Meteorology

Chapman's meteorological studies and his impact on meteorology are more than enough to secure him a position of lasting influence in this field, even though his major interests are concentrated on other branches of terrestrial and space physics.

Along with many others he felt that the word "meteorology" is an inappropriate name for this field, and in a letter published in 1946 entitled "A Plea for the Abolition of Meteorology" he suggested that it be renamed "aeronomy." As this suggestion was not accepted by meteorologists, he proposed later that the term "aeronomy" be adopted for the science of that part of the upper atmosphere "where dissociation and ionization become important."

It is impossible to separate Chapman's contributions to the fields of meteorology and aeronomy, since they overlap, but three of his accomplishments of particular interest to modern meteorologists may be singled out for special mention. These are his work on diffusion in the earth's atmosphere, his studies of the photochemistry of atmospheric oxygen as it applies to ozone, and his work on atmospheric tides and oscillations.

It is not surprising, in view of his outstanding contributions to the kinetic theory of gases, that Chapman, in collaboration with his pupil E. A. Milne, considered (in 1920) the composition, ionization, and viscosity of the atmosphere at great heights. At that time the stratosphere was still widely believed to be devoid of appreciable turbulent mixing. Moreover, since the composition of the atmosphere at greater heights was largely unknown, Chapman and Milne had to consider the possibility that free hydrogen might be present in the stratosphere, although they were inclined to "the assumption that the stratosphere contains no free hydrogen." One of the conclusions of their paper was that, notwithstanding the uncertainty about the lower boundary of the diffusion layer, or the "turbopause" as we call it today, the density and pressure distribution up to 100 km could be computed with good accuracy, assuming that the vertical temperature distribution is known. The information about the upper atmosphere acquired since that time has of course superseded the numerical results of that paper, but it has long served as a model for similar calculations.

Among Chapman's many papers applying diffusion theory to stellar and planetary atmospheres, special mention must be made of his and Kendall's theory of the origin of noctilucent clouds, published in 1965. This theory, based on a systematic investigation of the upward diffusion of water vapor and the downward diffusion of meteoric dust, attributes the appearance of these clouds to the simultaneous occurrence of a descent of the turbopause to the mesopause, a low mesopause temperature, and the presence of moist air below the mesopause. While acceptance of their theory will depend on the observation of the three required conditions when noctilucent clouds are present and absent, the theoretical discussion of the diffusion and convection and of their effect on the formation of noctilucent clouds will remain a guide for future studies.

At the Paris Conference on Ozone in 1929, Chapman presented a paper, the detailed results of which were published the following year in the Memoirs of the Royal Meteorological Society under the title, A Theory of Upper Atmospheric Ozone. Owing to the then fairly new discovery of the large amounts of total ozone present in the atmosphere at high latitudes, it was considered possible that solar corpuscular radiation might be a major factor in the production of atmospheric ozone. Little was known at that time about the vertical distribution of ozone except that ozone is mainly concentrated in a layer wall above the ground. Also, little if anything was known about the shortwave solar spectrum and the parameters determining the dissociation and recombination of O₁, O₂,and O₃. With a judicious choice of coefficients, partly based on the empirical fact that no diurnal variation was observed in the total ozone amount, Chapman showed that all the known facts concerning atmospheric ozone could be explained by a photochemical theory. In this theory no account was taken of the transport of ozone in the atmosphere, since at that time it could not be quantitatively determined, but even at that early stage in ozone research, Chapman anticipated the potential importance of such effects. Since that time, Chapman has continued his contributions to our understanding of the photochemistry of atmospheric oxygen and of the ozone distribution in the atmosphere, but his first paper on the subject deserves special mention because it was the first theoretical interpretation of the distribution and time variation of ozone, and it set the pattern for later work, quickly leading him to the prediction that in the ionosphere the oxygen is largely dissociated, a conception then entirely novel.

Chapman's studies of geomagnetic variations led to his interest in atmospheric tides and oscillations. Around the middle of the nineteenth century the existence of the barometric tide was demonstrated for tropical stations, where the irregular barometric changes are much smaller and the tidal amplitudes larger than in temperate latitudes. The difficulties of determining the barometric tide at extratropical stations led Airy (1877) to conclude that the tidal effect could not be found from the Greenwich data; but Chapman, in his paper on the lunar tide in 1918, made the first successful determination of the lunar tide at the extratropical station (Greenwich). The lunar barometric tide has now been determined at about ninety stations, with nearly two thirds of the determinations by Chapman and various collaborators. Apart form the intrinsic interest in the lunar atmospheric tide as an oscillation whose generating force is completely known, its determination also represents a remarkable demonstration of the power of the statistical procedures used for elimination of random noise. In 1932, Chapman was able to demonstrate the power of this technique even more strikingly by determining for Batavia the lunar tidal temperature variation, which has an amplitude of less than 0.01^oC. It is important to tidal oscillation theory that this determination of the lunar temperature oscillation showed that the lunar pressure variations are adiabatic within the accuracy of the determination. Chapman, however, questioned whether the changes would be adiabatic near the ocean surface, where heat can be readily conducted away. This questions has not yet been answered.

Chapman also made the first determination (and until recently the only one) of the lunar tidal wind at the earth's surface, at Mauritius. He also investigated tidal oscillations with periods of fractions of a solar day to one solar day. These studies have added much to our theoretical understanding of these oscillations, to our knowledge of their worldwide distribution, and their variations with time.

Chapman made his impact on meteorology not only with his scientific publications, but also by calling the attention of meteorologists to the problems of the upper atmosphere. His presidential addresses to the Royal Meteorological Society in 1933 and 1934 and to the Meteorological Association at the IUGG meetings in 1939 (Washington) and 1948 (Oslo) are some examples of this. He was also instrumental in the creation of the Meteorological Research Committee in Great Britain, and served as its first chairman from 1941 to 1947. In recognition of his significant contributions to meteorology, Chapman was appointed by the American Meteorological Society as the first Wexler Memorial Lecturer in 1964. It is impossible to assess fully the influence he has had and continues to have on his many colleagues and students, but it is certain that meteorology has gained much because of Chapman's interest in its problems.