CHAPTER XIV THE ANALYSIS OF AIR-MOVEMENT INTO THE Dates of the first daily weather-maps published by the meteorological services 1861. Synoptic weather charts of the Indian Ocean for the months of Jan., Feb. Sept. 1873-Nov. 1876. Hoffmeyer's charts of the North Atlantic 1877-87. Charts of the Northern Hemisphere published by the U.S. Signal 1880-1910. Daily synoptic charts of the N. Atlantic published by the Danish Aug. 1882-Aug. 1883. Daily charts of the N. Atlantic published by the Meteoro- 1900-14. Dekaden Bericht of the Deutsche Seewarte-daily charts of the North 1912-14. Daily charts of the North Atlantic issued with the Weekly Weather 1914. Daily charts of the Northern Hemisphere. U.S. Weather Bureau 1923. Daily charts of the Pacific Ocean published by the Imperial Marine Daily charts of the South Atlantic Ocean for the period of twelve consecutive THE subject of this chapter has been treated by Hildebrandsson and Teisserenc de Bort1 in a manner which is exemplary in every sense of the word. The first volume of their treatise is devoted to it and is illustrated by lithographic reproductions of maps from the original documents which make the work of peculiar value for the student of meteorology. Meteorological publications are of a special character: they are issued as official documents and are distributed much more by presentation and exchange than by sale; they are international and the international exchange extends only to the few libraries in any single country that can reciprocate the exchange. The library of the British Museum, for example, is not 1 Les bases de la météorologie dynamique, tome 1, Gauthier-Villars et Fils, Paris, 1907. entitled to receive the official publications of the meteorological services of foreign countries, neither are the libraries of the provincial towns nor those of the universities. Such publications may be found in a few specialised libraries but even there the multiplicity, and variety in form, are so great that the recipients do not always keep their rare documents accessible to the student. Consequently we may pay a tribute of admiration to the public spirit of the two distinguished meteorologists which led them to put together in book-form specimens of many of the original documents of which the copies are now out of reach. It is a contribution to science of a kind which is rare, and quite intelligibly so, because no one can suppose that it is remunerative in any other way than from the satisfaction of having made a permanent contribution to a science of great interest but of no less great difficulty. Here we must restrict ourselves to a brief abstract of the story which is set out in the first volume of Les Bases, and advise the reader to refer to the volume itself for further particulars and details. EARLY CHARTS OF THE DISTRIBUTION OF THE The story begins with Halley's map of the winds of the globe (fig. 102) taken from his celebrated memoir in the Philosophical Transactions of 1686, entitled 'An historical account of trade-winds and monsoons observable in the seas between and near the tropics with an attempt to assign the phisical cause of the said winds.' The ordinary facts about trade-winds and monsoons are now a commonplace of physical geography, but Halley adds to the conventional description two notes that are worth repeating; first that off the coast of Africa in the Gulf of Guinea, close to the region of the South East tradewinds, the predominant direction of the wind is from West, and secondly, that in the China Sea the alternation of wind in the monsoon is, between South in the summer season and North in the winter, instead of between South West or West and North East or East as in the Indian Ocean. Connecting these with the etesian winds of the Mediterranean and Egypt which blow during the summer months from the North, we get three sides of a summer circulation which proclaims the monsoon to be part of the chief cyclonic circulation of the whole globe. Halley's proposed physical cause is the ascent of warm air in the hotter regions of the globe, or more generally the distribution of solar radiation which causes a belt of maximum temperature near the equator and consequent ascent of warm air with a flow thitherward along the surface from either side, and calm or doldrums in the belt itself. An extension of the idea of air rising in the hotter regions to be replaced by air flowing along the surface expresses Halley's explanation of the West winds of the Guinea coast, and the alternation of the monsoons in correlation with the alternation of the relative temperatures of land and sea in the East Indian region. Halley's leading idea of the rising of warm air also passed into the literature of science; thermal convexion is its technical name. It has been regarded as a sufficient basis of many expositions of meteorological phenomena, including indeed those of the tropical revolving storm. The study of the upper air has taught us to ask how far in ordinary circumstances such convexion can be expected to extend, and further to ask for the study of details in respect