A planned COSPAR publication contains a set of atmospheres for several latitudes and times of year for the altitude region 30 to 80 km developed by G.V. Groves. A second set of atmospheres was developed by W. Priester and I. Harris ('The upper atmosphere in the range from 120 to 800 km, ' Proposed CIRA, 1964) for the region between 120 and 800 km for various solar fluxes and times of day. For the purpose of connecting these two sets of atmospheres and also to provide average values of the physical properties, a mean reference atmosphere was prepared as described in this report. The Mean Reference Atmosphere was defined as follows: Between 30 and 80 km the model represents mean conditions throughout the year for latitudes near 30 degrees. Between 120 and 300 km the model represents mean conditions over a 24-hour period for a 10.7-cm wavelength solar flux F of 150 X 10 to the minus 22d power w/sq m/c/s. Between 80 and 120 km a model was developed which provides a smooth connection between the above two parts of the mean atmosphere. (Author).

This Encyclopedia aims, basically, at summanzmg the wealth of well established facts and outlining the relevant theories in the different branches of physics. With this as goal, the writers were asked to present their specific field in such a way that access is possible to any scientist without special a priori information in that field; the basic concepts of physics are assumed to be known to the reader. The survey given in each paper was also to be long lasting, so that even a few years after publication, each volume would be useful, for example as an introduction for newcomers or as a source of information for workers in a neighbouring field. In the field of geophysics, dealt with in Vols. 47--49 of the Encyclopedia, this task is difficult to achieve because during the last decades there has been a much faster development of basic information and theory than during the decades before. When I came to contribute to this work the famous Julius Bartels, then editor of the geophysical part, told me that Vol. 49 should certainly take into account the results of the "International Geophysical Year" 1957/58 (I. G. Y. ), and that we had better wait until these were accessible than produce a kind of information which might be obsolete in a short time.