Downloaded 10/09/21 07:23 PM UTC Sydney Chapman 1 Harry Wexier, 1911-1962, First Wexier Memorial Lecturer Excellent and Devoted Student Ol Our Wonderful

Mthe r Memorial lecture

At its meeting in October 1963, the Council of the American tinguished Public Service Award, the National Civil Service Meteorological Society approved establishment of an annual League Career Service Award, and (posthumously) the Carl- lectureship in honor of the late Dr. Harry Wexler. At a sub- Gustaf Rossby Award of the American Meteorological Society sequent meeting, January 1964, the Council voted that the "for his contributions to knowledge of the atmosphere heat Wexler Lecture be given at the Annual Meeting of the So- balance and dynamic anticyclogenesis, for his interdisciplinary ciety, preferably in the evening of the first day of scientific studies in meteorology, oceanography, and glaciology, and for sessions. Dr. Sydney Chapman was invited to present the his outstanding leadership in international programs in the first Wexler Lecture on Monday evening, January 25, 1965. atmospheric sciences." Dr. Wexler, Director of Meteorological Research for the Dr. Sydney Chapman, internationally esteemed geophysicist, United States Weather Bureau for fifteen years, was taken is a fitting choice to be the first Wexler Lecturer. Staff mem- from the research scene at the time when so much of the ber of the High Altitude Observatory, Boulder, Colorado, research that he had inspired and participated in was just beginning to come to fruition. He was one of the first to advisory scientific director of the Geophysical Institute, Uni- consider and advocate the use of artificial satellites for me- versity of Alaska, and senior research scientist at the Institute teorological purposes as well as one of the first scientists of of Science and Technology, University of Michigan, Dr. Chap- stature to express concern about the effect of rocket and man has made investigations of the atmospheric tides, of the nuclear explosions on the atmosphere. He served on many composition of the atmosphere, of the emission of nocturnal national and international committees. His interest in the light and of the ionization of the higher atmosphere, all atmosphere extended from pole to pole—he was Chief Sci- testifying to the extent and breadth of his interest in the entist for the U. S. Antarctic Program of the International atmosphere. Geophysical Year—and from climatology to hurricanes—he As president of the Special Committee of the International participated in the first aircraft penetration of an Atlantic Geophysical Year, he has exemplified the spirit of interna- hurricane. His many awards in recognition of his notable tional scientific cooperation. He is an Honorary Member of contributions to the atmospheric sciences included the Losey the American Meteorological Society, as well as of the so- Award of the Institute of the Aerospace Sciences, the U. S. cieties and academies of many other nations. Most recently Air Force Award for Exceptional Service, the Department of he has been presented with the Copley Medal, the highest Commerce Exceptional Service Medal, the U. S. Navy Dis- award of The Royal Society, London.

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His background To all those of us who had enjoyed the privilege of personal friendship with Harry and contributions Wexier, the news that this Society had created a Memorial Lectureship in his honor was both agreeable and in no way surprising. His untimely death, which we can never cease to regret, took some decades from the expected duration of his career. But despite this, he achieved high distinction in our great branch of natural science, and played a mem- orable part in American and World meteorology. There may, indeed, be some encour- agement for younger scientists in the example he gave of how much can be achieved, how permanent a mark may be made, in a career so relatively brief. No one, I feel sure, will dispute the fitness of the designation of Harry Wexier, in the title of this first memorial lecture of the series, as an excellent and devoted student of our wonderful atmosphere. Let me recall some phrases from a tribute by R. C. Sutcliffe, F.R.S. (1962), his opposite number in British meteorology, who is certainly more capable than I am of fully understanding and appreciating Harry Wexler's work. He wrote of Harry Wexier as a major figure in the meteorological world, an original scientist of high rank, with a strong urge towards mathematical physics and theoretical understanding, who amidst his wide interests was recognizably a weatherman and climatologist. Par- ticularly cited were his work on anticyclones (Wexier, 1937, 1943, 1951a), his discussion i Dr. Chapman's current affiliations are: High Altitude Observatory, Boulder, Colo.; Institute of Science and Technology, The University of Michigan, Ann Arbor, Mich.; and Geophysical In- stitute, University of Alaska, College, Alaska.

Susan Wexier, daughter of the late Dr. Harry Wex- ier, and Dr. Raymond Wex- ier, his brother, examine the meeting program with Dr. Sydney Chapman (center), just before the First Wexier Memorial Lecture.

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Unauthenticated | Downloaded 10/09/21 07:23 PM UTC Vol. 46, No. 5, May 1965 of the normal atmospheric regions of heating and cooling (Wexier, 1944), and his paper "The Antarctic convergence—or divergence" (Wexier, 1959). Harry Wexier was trained at Harvard in mathematics, the subject of his first degree. It was a good basis for his M.I.T. graduate work in meteorology. For a brief period, students of meteorology at the University of Chicago were fortunate in experiencing his influence as their professor. Apart from that, during the 27 years of his working career he was in government service, in a variety of capacities. They included service with the Army Air Force as instructor and research executive. In some circles government service is thought to be deadening; but Harry Wexier was far too alive and enthusiastic, too original and vitally human, to be deadened. Instead he enlivened his surroundings and fellow workers. For the last fifteen years of his life he directed the research program of the U. S. Weather Bureau, in which he had held his first professional post as a young man. From 1946 onwards he initiated and guided many of the Weather Bureau's most important research projects. Their results are recorded in the impressive series of papers that have issued from the Weather Bureau during the last two decades, by his colleagues and himself. One manifestation of his friendly and cooperative nature, as well as of the team nature of much modern research, is the number of his joint papers. They are about 30 per cent of his total production, of nearly a hundred papers. His work in During the coming years one may expect that successive Harry Wexier Memorial Lec- the upper turers will review particular parts of his work, which gives ample scope for such treat- atmosphere ment. Obviously no one lecture could cover his achievements even in bare outline. On this occasion I shall refer specially, though briefly, to his work and interest in the upper atmosphere. It is natural, indeed necessary, that many meteorologists should confine their studies almost exclusively to the tropospheric weather region of the atmosphere. That region was also the main concern of Harry Wexier. But his interests extended to all levels of our atmosphere, and also to the atmospheres of other planets. I will limit myself to a few examples. He recognized that ozone, generated and mainly located in the stratosphere, is a valuable trace substance whose variations at and above ground level could throw light (Wexier, Moreland and Weyant, 1960) on Antarctic meteorology, and on interchanges between stratosphere and troposphere, across the gap in the tropopause that is associated with the mid-latitude jet stream (see Fig. 1); he discussed also (Wexier, 1950a, 1951b) the possible effects of ozonospheric heating on sea-level pressure.

