Meteorology
From LoveToKnow 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.
| Table of contents |
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, o 04; ammonia and all other gases, less than o oi 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 0 000 239° 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 (o 000 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 o 4 at the violet end of the spectrum and about o 8 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 |
| i 60 | 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 o 2375-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 0 001 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 o 18 according to Stefan, or o 1980 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+o o03 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 - o 0026 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 | 6 o | 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 | 6 o | 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 | + 1 4 | + 4'7 | +13.6 | + 7'7 |
| I. O | - O. 2 | + 2.4 | +11.8 | + 6.I |
| I. 5 | - 0.2 | + o I | 9.7 | + 4.0 |
| 2.0 | - 1.4 | - 2.1 | 7.3 | + 2 2 |
| 2 '5 | - 3'7 | - 4.3 | 5'0 | + 0'4 |
| 3. o | - 6. o | - 6.4 | 2.1 | - 1 7 |
| 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 |
| 6 o | -23.7 | -25.2 | -14.8 | - 18.7 |
| 7 o | -31.5 | -32.0 | -2 I .7 | -25.8 |
| 8 o | -39'0 | -39.0 | -29.3 | -33'5 |
| 9.0 | -46.9 | -46.7 | -38 o | -41.4 |
| Io o | -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 | -I 7 | 179 | -1.1 | 195 | 10.0 |
| 3,000 „ | 55 | -3'9 | 79 | -3'6 | 119 | -2.8 | 127 | Io 7 |
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. | 2 0 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 | 6 o | 5.5 | + 5.3 | - 1.4 | +15. I | 5'0 |
| 2 | + 0.5 | 1.7 | + 0.3 | + 0.7 | - 3.6 | + 10 2 | 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 | -22 I | -23.0 | -21.9 | -28.9 | -13.3 | -22.0 |
| 7 | -30.2 | -29 I | -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 | o I 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 | o 67 | Boo | 2.20 | 2.82 | 7.55 | 6'65 |
| August | o 86 | o 62 | 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 | o 36 | 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 10 7 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. | o 82 | 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 | o 68 | 0.44 | 0.59 | - | - | 4 |
| (Hammon.) | |||||||||
| Balloons. | 0.89 | o 83 | o 80 | o 78 | o 67 | o 46 | 0.44 | - | 2 |
| (Hazen.) | |||||||||
| Balloons. | o 84 | 0.80 | o 66 | 0 61 | 0.50 | 0.54 | 0.4 1 | 0.37 | 15 |
| (Hann.) | |||||||||
| Mountains | 0.83 | 0.81 | o 80 | o 66 | 0.61 | 0.58 | 0.55 | 0.47 | 6 |
| (Hann.) | |||||||||
| Computed by Hann. | 0.85 | 0.81 | 0.72 | 0.65 | o 58 | 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 | 1 00 | - | 77 |
| 0.5 | - | 65 | 71 |
| 1 0 | 0.76 | 65 | 71 |
| 1.5 | 0.65 | 59 | 62 |
| 2 o | 0.55 | 59 | 57 |
| 2.5 | 0.47 | 45 | 58 |
| 3. o | 0.39 | - | 55 |
| 3.5 | - | - | 49 |
| 4.0 | o 26 | - | 53 |
| 4.5 | - | - | 54 |
| 5. o | 0.17 | - | - |
| 5.6 | O II | - | - |
| 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 | I 3 | I. 0 | 0.7 | 0.5 |
| 12,000 | 0.275 | 3.01 | 2.20 | 1.58 | I. 12 | 2 I | 1 5 | 1. I | o 8 |
| 18,000 | 0.1 44 | 1.58 | 1 15 | 0.83 | 0.59 | 2.5 | I 8 | 1 3 | 0.9 |
| 24,000 | 0.075 | o 82 | o 62 | 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 | 6 o | 7.0 | 8 o |
| mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm. | |
| Suring. | o 83 | o 68 | 0.51 | 0.41 | 0.34 | 0.26 | 0.20 | 0.17 | 0.14 | 0 I I | 0.0 54 | 0.028 | 0.013 |
| Hann . | o 83 | 0.70 | 0.58 | 0.48 | 0'40 | 0.34 | o 28 | 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 980 8 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.
' 101 11101= ' 'Il 11 ,? ? ? B?
.
, 1 44 1
111111111
°^ °? ??
?? ???
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 | II 8 | 5.5 |
| Storlien.. .. . | 6 3'5 | 8.3 | 1 8 | - | - |
| Upsala, 1884-1885. . „ 1896-1897. . | 60 60 | 8.9 8.2 | 2.3 I 8 | 13'4 | 3'6 |
| Pavlosk. ... . | 60 | 8.8 | 1.9 | 11.7 | 4.7 |
| Dantzig.. .. . | 54.5 | 10 0 | 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 | i 6 | 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 | 10 9 | 2.0 | 18 o | 4.0 |
| 6 8 | |||||||
| m. | m. | m. | m. | m. | m. | ||
| Bossekop. . | 6.5 | 7.3 | 12 5 | 1 5.4 | 1 9.0 | 2 4.4 | - |
| Upsala. . | 9-I | 8.7 | 16.0 | 20 4 | 26.6 | - | - |
| Potsdam. . | 9.3 | 10 3 | 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 | 30 8 | 28.8 | - |
| Washington 1 | (8.6) | 14.6 | 17.3 | 20 3 | 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 | 8 o | 13.6 | 13 o | 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 g