of that displacement of the risen air of the trade-winds and monsoons; and altogether the convexion of warm air has proved to be a subject requiring all the investigation that is to be expressed in Vol. III of this book instead of being a simple process that even a school-boy could easily understand and apply. As between the tropical "high-pressure" and the equator a sufficient proximate cause is the relatively lower pressure of the equatorial belt, but to claim that the low pressure of the equator compared with the high pressure of the tropic is the simple expression of the difference of temperature between the two would contravene all the laws of meteorological procedure. The direction of the trade-wind was the next subject for inquiry. The replacement of Halley's convected air by flow inwards from both North and South would, it was supposed, give rise to a North wind in the Northern hemisphere and a South wind in the Southern, but the winds were North East and South East, not North and South. Halley merely suggested that the wind followed the sun from East to West. An explanation was offered by George Hadley in 1735, also in a paper in the Transactions of the Royal Society. The physical cause in this case was given as the rotation of the earth from West to East with the consequent more rapid motion of the equatorial belt as compared with the tropics. The explanation was subsequently re-invented by John Dalton, who writes as follows1: Now in order to perceive the reason of these facts, it must be remembered, that the heat is at all times greatest in the torrid zone, and decreases in proceeding Northward, or Southward; also, that the poles may be considered as the centres of cold at all times: hence it follows, that, abstracting from accidental circumstances, there must be a constant ascent of air over the torrid zone, as has been observed, which afterwards falls Northward and Southward, whilst the colder air below is determined by a continual impulse towards the equator. And, in general, wherever the heat is greatest, there the air will ascend, and a supply of colder air will be received from the neighbouring parts. These, then, are the effects of the inequality of heat. The effects of the earth's rotation are as follow: the air over any part of the earth's surface, when apparently at rest or calm, will have the same rotatory velocity as that part, or its velocity will be as the co-sine of the latitude; but if a quantity of air in the Northern hemisphere receive an impulse in the direction of the meridian, either Northward or Southward, its rotatory velocity will be greater in the former case, and less in the latter, than that of the air into which it moves; consequently, if it move Northward, it will have a greater velocity Eastward than the air, or surface of the earth over which it moves, and will therefore become a SW wind, or a wind between the South and West. And, vice versa, if it move Southward, it becomes a NE wind. Likewise in the Southern hemisphere, it will appear the winds, upon similar suppositions, will be NW and SE respectively2. 1 Meteorological Observations and Essays, by John Dalton, D.C.L., F.R.S., 2nd edition, Manchester, 1834, p. 85. 2 "M. De Luc is the only person, as far as I know, who has suggested the idea of the earth's rotation altering the direction of the wind, which idea we have here pursued more at large. Vide Lettres Physiques, etc. Tom. v, Part 2, Let. cxlv." In the Preface to the same volume Dalton remarks: The second essay, containing the theory of the trade-winds, was, as I conceived when it was printed off, original; but I find since, that they are explained on the very same principles, and in the same manner, in the Philosophical Transactions for 1735, by George Hadley, Esq., F.R.S.... I cannot help observing here, that the following fact appears to be one of the most remarkable that the history of the progress of natural philosophy could furnish. Dr Halley published in the Philosophical Transactions, a theory of the trade-winds, which was quite inadequate, and immechanical, as will be shown, and yet the same has been almost universally adopted; at least I could name several modern productions of great repute in which it is found, and do not know of one that contains any other. The same gentleman published, through the same channel, his thoughts on the cause of the aurora borealis [associated with the earth's magnetic field], as mentioned above, which must then have appeared the most rational of any that could be suggested, and yet I do not find that anybody has afterwards noticed it, except Amanuensis." On the other hand, G. Hadley, Esq. published in a subsequent volume of the said Transactions, a rational and satisfactory explanation of the trade-winds; but where else shall we find it? We have already noticed a glimpse of a circulation in the upper air, opposite to that at the surface, in the power of attracting clouds attributed by Theophrastus to Kaikias, the North Easter. The next step in our progress to a knowledge of the general circulation is the repetition of this