FIG. 1. Schematic rep- resentation of winter ozone cycle in Antarctica. (From Wexier, Moreland and Weyant, 1960, courtesy of Monthly Weather Review)

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Unauthenticated | Downloaded 10/09/21 07:23 PM UTC Bulletin American Meteorological Society The second example is his interest in dust. It was the subject of one of his earliest papers (1936), and of three later ones (Wexler, 1951c, d, 1952). The last of these is an excellent account of volcanoes and their possible influence on world climate, for general scientific readers; it gives a sympathetic but critical account of the theory proposed by his eminent Weather Bureau predecessor Humphreys, that volcanic dust may be an important factor in long-range climatic variations. The third example is provided by his discussions of the vertical propagation of storms —either downward from the outer fringes of the atmosphere, where solar radiation first encounters appreciable absorption, or upward from the earth's surface, where about half of the incoming radiation is finally absorbed, and transformed into potential, kinetic, and internal energy of the air (1958; see also 1953). My fourth and last example is his discussion of annual and diurnal temperature variations in the upper atmosphere (Wexler, 1950b). He concluded that in the upper ozonosphere in the higher latitudes the annual variations might be as great as 75C, and that the average diurnal range there might rise to from 10 to 15C. These conclusions are now being borne out by the extensive rocket measurements made in recent years (Webb, 1965). Harry Wexler's wide knowledge of the upper atmosphere was indicated by the 1957 Report of the American Geophysical Union Committee on this subject; he was the Chair- man, the members being Johnson, Kaplan, Kellogg, Newell and Whipple, with Fritz as consultant. There and elsewhere Harry Wexler showed that his interests included ionospheric phenomena and the geomagnetic variations. His role in A cooperative man in his position, with such wide learning and interests, was happily national and and inevitably drawn into national and international meteorological planning. His international membership of many committees for such work is indicated in several biographical no- planning tices (see the lists in Monthly Weather Review, 1963, pp. 481, 657). I will mention only three of such connections. He was a member of the U. S. National IGY Committee, which appointed him as Chief Scientist for its Antarctic program; he was the World Meteorological Organization Reporter on Meteorological Satellites, and initiated the concept of the World Weather Watch, linked with the United Nations Organization; and he served on the Antarctic and Space Research Committees (SCAR and COSPAR) of the International Council of Scientific Unions. He also gave devoted service in many ways to this Society. He was deeply concerned that the advance of Antarctic and space science should be devoted to peaceful purposes. Harry Wexler, so involved in wide interests and multifarious scientific and admin- istrative activities, had nevertheless a warm humanity, most evident among family, friends and colleagues: a sense of the beauty and wonder of Nature: and a spirit of adventure, shown by his aircraft flights into an Atlantic hurricane, and in his Antarctic travels. Moreover he possessed the gift of lucid and interesting exposition in speech and writing, which he exercised in lectures and reviews, as well as in his continued flow of original scientific work. Naturally numerous awards and distinctions were conferred on him, to the delight of his many friends. The most recent (1960) was the Distinguished Public Service Award of the U. S. Navy. Meteorological In his latest years he was particularly concerned with meteorological satellites. As early satellites as 4 May 1954, for the Third Symposium on Space Travel, at the American Museum's Hayden Planetarium in New York, he had outlined the advantages of space research irj meteorology, and indicated some possible plans for it (Wexler, 1954). This was at a time when to many people, perhaps to most, such ideas seemed fantastic and almost cranky. Harry Wexler's advocacy of satellites for scientific research slightly ante-dated the address given by Professor S. F. Singer to a committee of URSI—the International Union for Scientific Radio—at the Hague, which led not many days later, at Rome, to the adoption of satellite research as part of the program of the International Geophysical Year (IGY). This in turn induced first the United States, in 1955, and the next year the USSR, to announce that they would include satellite research in their national IGY programs. The early satellites explored the magnetosphere, and revealed the existence of the Van Allen belts of energtic particles. Subsequently such satellites have become im-

