I created a table of content below; which was not part of the original scientific publication. The 56 pages publication is some 250Kb and I broke it in several files for acceptable web access speed.
Please go to the Science section of this website for more scientific papers by James E. McDonald and other scientists, plus comments and information regarding scientists' work and position about the UFO phenomenon.
Since the bulk of UFO reports involve objects reportedly seen in the air, it is not surprising that many attempts to account for them have invoked principles of atmospheric physics. Over the past twenty years, many of the official explanations of important UFO sightings have been based on the premise that observers were misidentifying or misinterpreting natural atmospheric phenomena. Dr. D.H. Menzel, former director of Harvard Observatory, in two books on UFOs (Ref. 24, 25), has leaned very heavily on atmospheric physics and particularly meteorological optics in attempting to account for UFO reports. More recently, Mr. Philip J. Klass, Senior Avionics Editor of Aviation Week, has written a book (Ref. 39) purporting to show that most of the really interesting UFO reports are a result of unusual atmospheric plasmas similar to ball lightning. Over the years, many others have made similar suggestions that the final explanation of the UFOs will involve some still not fully understood phenomenon of atmospheric physics.
As a scientist primarily concerned with the field of atmospheric physics, these suggestions have received a great deal of my attention. It is true that a very small fraction of all of the raw reports involve misidentified atmospheric phenomena. It is also true that many lay observers seriously misconstrue astronomical (especially meteoric) phenomena as UFOs. But, in my opinion, as has been emphasized above and will be elaborated below, we cannot explain-away UFOs on either meteorological or astronomical grounds. To make this point somewhat clearer, I shall, in the following, remark on certain past attempts to base UFO explanations on meteorological optics, atmospheric electricity, and radar propagation anomalies.
Mirages, sundogs, undersuns, and various refraction and refraction phenomena associated with ice crystals, inversions, haze layers, and clouds have been invoked from time to time in an attempt to account for UFO observations. From my study of the past history of the UFO problem and from an examination of recent "re-evaluations" of official UFO explanations, I have the strong impression that many alterations of explanations for classic UFO cases that have been made in the official files in the last few years reflect the response to the writings of Menzel (especially Ref. 25). I have elsewhere (Ref. 2) discussed a number of specific examples of what I regard unreasonable applications of meteorological optics in Menzel's writings. Some salient points will be summarized here.
A principal difficulty with Menzel's mirage explanations is that he typically overlooks completely stringent quantitative restrictions on the angle of elevation of the observer's line of sight in mirage effects. Mirage phenomena are quite common on the Arizona desert, but both observation and optical theory are in good accord in showing that mirage effects are confined to lines of sight that do not depart from the horizontal by much more than a few tens of minutes of arc. Under some extremely unusual temperature conditions in the atmosphere (high latitude regions, for example), one may get miraging at elevation angles larger than a degree, but these situations are extremely rare, it must be emphasized. In Menzel's explanations and in certain of the official explanations, however, mirages are invoked to account for UFOs when the observer's line of sight may depart from the horizontal by as much as five to ten degrees or even more. I emphasize that this is entirely unreasonable. If it were the case that all UFOs were reported essentially at the observer's horizon, then one would have to be extremely suspicious that we were dealing with some unusual refraction anomalies. However, as has been shown by many cases cited above and has been long known to serious investigators of UFO phenomena, no fixed correlation exists. Some of the most interesting UFOs have been seen at close range directly overhead, quite obviously ruling out mirage explanations. The 1947 sighting by Arnold near Mt. Rainier is explained officially and by Menzel as a mirage, yet the objects which he saw (9 fluttering discs) changed angular elevation, moved across his view through an azimuthal range of about 90 degrees, and were seen by him during the period when he was climbing his own plane through an altitude interval that he estimates to be of the order of 500 to 1000 ft. Anyone familiar with mirage optics would find it utterly unreasonable to claim that such an observation was satisfactorily explained as a mirage. Similarly, as has been noted above, the 1948 sighting by Eastern Airlines pilots Chiles and Whitted, once explained by Menzel as a "mirage", involves quantitative and observational factors that are not even approximately similar to known mirage effects, There are some extremely rare and still not well-explained refractive anomalies in the atmosphere, such as those that have been discussed by Minnaert, but good UFO observations are so much more numerous than those types of rare anomalies that it is quite out of the question to explain the former by the latter.
