnestly recommended by Abbe in 1883, and in fact tried at that time; but the apparatus and records were unfortunately burned up. This so-called dynamic, as distinguished from the static, method was first applied by Pouillet in 1838 in using his pyrheliometer, which was the first apparatus and method that gave approximate measures of the radiant heat received from the sun. In order to improve upon Pouillet's work more delicate apparatus has been constructed, but the fundamental methods remain the same. Thus Angstrom has applied both Langley's bolometer and his own still more sensitive thermoelectric couple and balance method; Violle uses his absolute actinometer, consisting of a most delicate thermometer within a polished metal sphere, whose temperature is kept uniform by the flow of water; while Crova, with a thermometer within an enclosure of uniform temperature, claims to have attained an accuracy of one part in a thousand. Chwolson has reviewed the whole subject of actinometry, and has shown the greater delicacy of his own apparatus, consisting of two thin plates alternately exposed to and shielded from sunshine, F.whose differences of temperature are measured by electric methods.
As none of the absolute methods for determining the solar radiation in units of heat lend themselves to continuous registration, it is important to call attention to the possibility of accomplishing this by chemical methods. The best of these appears to be that devised by Marchand, by the use of a device which he calls the Phot-antitupimeter. In this the action of the sunlight upon a solution of ferric-oxalate and chloride of iron liberates carbonic acid [[[Apparatus And Methods]] gas, the amount of which can be measured either continuously or every hour; but in its present form the apparatus is affected by several serious sources of error.
FIG. 3. - Abbe's Marine Nephoscope. Horizontal Projection of Mirror.
The electric compensation pyrheliometer, as invented by Knut Angstrom (Ann. Phys., 1899), offers a simple method of determining accurately the quantity of radiant energy. He employs two blackened platinum surfaces, one of which receives the radiations to FIG. 4. - Abbe's Marine Nephoscope. Horizontal Projection of Compass.
be measured, while the other is heated by an electric current. The difference of temperature between the two disks is determined by a thermocouple, and they are supposed to receive and lose the same amount of energy when their temperatures are the same. A Hefner FIG. 5. - Abbe s Marine Nephoscope. Vertical Section.
lamp is used as an intermediate standard source of radiation, and alternate observations on any other source of radiant heat give the means of determining their relation to each other. By means of two such instruments Angstrom secured simultaneous observations on the intensity of the solar radiation at two points, respectively, 360 and 3352 metres above sea-level, and determined the amount of heat absorbed by the intermediate atmosphere. An accuracy of 1 - 1000 appears to be attainable, and this apparatus is now being widely used. The records of1901-1905have already given rise to the belief that there is a variation in our insolation that may eventually be traced back to the sun's atmosphere.
Meteorograph
The numerous forms of apparatus designed to keep frequent or continuous register of the prevailing pressure,. temperature, moisture, wind, rainfall, sunshine, evaporation, and other phenomena are instruments that belong peculiarly to meteorology as distinguished from laboratory physics. Such apparatus may be broadly divided into several classes according as the records are obtained by the help of photography, or electricity, or by direct mechanical action. The prevailing tendency at present is in favour of apparatus in which the work of the recording pen is done by a falling weight, whose action is timed and limited by the making and breaking of electric currents by the meteorological apparatus proper. The most serious defect in such instruments, even when kept in good working order, is a want of sensitiveness commensurate with the desired openness of scale. It is very important that a fraction of a minute of time should be as recognizable as one-tenth of a degree of temperature; one thousandth of an inch of barometric pressure, and velocities of one hundred miles per hour, as well as rapid changes in all these elements, must be measurable. But instruments whose scales are large enough to record all these quantities are usually so sluggish as regards time that the comparison of the records is very unsatisfactory. In order to study the relationships between temporary and fleeting phenomena, it is necessary that all instruments should record upon the same sheet of paper, so that the same time-scale will answer for all.
The instruments that respond most nearly to the general needs of meteorology are the various forms of meteorographs devised by Wild for use at St Petersburg, by Sprung and Fuess for use at Hamburg and Berlin, and by Marvin for Washington. The photographic systems for pressure and temperature introduced many years ago at stations in Great Britain and the British colonies are not quite adequate to present needs. The portable apparatus manufactured by Richard Freres at Paris is in use at a very large number of land stations and on the ocean, and by giving special care to regular control-observations of time, pressure and temperature, important results may be obtained; but in general the timescales are too small, and the unknown sources of error too uncertain, to warrant implicit reliance upon the records.
ZWI signed:
KSF