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Unauthenticated | Downloaded 10/09/21 07:23 PM UTC Vol. 46, No. 5, May 1965 portant adjuncts to solar, planetary and general astronomical research. But also, to quote R. C. Sutcliffe again, "the United States went ahead with meteorological satellites, as it seemed without any kind of hesitation, as though the successful outcome was a foregone conclusion—and in this respect, at least, far ahead of their Russian rivals." He added that "this must owe a great deal to Wexler's enthusiasm and faith, already well justified and rewarded." In his last two years Harry Wexier wrote at least a dozen ar- ticles, alone or with colleagues, on the TIROS results and their interpretation (see pp. 479-481 of the bibliography of his publications in Monthly Weather Review, 1963). Such a man, so all-round in interests and capacities, could not but look backward, as well as around and forward, in his science. Harry Wexler's concern with the historical aspects of meteorology was shown on more than one occasion (cf. Wexier, 1958, 1962). He felt justly proud of the American pioneers, such as Benjamin Franklin, Espy and Maury, to name but a few. Sharing this interest, I invite you to join me in some historical contemplations on meteorology. Nomenclature First, as to the name: the best-known ancient treatise on our science is the METEftP0A0ri- KW (in Latin Meteorologica) of Aristotle (384-322 B.C.). The Oxford English Dic- tionary gives an Elizabethan reference to this in 1588 as Aristotle's book of meteorologicks. The first reference it gives to the adjectival form meteorological was even earlier, in 1570, and that to the name meteorology was rather later, in 1620. The word meteorology sur- vived competition with a rival term meteorography (1736). As regards ourselves, the name meteorologist appeared already in 1621, but other names were also used, two of them still earlier, meteorologician in 1580 and meteorologian in 1614: and somewhat later, meteorologer (1683). Less than half of Aristotle's Meteorologica dealt with weather science; the rest was concerned with shooting stars, astronomy and oceanography. Still in the 17th century meteorology included both weather science and shooting stars, as shown by these quota- tions: "The watchful and industrious meteorologer, who makes it his work to attend the motions of winds, rain, thunder . . ." (1683); and "The trajectories and shooting of the stars, of which meteorologers write" (1686). The word meteor had originally a very general meaning, which certainly in Shake- speare's time included weather phenomena, as witness this quotation from King John (act 3, scene 4, 145-147): No common wind, no customed event, But they will pluck away his natural cause And call them meteors, prodigies and signs. But by the strange accidents that determine the development of language, this word gradually became restricted to mean a shooting star, while the word meteorology was limited to mean weather science. Such a linguistic disconnection must be very puzzling to, say, an African learning English. This circumstance, and the ponderosity of the word meteorology and its derivatives, led me to suggest in 1945 in the correspondence columns of WEATHER that we abandon the use of these words, and substitute for them the words aeronomy, aeronomer, aero- nomic. There were obvious alternatives, namely aerology and its derivatives; but this name had already become adopted for a specialized branch of our science. Perhaps because I have too little of the politician about me, and did not make the right moves to gain consideration for my proposal; this met with almost no response, and certainly to no action. Hence later (1953) I suggested a different, more limited, meaning for the word aeronomy, namely that it should signify the science of that part of the upper atmosphere in which dissociation and ionization are important. This proposal was offi- cially adopted, and aeronomy is now a word widely used. I continue to think it desirable to abandon the word meteorology and its derivatives. It can be replaced by aerology, used in this wider sense; and I venture to do so in the remaining part of this lecture. Early astronomy Let us next consider some points in the history of our science itself. When conscious and aerology thought slowly developed in the mind of prehistoric man, the stars and the weather both drew his attention, and astronomy and aerology had their primitive beginnings. Among the most ancient data of astronomy are the records of occultations and eclipses, that is, of coincidences in the sky; their value for modern astronomy depends on their chrono-

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Unauthenticated | Downloaded 10/09/21 07:23 PM UTC Bulletin American Meteorological Society logical interpretation. Gradually geometrical measurements, of direction of the celestial objects, were undertaken, with slowly increasing accuracy; and in aerology the wind vane was invented, to show the direction of the air motion. These measurements were associated with recorded times. Whereas the motions of the heavenly bodies, particularly of the few planets, are slow and systematic, the wind, on the contrary, is in many places irregularly variable and local. In the course of about two millennia, the work of such men as Ptolemy, Copernicus and Kepler, on the basis of the astronomical data, gradually led to a general and con- cise approximate description of the planetary motions, by Kepler's three laws. These are mathematical and numerical; they were developed over the years 1607 to 1620. The first synthesis of the wind data was the map of the trade winds by Halley (1686a). This was original and valuable, but of course lacked the simplicity and definiteness of Kep- ler's laws. Measurements of direction (and of luminosity) continued to be the staple material of astronomy for over three centuries more, until the development of spectral analysis, and its application to the sun, by Kirchhoff in 1859, gave birth to the new science of astrophysics. But aerology needs much more than measurements of the wind direction. In Kepler's time neither the temperature nor the pressure of the air could be reliably measured. Galileo (1564-1642) did devise a primitive form of thermometer (1593), but its readings were affected by changes of pressure. Surprisingly, Galileo denied the existence of pressure in the air (1612) (M5).2 Aerology and In the century after 1600 the progress of aerology was fundamental progress in the uni- physics versal science of physics. At that time the Church had firmly adopted the assertion by Aristotle that a vacuum is impossible, and abhorred by Nature. The clarification of ideas on the pressure of the air and its weight, and on the vacuum, was achieved quietly by a few men, and was linked with the introduction of the barometer. Middleton, in his recent book, The History of the Barometer (1964),3 describes in fascinating detail the growth of thought and knowledge on these subjects, and makes clear the outstanding part played by Torricelli (1608-1647) in 1644. Had his work and the inferences from it, as to the achievement of the vacuum, been publicly announced, aerology might have had its martyrs; but warned by the experiences of Bruno and Galileo, the pioneers in Italy lay low (M30, 31). Mersenne, the priest-scientist of Paris, was shown the Torri- cellian experiment in Florence in 1644 (M37). Through him it became known about 1645 to the group of Englishmen who at that time met in London to discuss science, and who later formed the Royal Society, to which in 1660 King Charles gave a royal charter and its name. These men discussed, among other subjects, "the weight of the air, the possibility or impossibility of Vacuities, and Nature's abhorrence thereof, the Torricellian experiment in quicksilver, . . (Wallis, 1678; M55). The experiment also became known to Pascal in Paris in 1646 (M38), and at his suggestion his brother-in-law at Puy-de-Dome in 1648 measured the height of the barometer at different levels on the mountain (M51), and showed that the barometer fell with increasing altitude. This was confirmed in England in 1653 (M59) and on Snowdon in Wales in 1686 and 1697 (MP67, 275). The news of Torricelli's experiment was first made known in print in 1660 by Boyle (PT 2, 155, 1666; M65, 66), and it was he who gave the name barometer to the instru- ment (1663); another name, baroscope (1664), was also in use for a time (M71). Boyle, Hooke and Halley were zealous investigators of the air. Hooke devised the wheel barometer (1664) (M94), and in 1686 improved it to give the first type of micro-barograph (MP61). The introduction of the barometer aroused extraordinary interest and enthusiasm in learned circles in England at that time. An early contributor to the Philosophical Transactions of the Royal Society, Dr. John Beal (1666), who thought the barometer "one of the most wonderful instruments that ever was in the world," wrote thus: "Who 2 See footnote 3. s In the text, reference is made to this book by the letter M and the page number; likewise references to the book on Halley by MacPike (1932) are indicated by MP followed by the page number; PT refers to early volumes of the Philosophical Transactions of the Royal Society.