Sundogs, or parhelia, are a quite well-understood phenomenon of meteorological optics. Refractions of the sun's rays on horizontally falling tabular ice crystals produce fuzzy, brownish-colored luminous spots at about 22 degrees to the left and right of the sun when suitable ice-crystal clouds are present. Rarer phenomena, produced by the moon rather than the sun, are termed paraselenae. Sundogs are relatively common, but it is probably true that many laymen are not really conscious of them as a distinct optical phenomenon. For this reason, it might seem sensible to suggest that some observers have been misled by thinking that sundogs were UFOs. However, anyone with the slightest knowledge of meteorological optics talking directly to such a witness would, within only a few moments of questioning, establish what was involved. Instead of dealing with anything like a sharp-edged "object", one would quickly find that the observer was describing a very vague spot of light which he saw to the left or right of the sun, probably very near the horizon. To blandly suggest, as Menzel has done, that Waldo Harris in the 10/2/61 sighting near Salt Lake City was fooled by a sundog is to ignore either all of the main features of the report or to ignore all of what is known about sundogs.
Undersuns, or sub-suns, can be seen rather frequently when flying in jet aircraft at high altitudes. They are a reflection phenomenon produced by horizontally floating ice crystals, which reflect an image of the sun (or at night the moon) and can give surprisingly sharp solar images in still air where turbulence does not cause appreciable tilting of the ice crystals. Here again, it is probably true that many laymen may be sufficiently unaware of this optical phenomenon that they could be confused when they see one. But, as with sundogs, the stringent quantitative requirements on the location of this optical effect relative to the sun would permit any experienced investigator to quickly ascertain whether or not an undersun was involved in this specific sighting. The effect involves specular reflection of the sun's rays, whence the undersun is always seen at a negative angle of elevation in which the observer's line of sight to the undersun is just as far below the horizon as the sun momentarily lies above that same observer's horizon. Clearly, many of the UFO cases that have been cited in examples given above do not come anywhere near satisfying the angular requirements for an undersun. In my own experience, I have only come across two or three reports, out of thousands that I have examined, where I was led to suspect that the observer was fooled by an undersun.
"Reflections off clouds" have been referred to repeatedly in Menzel's writings, never with any quantitative discussion of precisely what he means. But the impression is clearly left that many observers have been and are continuing to be fooled by some kind of cloud-reflections. Aside from the above-described undersun, I am unaware of any "cloud-reflection phenomenon" that could produce anything remotely resembling a distinct object. Clouds of droplets or ice crystals do not provide a source of specular reflection (except in the case of horizontally-floating ice crystals observed from above with a bright luminary, such as sun or moon, in the distance - undersun). What Menzel could possibly have in mind when he talks loosely about such cloud reflections (and he does so on many different places in his books), I cannot imagine.
Inversions are invoked by Menzel, and in official evaluations, to account for certain UFO sightings. Inversions produced by radiational cooling or by atmospheric subsidence are relatively common meteorological phenomena. In some cases, quite sharp inversions with marked temperature differences in rather small vertical distances are known to occur. It is such inversion layers that are responsible for some of the most striking desert mirages of the looming type. To experience a looming mirage, the observer's eye must be located in or near the atmospheric layer wherein the temperature anomalously increases with height (inversion layer), and the miraged target in the object-field must also lie in or near the inversion layer. Inversion layers are essentially horizontal, and the actually-encountered values of the inversion lapse rates are such that refraction anomalies are confined to very small departures from the horizontal, as noted above under remarks on mirages. All of these points are well-understood principles of meteorological optics. However, Menzel has attempted to account for such UFOs as Dr. Clyde Tombaugh saw overhead at Las Cruces in August 1949 in terms of "inversion" refraction or reflection effects. Since I have discussed the quantitative unreasonableness of this contention elsewhere, I will not here elaborate the point, except to say that if inversions were capable of producing the optical disturbances that Menzel has assumed, astronomers would long since have given up any attempt to study the stars by looking at them through our atmosphere.