curious behaviour of the North Easter in the case of the North East trade-wind, which by the observations of von Humboldt in 1798 and subsequently by Leopold von Buch in 1815 and others was found to be reversed at the Peak of Tenerife. Similar phenomena were observed by Fouqué on the mountains of the Açores and by Goodrich and others on the peak of Mauna Loa in the Sandwich Islands. This led to the further suggestion, we know not exactly on what grounds, that the South Westerly wind over the top of the North Easter came to the earth's surface where there were known regions of prevailing Westerlies and South Westerlies, and that the main currents over the earth's surface were from South West and North East. In this manner H. W. Dove, at one time a meteorological king, “the founder of the entire superstructure of accurate climatological knowledge," was led to look upon the general circulation of the atmosphere as consisting of an equatorial current and a polar current, and to regard the vicissitudes of weather in the temperate zone as incidents in the conflict between these two currents. The theory is given in the very celebrated meteorological book Das Gesetz der Stürme or Law of Storms. A special ground for taking this view of the importance of the conflicting currents is the law of gyration which shows how during the passage of a storm the wind changes from the equatorial to the polar side. We are thus confirmed in the suggestion that the local circulations, which may be tropical hurricanes or cyclonic depressions with all the complications of their form, are to be regarded rather as incidents in the general circulation than as separate and independent phenomena. In spite of the fact that the general circulation must always be regarded as having its part in every meteorological problem we shall take the liberty of passing over the representations of the general circulation which were elaborated by Maury1, Ferrel2 and James Thomson3; first because they divide the earth into zones of latitude in which parallel transverse winds are shown all round the several zones, regardless of the difficulty or impossibility of adjusting such an arrangement to any admissible scheme of pressure; and secondly because they show, similarly distributed, an exchange between upper and lower air. In presenting a notion of the general circulation of the atmosphere we desire to rest upon observations, and we regard observations of pressure as among the most important. Dove rendered a signal service to the observational representation of the general circulation by producing monthly maps of isotherms of the globe which are to this day the foundation of the maps of temperature in many atlases. His monthly isotherms which reviewed observations of temperature from 1729 onwards were preceded, as we have already noticed, by Humboldt's chart (fig. 103), also by a chart of mean isotherms for the year by L. F. Kämtz in his Lehrbuch der Meteorologie (Halle, 1832), which included isogeotherms by Kupffer5. Kämtz also noted the existence of two poles of cold in the Northern hemisphere, and in this view he was in agreement with Sir David Brewster and Sir John Herschel. Dove however controverted the view. We also find two charts by Mahlmann, 1836 and 1840, two others by Berghaus, 1838 and 1839, and preliminary charts by Dove in 1848. Here we add the titles of other illustrations of the normal distribution of the meteorological elements which have been reproduced by Hildebrandsson and Teisserenc de Bort. They represent successive steps in the advancement of our knowledge of the normal general circulation of the atmosphere. The general result of these contributions forms in great part the subject of Vol. II in which a representation of the present state of our knowledge will be attempted. Temperature: Mean annual temperature by Buchan 1868; monthly and annual charts 1871 and 1889 (Challenger Report). Teisserenc de Bort 1881 (4 maps). Annual and monthly charts by J. Hann 1895 and 1901. North polar charts, Mohn 1905. Monthly isanomalies, Teisserenc de Bort and Wild 1879. Annual amplitudes, Supan and Wild (Hann's edition of Berghaus's Atlas). Isotherms and Isanomalies for Russia and Europe by Wild 1881, for Spain by Teisserenc de Bort 1880, for Europe by Ekholm 1899. Chart illustrating the march of the isotherm of o° C and 12° C in spring and autumn, Hildebrandsson and Högbom 1880 and 1883. Annual isotherms and isanomalies for the sea-surface by W. Köppen. 1 The physical geography of the sea, by M. F. Maury, London, Sampson Low, Son & Co. 1855. 2'Essay on the winds and currents of the ocean,' by W. Ferrel, Nashville Journal of Medicine and Surgery, vol. xI, nos. 4 and 5, 1856. The paper is reprinted in Professional papers of the Signal Service, no. 12, Washington, 1882. The grand currents of atmospheric circulation,' British Association meeting, 1857. 4 H. W. Dove, Verbreitung der Wärme, 1852. 5 Über die mittlere Temperature der Luft und des Bodens auf einige Punkten des östlichen Russlands,' Pogg. Ann. 1829. • Physikalischer Atlas, Gotha, 1838; 2te Auflage, 1849. |