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Unauthenticated | Downloaded 10/09/21 07:23 PM UTC Vol. 46, No. 5, May 1965 did believe that by palpable evidence we should be able to prove, the serenest air to be the most heavy, and when darkest clouds hang nearest to us, ready to dissolve, or drop- ping, then to be lightest." In 1661 Boyle demonstrated to the Society his law that the pressure and the volume of the air are in reciprocal proportions (M69). Halley (1686b) used this law, treating the atmosphere as isothermal, to calculate the density of the air at different heights. He estimated the height of the atmosphere, were the air of uniform density like water, to be no more than 5.1 miles. This value, 8.2 km, is a good estimate of what was long called the "height of the homogeneous atmosphere," now known as the scale height, a term applicable at all levels. Halley's table of densities up to 45 miles was the fore- runner of countless later atmospheric tables. From twilight optical phenomena he be- lieved that the air does extend to about this height (PT 16, 107). He remarked that upon his "suppositions, it appears that at the height of 41 miles the air is so rarified as to take up 3000 times the space it occupies here, and that at 53 miles high it would be expanded above 30,000 times; but 'tis probable, that the utmost power of its spring cannot exert itself to so great an extension, and that no part of the atmosphere reaches above 45 miles from the surface of the earth." He marvelled at the great expansion he calculated for the height of 45 miles. "Now what texture or composition of parts shall be capable of this great expansion and contraction seems a very hard question: and which, I suppose, is scarce sufficiently to be accounted for, by the comparing it to wool, cotton and the like springy bodies." The nature of air had already been discussed by many authors. In 1621 one author likened the atmosphere to an immense sponge (M6). Boyle in 1660 rejected this idea (M66), saying that the atmosphere is "not like a sponge, an entire body, but a number of slender and flexible bodies, loosely complicated." Already Descartes (1596-1650) in 1631 had compared the air to wool (M7); Torricelli (M28) and Boyle (M66) also used the same conception, apparently independently. Halley, in his paper on the height of the atmosphere, further remarked: "Nor can the rarefaction proceed ad infinitum; for supposing the spring whereby it dilates itself, occasioned by what texture of parts you please, yet must there be a determinate magnitude of the natural state of each particle, as we see it is in wool and the like, whose bodies being compressible into a very small space, have yet a determinate bulk which they cannot exceed, when freed from all man- ner of pressure." These misconceptions about the expansibility of air were not fully cleared up until after about two centuries the kinetic theory of gases, and of matter in general, was developed. This was also the long-deferred clue to another mystery discussed by Halley (1688), in a paper on evaporation from water. To explain this, Halley adopted the view expressed by Newton, that matter as we know it has in it a great proportion of empty space. Halley pictured the particles of water and air as spherical bubbles, with skins proportionally far thinner than those of soap bubbles, and thinner for air than for water, so that the particles of air are the more compressible and expansible, and lighter. He thought that the bubbles might be filled with "a very refined matter next to vacuity," and that heating would distend the bubbles and lessen their specific gravity. These conceptions, he wrote, might in some measure render more intelligible the rising of water particles into the air: "for that otherwise the difficulty would seem insuperable to explain how any matter 800 times heavier should be imbibed, elevated and sustained in any fluid" (MP212). Newton's mechanics, I have compared Kepler's descriptive laws of planetary motion with Halley's synthesis of Kepler's laws, and wind data in his trade wind chart. In 1684 Halley heard from Christopher Wren that the trade winds Kepler's laws had been explained by Newton (MP6). Halley hastened to visit Newton at Cambridge, and persuaded him to write up his work, which grew into the immortal Principia. This enunciated Newton's law of gravitation and his three laws of mechanics, and developed some of their astronomical, ballistic, tidal and other consequences. Hal- ley, then secretary of the Royal Society, saw the volume through the press in the Transac- tions, and even partly financed it himself. During this period Halley was also active in aerological research; and in his trade winds paper, besides portraying them by his map, he sought to give a physical explanation of them. He pictured the air as ascending, in equatorial latitudes, where the heating by the sun is most intense. This would cause a