Other atmospheric-optical anomalies have been adduced by Menzel in his UFO discussions. He has repeatedly suggested that layers of haze or mist cause remarkable enlargement of the apparent images of stars and planets. By enlargement, he makes very clear that he means radial enlargements in all directions such that the eye sees not a vertical streak of the sort well-known to astronomers as resulting from near horizon refraction effects, but rather a circular image of very large angular size. Menzel even describes a sighting that he himself made, over Arctic regions in an Air Force aircraft, in which the image of Sirius was enlarged to an angular size of over ten minutes of arc (one-third of lunar diameter). I have discussed that sighting with a number of astronomers, and not one is aware of anything that has ever been seen by any astronomer that approximates such an instance. In fact, it would require such a peculiar axially-symmetric distribution of refractive index, which miraculously followed the speeding aircraft along as it moved through the atmosphere, that it seems quite hopeless to explain what Menzel has reported seeing in terms of refraction effects.
Dr. Menzel's writings on UFOs have evidently had, in some quarters, a marked effect on attitudes towards UFOs. I regard that effect as deleterious. If I felt that we were dealing here with just a slight difference of opinion about rather controversial scientific matters on the edge of present knowledge, I would withhold strong comment. However, I wish to say for the record, that I regard the majority of Dr. Menzel's purported meteorological-optical UFO explanations as simply scientifically incorrect. I could, but shall not here, enlarge upon similar critique of official explanations that have invoked such arguments.
One phenomenon in the area of atmospheric electricity to which appeal has been made from the earliest years of investigations of the UFO phenomena is that of ball lightning. For example, a fairly extensive discussion of ball lightning was prepared by the U.S. Weather Bureau for inclusion in the 1949 Project Grudge report (Ref. 6). It was concluded in that report that ball lightning was most unlikely as an explanation for any of the cases which were considered in that report (about 250). Periodically, in succeeding years, one or another writer has come up with that same idea that maybe people who report UFOs are really seeing ball lightning. No one ever felt this idea worth pursuing very far, until P.J. Klass began writing on it. Although his ideas have received some attention in magazines, there is little enough scientific backup to his contentions that they are quite unlikely to have the same measure of effect that Menzel's previous writings have had. For that reason, I shall not here elaborate on my strong objections to Klass' arguments. I spelled them out in considerable detail in a talk presented last March at a UFO Symposium in Montreal held by the Canadian Aeronautics and Space Institute. Klass has, in my opinion, ignored most of what is known about ball lightning and most of what is known about plasmas and also most of what is known about interesting UFOs in developing his curious thesis. It cannot be regarded as a scientifically significant contribution to illumination of the UFO problem.
In the past twenty years, there have been many instances in which unidentified objects have been tracked on radar, many of them with concurrent visual observations. Some examples have been cited above. It is always necessary to approach a radar unidentified with full knowledge of the numerous ways in which false returns can be produced on radar sets. The physics of "ducting" or "trapping" is generally quite well understood. As with mirages, the allowed angle of elevation of the radar beam can only depart from zero by a few tens of minutes of arc for typically occurring inversions and humidity gradients. Ducting with beam angles in excess of a degree or so would require unheard of atmospheric temperature or humidity gradients. Care must be taken in interpreting that statement, since beam-angles have to be distinguished from angles of elevation of the beam axis. For the latter reason, a beam-axis elevation of, say, two degrees still involves emission of some radar energy at angles so low that some may be trapped, yielding "ground returns" despite the higher elevation of the axis. All such points are well described in an extensive literature of radar propagation physics.
In addition to trapping and ground return effects, spurious returns can come from insects, birds, and atmospheric refractive-index anomalies that generate radar echoes termed "angels". These are low-intensity returns that no experienced operator would be likely to confuse with the strong return from an aircraft or other large metallic object.
Also, other peculiar radar effects such as interference with other nearby sets, forward scatter from weak tropospheric discontinuities (see work of Atlas and others), and odd secondary reflections from ground targets need to be kept in mind.
When one analyzes some of the famous radar-tracking cases in the UFO literature, none of these propagation anomalies seem typical as accounting for the more interesting cases. Several examples have already been discussed above (Cases 32, 35, 36, 37, 38, 39).