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FIG. 2. Vertical loops called cells account for north-south circulation of air and are supposedly responsible in part for dif- ferences in the prevailing surface winds. Dotted lines: polar fronts. Dark gray wedges: low-pressure regions. Light gray wedges: high-pressure regions. (From Wexler, 1962, courtesy of Scientific American)

flow at ground level towards the equator, as shown by his map. Thus he had the idea of a circulation cell in the atmosphere (see Fig. 2). He confessed his inability to understand why the trade winds did not extend beyond 30° latitude. His explanation of the latitudinal component of the trade winds was un- satisfactory. He corresponded about it with Wallis, who held the chair of geometry at Oxford, to which Halley himself later succeeded. Wallis made objections to this part of Halley's theory, and Halley replied: "Your questioning my hypothesis . . . makes me less confident of the truth thereof, and I should be glad to see some other notion whereby more of the appearances would be naturally solved" (MP80). The new notion required was supplied in 1735 by Hadley, also in the Philosophical Transactions, by taking account of the earth's rotation. Halley attached no importance to this in his theory; he even said that he was very sure that the motion of the earth "has little effect on the tides" (MP75). Hadley's explanation was, of course, implicit in Newton's laws; but the extrac- tion of the aerological inferences from them is a slow and difficult process, still under way. Newton himself could err in such inferences, as in his calculation of the speed of sound in air; this had to be corrected by Laplace, who saw that the volume changes involved must be adiabatic, not isothermal as assumed in Newton's calculation. Astronomy and I now turn from this great period in the history of astronomy and aerology to some aerology today present-day questions. The circumstances of the two sciences could hardly be more different. The traditional measurements of astronomy serve certain important practical needs. One is time-keeping—although now new types of clock rival or outdo the performance of our slightly irregularly rotating earth. Another is the determination of position on the globe; this also can now be done otherwise. In the main, however, the pursuit of astronomy is part of the pride of man, a manifestation of that strong urge within the human race to search out hidden mysteries, to understand the scope and nature of the boundless universe, and to know what is our place in it. It probes the far extents of space and of past time, and tries to pierce the mists that veil the distant future. Its marvelous progress from its primitive beginnings is one of the glories of mankind.

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Unauthenticated | Downloaded 10/09/21 07:23 PM UTC Vol. 46, No. 5, May 1965 And despite its antiquity it has in our day renewed its youth, with the development of radio astronomy. Its discoveries open up vistas without end, for further enquiry. Its progress is being and will be enhanced by the advances of engineering capacity that en- able space vehicles to be launched and guided, and information to be transmitted over vast distances. By and large the effort and money devoted to astronomy are not much influenced by the practical day-to-day and year-to-year needs of society, as are those of weather science. Nor are the predictive attempts in astronomy in any way comparable with those in aerology, either in their term (millions or even billions of years rather than hours, days or months as in aerology), nor in their early known success or failure. Only in solar physics does the predictive element have some practical urgency, for telecommunications and space travel. A science that I may call heliology, or the aerology of the sun, is taking shape. The earth is a kind of distant TIROS viewing station for the sun, showing us at each moment a complete hemisphere, changing at a rate determined mainly by the sun's rotation, but also by our orbital motion. By solar spectroscopy we are able to probe different levels of the solar atmosphere. But the mechanisms that so much and so often change the face of the sun are internal, lying at depths beyond our powers of visual penetration. Attempts have been and are being made to foretell the growth and decay of sunspot groups, or the occurrence of solar flare storms; their imperfect achievements can be viewed by terrestrial aerologists with sympathetic understanding. Our weather scientists have tasks that are of immense economic and social importance, and, beyond the current need to predict, looms the possibility of future need and capacity to control. Predictive performance can be checked in daily detail by the man in the street, and good results tend to be less well remembered than signal failures or imperfect fulfillment. This, I think, affects the intellectual attitude taken towards aerology by fellow scientists such as astronomers and physicists. They would do well, however, to consider the extraordinary detail, scope and difficulties of weather science. Even one degree of arc on the earth is comparable with the scale length of some important weather phenomena; and on a sphere there are more than forty thousand square degrees. Many of these are in areas of little social concern, but what goes on there may affect regions of social importance. The atmosphere is a vast continuum, which must ultimately be treated as a whole, over the entire surface of the earth, and to considerable heights above the surface. Only in our generation is our data-gathering capacity beginning to cope with what is needed. The same applies to our capacity to transmit the data to regional and world centers for analysis, and to the capacity of computers and machine plotters to handle, represent and interpret the data. Even if the solar influences operating on the atmosphere were constant and fully known, the task of determining the course of world and local weather, on the basis of the known laws of mechanics, physics and chemistry, would be one of appalling complexity. But in fact we do not yet know in detail how the sun's radiations change, nor how significant the changes are for weather science. The astronomer in his quiet observatory or study, the physicist before his isolated experiment in the laboratory, might well marvel that men can be found to face such complicated and urgent tasks as confront the world aerolo- gist. But there are men who will tackle immense difficulties, doing their best possible. The worldwide There are, however, some problems of aerology on a large scale which, although in no atmospheric waY urgent, are of great interest, and worthy of active attention. As on other occasions oscillations elsewhere, I express the belief that among these are the worldwide oscillations of the atmosphere, thermal and tidal, that occur in the course of the solar and lunar day. (See Figs. 3-6.) There is a great contrast between the seas and the atmosphere in the relative magnitude of the solar to the lunar half-daily variations. In the oceans the lunar tidal influence is dominant, whereas in the atmosphere its effects, though present, are so small as to be difficult to determine (Chapman, 1951a; Wilkes, 1949). The outstanding magnitude of the solar half-daily barometric variation relative to the 24-hourly component led Kelvin to suggest that an important degree of resonance magnifies the former. The degree of respect for the value of this idea has fluctuated during subsequent decades, and the fairly recent calculations of the free periods of oscillation of the types concerned, by Jacchia and Kopal (1952), were not favorable to it. M. V. Wilkes has recently suggested to me that such calculations need further consideration, taking account of the possibility 234

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FIG. 3. Geographical distribution of the annual mean lunar semidiurnal air tide Ls in barometric pressure, indicated by dial vectors, each referring to the place at the mid-point of the arrow, whose length gives the amplitude Z2 on the scale shown, and whose direction gives the time of high air tide, as shown on a local mean lunar clock. (From Chapman, 1951)

FIG. 4. Harmonic dials (with probable error circles) indicating the annual change of the lunar semidiurnal air tide in barometric pressure. (a) Annual (Y) and J, E, D four-monthly determinations for Taihoku, Formosa (1897-1932). Also sets of twelve monthly mean dial points for (b) Taihoku (1897- 1932), (c) Batavia (1866-1895), (d, inset) mean of Potsdam (1893- 1922) and Hamburg (1884-1920), (e) Hong Kong (1885-1912), and (f, in center) the mean of Coimbra, Lisbon, and San Fernando. (From Chapman, 1951)

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FIG. 5. The distribution of the annual mean solar semi- FIG. 6. Harmonic dials indicating the annual change in diurnal barometric variation (S2) over North America. Each the solar semidiurnal barometric variation (S2) at four widely dial vector refers to S2 at the point at its thick end. (From spaced points in middle latitudes, (a) Washington, D. C., (b) Bartels, 1932) Kumamoto (33N, 131E) (c) mean of Coimbra, Lisbon, and San Fernando, (d) Montevideo, Uruguay. Compare with the much larger proportional changes of L2 shown in Fig. 4. (From Chapman, 1951)

that the adiabatic assumption may not be strictly justified at all heights as regards trans- fer of heat vertically; he points out that resonance calculations are very sensitive to the precise value assumed for 7, the ratio of the specific heats. Small and Butler (1963), taking account of the heating of the atmosphere in the ozonosphere as well as that in the troposphere, report success in explaining the solar half-daily barometric variation. It seems desirable that their work should be independently repeated and checked, perhaps with improved and fuller data. Confirmation of their results would still leave much scope for further studies in this field. As yet we have no explanation of the zonal part of the solar 12-hourly variation of the barometer, which in high latitudes is the main part. Nor do we have a reliable numerical theory of the lunar atmospheric tide, which shows a larger month-to-month change throughout the year than does the solar 12-hourly variation—contrary to what one would expect. It must be a consequence of some dy- namical characteristics of the atmosphere, on a large scale, of whose nature and meaning we are as yet ignorant. Any satisfactory theory of the lunar atmospheric tide must also be able to explain its changes of amplitude and phase with height in the atmosphere, about which we know something from ionospheric, geomagnetic and cosmic ray studies. Noctilucent clouds Another technical topic which at present has limited interest for weather forecasting is that of the noctilucent clouds. These clouds show a delicate filigree pattern of parallel silvery streaks or waves resembling cirriform clouds (Ludlam, 1957; Witt, 1962; Paton, 1964). They are at a height of about 80 km, and so are illuminated by the sun when this is well below the horizon, and most of the atmosphere overhead is in darkness. They are seen in latitudes 45° to 65°, mostly in summer. In the higher latitudes the more oblique descent of the sun below the horizon gives longer times in which the clouds, if present, can be seen. The observation of such clouds was a supplementary part of the IGY auroral program (Hoffmeister and Paton, 1957), although there is no physical rela- tion or similarity of form or causation between the two phenomena. Hoffmeister in Germany and Paton in Scotland have for some years kept watch for them, and have recorded many occurrences there. In this country little attention was paid to them, and few records of them were available, until recently Fogle (1964) of the University of Alaska Geophysical Institute took up the organization of their study in North America, by visual and photographic watch, with the support of the National Science Foundation, and with the cooperation of Canadian observers. In the southern hemisphere there is

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Unauthenticated | Downloaded 10/09/21 07:23 PM UTC Bulletin American Meteorological Society a dearth of such observations, and two weeks ago he went to Chile to watch for them and to prepare for a more organized watch there during the next southern summer, of 1965/66. In Sweden, by cooperation between the Air Force Cambridge Research Laboratories and the International Meteorological Institute of Stockholm, the clouds have been investi- gated by rockets, and plans are made for rocket research on them also in Alaska. In Sweden rockets were launched to pass through the level of noctilucent clouds both when they were present and when they were absent; the rockets carried coated plates which were exposed to catch particles in the air at the cloud levels. They found particles which appeared to be micrometeorites, nickel-based; they were many times more numerous, and the mesopause temperature was decidedly lower, on the occasion when the clouds were present than when not. When the clouds were present, the particles seemed to have had a coating around them, which was interpreted as an ice coating, though it had disappeared when the plates were recovered. It is supposed that the noctilucent cloud particles con- sist of water vapor condensed on meteoritic duct nuclei (Witt, Hemenway and Soberman, 1964). If this be so, the appearance of the clouds at or near the mesopause requires adequate amounts of both dust and water vapor, and a temperature low enough to favor condensa- tion. The clouds can sometimes be observed for some hours in the far north; they are often in rapid horizontal motion; they are thin, and have much detailed structure, which remains rather constant during the periods of observation. Any explanation of them must account for these features; one of the most demanding is the presence of enough water vapor at that great height. Doubtless large-scale mesospheric synoptic factors are involved in the whole phenome- non, such as a general upward convection of moist air above a certain latitude and height in the summer half-year, as Murgatroyd and Singleton (1961) have tentatively inferred. P. C. Kendall and I have recently concluded that it is not necessary to make a multiplicity of additional hypotheses to explain the detailed character of the clouds, and why in the favored latitudes and months sometimes they appear and sometimes not. It seems enough to make the basic supposition that the clouds are formed when the mesopause temperature minimum becomes unusually low. This implies a steepening of the temperature gradients both below and above the mesopause. On the underside the stability of the air will be lessened, and upward transport of water vapor by convection will be facilitated. Above the mesopause the stability will be increased. It appears likely (Nicolet) that there is generally some mixing of the air even above the mesopause; the level at which mixing effectively ceases, or the turbopause (Chapman, 1951b), thus usually lies above the mesopause. But a marked lowering of the mesopause temperature is likely to bring the turbopause down to a lower level, perhaps to the mesopause itself. We have shown that this will increase the dust density in that region. Our work is a development from that of Yu and Klein (1964), who have lately made a mathematical study of the dust distribution resulting from the brief entry of a limited dust cloud or layer into the atmosphere from above, into an initially dustfree atmosphere. After some time, depending on the rate of diffusion of the dust in still air, a quasi-steady state is approached above a lower boundary level, that steadily descends, with ever decreasing speed; in the upper region the dust density becomes proportional to the air density, and therefore (as we suppose for simplicity) exponential. For particles of the size and mass inferred from optical and rocket measurements of the noctilucent clouds, the exponential distribution of dust is attained down to a height of 80 km in about seven hours. These are the conditions in a stable atmosphere. If there is a turbulent region below some level, we can for simplicity take the transition to be sudden, and represent it as regards diffusion by supposing that at the interface there is a discontinuity of the diffusion constant, being a large increase from above to below, where the diffusion is by eddies. At the interface there will also be a discontinuity of dust density, a large drop from above to below, in the ratio of the two diffusion constants. We may suppose that ordinarily the mesopause, the limiting level to which water vapor can be raised by active convection, lies below the turbopause, and consequently in the region of reduced dust density. If because of a decline of the mesopause temperature the turbopause descends to the mesopause, in the course of a few hours the dust density in the thermosphere just above will be much increased. At the same time the steeper

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Unauthenticated | Downloaded 10/09/21 07:23 PM UTC Vol. 46, No. 5, May 1965 gradient in the mesosphere may bring the water vapor below the mesopause to unusual abundance. By diffusion, mainly molecular, this water vapor will rise into the thermosphere and there condense on the dust, forming noctilucent cloud. The cloud will be thin, partly because of the upward decrease of dust density in the thermosphere, but mainly because of the upward decrease of the water vapor; this will be more concentrated towards the mesopause than the dust, because the vapor is subject to the dissociating influence of solar ultraviolet radiation, which penetrates down to about the level of the mesopause. Using the tentative values of the height distribution of the water vapor given by Bates and Nicolet (1950), we infer that the effective scale height of the water vapor in the lowermost thermosphere will be only about 2 km. For this reason the cloud layer must be very thin. We attempt no explanation of the supposed decrease of the mesopause minimum temperature. But if our theory is correct, the appearance of noctilucent clouds in any region will be an indication of such a temperature reduction, and in time, if not now, this will be helpful in forming a complete picture of atmospheric conditions at those levels. Our wonderful The noctilucent clouds are among the many beauties and wonders of what (in the title atmosphere of this lecture) I have called our wonderful atmosphere. I wish to conclude on this note. I have said elsewhere, in connection with another atmospheric beauty, the aurora, that however objectively we study it, when we behold it we do not lose our sense of awe and wonder at its majesty and mystery. Harry Wexier had this feeling about our atmosphere. He illustrated its almost unbelievable vastness (Wexier, 1955) by saying that "if it were divided up for observation among all the human beings on earth, each person would be responsible for watching approximately two million tons of air." Thus even if the "population explosion" were to leave mankind with "standing room only," we should at least not be short of air to breathe! The wonders of the atmosphere are both visual and intellectual. As to the latter, I have already mentioned how in 1666 John Beal marvelled that when the sky is laden down with water, the air weighs least heavily upon the ground. Another wonder, both intellectual and visual, is the blue sky. This is so familiar to us all that few ask why or what it is—but the genius of the 4th Lord Rayleigh (1899), who showed that it is caused by molecular light-scattering, made it possible to deduce from it the incredibly vast number and small size of the air molecules—a problem that teased the 19th century physicists for decades before they achieved equally good estimates. To me one of the most astonishing intellectual wonders of the atmosphere is that merely from moist air and dust in motion come thunder and lightning. What physicist or chemist in his laboratory, able to experiment at will on these substances, but shut from any outside view, would have dared to conclude that they could generate such vio- lent, varied and impressive phenomena? I quote from Wordsworth: The gathering clouds grew red with stormy fire Guilt and Sorrow 19 Air blackened, thunder growled, fire flashed from clouds that hid the sky Poet's Dream 3 The lightning, the fierce wind, and trampling waves Peele Castle 52 and from Shakespeare: the thunder That deep and dreadful organ-pipe Tempest 3.3.97/8 Heaven's artillery Taming of the Shrew 1.2.202 Such sheets of fire, such bursts of horrid thunder, Such groans of roaring wind and rain King Lear 3.2.46/7

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Unauthenticated | Downloaded 10/09/21 07:23 PM UTC Bulletin American Meteorological Society and from Milton: the thunder Winged with red lightning and impetuous rage, Perhaps hath spent his shafts, and ceases now, To bellow through the vast and boundless deep. Paradise Lost, I, 174/77 Another wonder of our atmosphere is the immense range of its moods: contrast Milton's: The breath of heaven, fresh blowing, pure and sweet, With dayspring born Samson Agonistes 10/11 While the still morn went out with sandals grey Lycidas 187 with Shakespeare's lines: the dreadful spout, Which shipmen do the hurricano call, Constringed in mass by the almighty sun. Troilus and Cressida 5.2.171/73 These are some of the wonders of our atmosphere, whose excellent and devoted student, Harry Wexler, we honor today.

Cited papers by 1936: A note on dust in the atmosphere. Bull. Amer. Meteor. Soc., 17, 303-305. Harry Wexler 1937: Formation of polar anticyclones. Mon. Wea. Rev., 65, 229-236. 1943: Some aspects of dynamic anticyclogenesis. Miscellaneous Reports No. 8, University of Chi- cago, Institute of Meteorology. 1944: Determination of the normal regions of heating and cooling in the atmosphere by means of aerological data. J. Meteor., 1, 23-28. 1950a: Possible effects of ozonosphere heating on sea-level pressure. J. Meteor., 7, 370-381. 1950b: Annual and diurnal temperature variations in the upper atmosphere. Tellus, 2, 262-274. 1951a: Anticyclones. Compendium of Meteorology, Boston, Amer. Meteor. Soc., 621-629. 1951b: Possible effects of ozonospheric heating on sea-level pressure. Trans. New York Acad. Sci., II, 31, 282-287. 1951c: On the effects of volcanic dust on insolation and weather, (I). Bull. Amer. Meteor. Soc., 32, 10-15. 1951d: Spread of the Krakatoa volcanic dust cloud as related to the high-level circulation. Bull. Amer. Meteor. Soc., 32, 48-51. 1952: Volcanoes and world climate. Sci. Amer., 186, 74-80. 1953: Radiation balance of the earth as a factor in climatic change. Climatic Change, Chap. 5, Harvard University Press, 73-105. 1954: Observing the weather from a satellite vehicle. J. British Interplanetary Soc., 13, 269-276. 1955: The circulation of the atmosphere. Sci. Amer., 193, 114-124. 1958: A meteorologist looks at the upper atmosphere. Atmospheric Explorations, Chap. 4, Amer. Acad. Arts and Sciences, Mass. Inst. Tech., Cambridge, 79-100. 1959: The Antarctic convergence—Or divergence? The Atmosphere and the Sea in Motion, Sci- entific Contributions to the Rossby Memorial Volume, Rockefeller Institute Press and Oxford University Press, New York, 107-120. 1960: (with W. B. Moreland and W. S. Weyant) A preliminary report on ozone observations at Little America, Antarctica. Mon. Wea. Rev., 88, 43-54. 1962: Dedication to Matthew Fontaine Maury, Antarctic Research—The Maury Memorial Sym- posium. Geophysical Monograph, No. 7, Amer. Geophys. Union, 1-3. References Bartels, J., 1932: Tides in the atmosphere. Sci. Monthly, 35, 110-130 (p. 119). Bates, D. R., and M. Nicolet, 1950: The photochemistry of atmospheric water vapor. J. Geophys. Res., 55, 301-327. Beal, J., 1666: Phil. Trans. Roy. Soc., 2, 155. Butler, S. T., and K. A. Small, 1963: The excitation of atmospheric oscillations. Proc. Roy. Soc., A, 274, 91-121. Chapman, S., 1945: A plea for the abolition of "meteorology." Weather, 1, 146-147. } 1951a: Atmospheric tides and oscillations. Compendium of Meteorology, Amer. Meteor. Soc., Boston, 510-530. 239

Unauthenticated | Downloaded 10/09/21 07:23 PM UTC Vol. 46, No. 5, May 1965 , 1951b: Upper atmospheric nomenclature. Addendum, J. Atmos. and Terr. Phys., 201. , 1958: Nomenclature in meteorology. Weather, 8, 62. , and P. C. Kendall, 1965: Noctilucent clouds and thermospheric dust: their diffusion and height distribution. Quart. J. Roy. Meteor. Soc., 91 (in press). Fogle, B. T., 1964: Noctilucent clouds in the southern hemisphere. Nature, 204, 14-18. Halley, E., 1686a: An historical account of the trade winds, and monsoons, observable in the seas between and near the tropics, with an attempt to assign the physical cause of the said winds. Phil. Trans. Roy. Soc., 16, 153-168. , 1686b: A discourse of the rule of the decrease in the height of the mercury in the barometer, according as places are elevated above the surface of the earth; with an attempt to dis- cover the true reason of the rising and falling of the mercury. Phil. Trans. Roy. Soc., 16, 104- 116. , 1688: A discourse tending to explicate the modus of the rising of vapours out of water. Read April 11, 1688, but not published by the Royal Society; MacPike (1932) pp. 140-142, 212. Hoffmeister, C., and J. Paton, 1957: Visual observations of the airglow and other non-auroral luminosities of the sky. I.G.Y. Annals, 4f 110-114. Jacchia, L. G., and Z. Kopal, 1952: Atmospheric oscillations and the temperature profile of the upper atmosphere. J. Meteor., 9, 13-23. Ludlam, F. H., 1957: Noctilucent clouds. Tellus, 9, 341-364. MacPike, E. F., 1932: Correspondence and Papers of Edmond Halley. Oxford, Clarendon Press, xiv and 300 pp. Middle ton, W. E. K., 1964: The History of the Barometer. Baltimore, Johns Hopkins Press, xx and 489 pp. Murgatroyd, R. J., and F. Singleton, 1961: Possible meridional circulations in the stratosphere and mesosphere. Quart. J. Roy. Meteor. Soc., 87, 125-135. Paton, J., 1964: Noctilucent clouds. Meteor. Mag., 93, 161-179. Rayleigh, 4th Lord, 1899: On the transmission of light through an atmosphere containing small particles in suspension, and on the origin of the blue of the sky. Phil. Mag., 47, 375-384; also in Scientific Papers, 4, 397-405, Cambridge University Press. Sutcliffe, R. C., 1962: Obituary notices, Dr. H. Wexler. Quart. J. Roy. Meteor. Soc., 88, 565-566. Webb, W. L., 1965: Personal communication. Wilkes, M. V., 1949: Oscillations of the earth's atmosphere. Cambridge University Press, 74 pp. Witt, G., 1962: Height, structure and displacements of noctilucent clouds. Tellus, 14, 1-18. , C. L. Hemenway, and R. K. Soberman, 1964: Collection and analysis of particles from the mesopause. Proc. 4th International Space Science Symposium (Warsaw, June 1963). Amster- dam, North Holland Publishing Company, 197-204. Yu, K., and M. M. Klein, 1964: Diffusion of small particles in a nonuniform atmosphere. Phys. Fluids, 7, 651-657.

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