Stereo-Pair Photographs Charles Warner Department of Environmental Sciences of Monsoon University of Virginia Charlottesville, VA 22903

Abstract 2. Viewing stereo-pair photographs

A technique for viewing stereo-pairs of photographs is described. When viewing objects less than 3 m away, a person's two eyes Stereo-pairs are used to illustrate the slender structure of cumulus are sufficiently far apart that a slightly different view is seen clouds generally observed during the Winter and Summer Monsoon by each eye. Good results follow if the ray directions differ by Experiments; the visual appearance of "hot towers'"; the prevalence of thin stratus layers, including the existence of discontinuities in the roughly 8°. Ordinary stereo vision involves a wide range of vertical through them and the growth of altocumulus from them; angles and distances to which one becomes accustomed; the and the delicate structure and considerable depth of elements of same applies to stereo-pair pictures. Poggio (1980) gives an cirrus. appropriate account of human binocular vision. Clouds seen from an aircraft generally are tens of kilometers away. One may use a camera to take two pictures separated by an inter- 1. Introduction val of a few seconds in order to yield views from two "eyes" separated by a baseline along the flight path. Combining the This article is written for two purposes: first, to promote 3- two views in stereo, the field appears as though minia- dimensional stereo-pair photography of clouds, and second, turized on a table top. One then may gaze at leisure at fine to illustrate by this means, and to discuss, some common details of the cloud structure that are not discernible by other cloud forms recorded over tropical oceans during the Winter means. No special ability or expertise is required to do this. and Summer Monsoon Experiments (MONEX). In ordinary vision, the rays to each eye originate at the ob- On the July 1981 cover of the BULLETIN, Hasler et al. pre- ject, and focusing is at the range of the object. The muscles sented photographs of Hurricane Allen for viewing in stereo using colored glasses. They used temperature as sensed in the to shift picture elements recorded by geostationary satellite; the image with shifts was superimposed in a differ- ent color upon an original. Use of the colored glasses yielded stereo vision of height as determined by temperature. Their product showed tilted cumulus towers organized in bands, with plumes of cirrus, continuous cloud around the central eye, and many different levels and textures of stratus and alto- cumulus clouds. Much of the internal structure of the hurri- cane was exposed to view. The creation of stereos such as those by Hasler et al. re- quires special processing not available to everyone. How- ever, most people do have muscular coordination sufficient to see common objects in three dimensions when ordinary photographs are taken as stereo pairs. The delicate structures of clouds recorded from aircraft are seen completely and eas- ily by this means. Writing in Weather, Fraser (1968) gave an ample discus- sion of stereo-pair photographs, including a historical re- view; Warner (1983) briefly described the cross-eye method of viewing, and used stereo-pairs to highlight features of cumulus convection. A brief description of stereo viewing is offered here, before going on to use stereo pairs to portray characteristics of clouds over tropical oceans, characteristics that commonly have been recorded in the data gathered dur- ing recent experiments.

FIG. 1. Stereo-pair (top) and geometry of viewing by the © 1984 American Meteorological Society cross-eye method (bottom).

344 Vol. 65, No. 4, April 1984

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FIG. 2.

FIG. 3.

FIG. 4.

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FIG. 5.

FIG. 6.

FIG. 2. (Preceding page, top): Stereo-pair looking southeastward where the rays cross. Point F is found by winking each eye over the coast of Borneo, taken at 109.5°E, 1.0°S, 500 hPa, at 0751 alternately and moving the finger until aligned similarly on GMT, 6 December 1978. each image. Then one looks at the finger: behind, three im-

FIG. 3. (Preceding page, center): Stereo-pair looking northeast- ages are seen, the middle one a superimposed pair. Concen- ward over southwest Borneo, taken at 111.0°E, 1.2°S, 500 hPa, at trating on the picture, one slightly relaxes the focusing mus- 0727 GMT, 6 December 1978. (In this pair, the aircraft wing appears cles, and the 3-dimensional structure is seen completely as at different elevations due to a slight net roll between the exposures. the brain locks on to the object. This requires a little practice, This is easily ignored when attention is concentrated on the clouds.) and is best tried for short intervals over a day or two. FIG. 4. (Preceding page, bottom): Stereo-pair looking westward, taken at 97.3°E, 7.0°S, 445 hPa, at 0521 GMT, 9 December 1978.

FIG. 5. (This page, top): Stereo-pair looking westward, taken at 3. Stereo-pairs from MONEX 90.0°E, 11,0°N, 327 hPa, at 0915 GMT, 5 July 1979. (Ignore the image of the camera diaphragm.) The Winter Monsoon Experiment (WMONEX) has been

FIG. 6. (This page, bottom): Stereo-pair looking southwestward, described by Greenfield and Krishnamurti (1979). U.S. re- taken at 89.1°E, 20.3°N, 328 hPa, at 1058 GMT, 5 July 1979. search aircraft reconnoitered the northeast winter monsoon over the South China Sea and environs during December 1978. The author photographed 1500 views of clouds during controlling these two functions are independent, and are 15 missions. Cumulus clouds typical of unstable stratifica- coordinated by a conditioned reflex. To view stereo-pair tion are shown in Fig. 2. Stereo viewing brings out the small photographs, as in Fig. 1, one must acquire a new condi- widths of these cumulus in comparison with their height. tioned reflex: the right eye looks at the "right eye" picture, Scorer and Wexler (1963, Plate 16) refer to this shape as cas- placed to the left, and vice versa; focusing is on the pictures, tellanus, and note that these cumulus generally originate at a range greater than that where the rays cross. To view the near the level of cloud base. Here we infer growth in unstable stereo pair in Fig. 1, one may place a finger at the point (F) stratification over the coast, while offshore over the Java

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Sea, the cumulus were shallow (see the right background in of varying amounts of and droplets with appro- Fig. 2). In a single photograph, the cumulus seem relatively priate care. massive. In stereo, clear spaces between towers are perceived, In Fig. 6, the stereo-pair shows cirrus above flight level, and the fractional area coverage by cumulus on a horizontal and makes clear that the cirrus elements occupy a consider- plane is seen to be small: measurements made in similar situ- able depth in the vertical. At the top right, the base of the cir- ations (for instance Warner, 1981) show area coverage of rus has the form of a curved veil. Stereo measurements only a few percent. (Warner, 1981) from time-lapse films taken on this day A cumulonimbus, sometimes called a "hot tower," is showed that cirrus of similar appearance existed between shown in Fig. 3. may be seen around the remains of a 13.5 and 11.5 km (—170 and —230 hPa). Highest cirrus was tower that has recently stopped rising and spread sideways. measured at nearly 15 km (—130 hPa). Just below, new rising towers may be seen. An anvil has The view in Fig. 6 is looking southwestward from a posi- formed to the northwest (left), extending downwind out of tion near the center of the depression, which intensified dur- the picture. ing the period 3-7 July. The distant cumulonimbus buildup Just below flight level, the presence of many elements of on the right was occurring west of the center; the depression stratus indicates a stable layer. Stereo viewing shows that was propagating westward. Thin altostratus layers occur much of the stratus is very thin. Side-camera films exposed below flight level, with some fine structure of shallow alto- during ascending and descending aircraft traverses have cumulus. No sign of cumulus appears at levels below the alto- shown that stratus such as this is generally only a few meters stratus. Studies of this and other monsoon depressions indi- thick. Below the stratus, many cumulus fractus and slender cate subsidence near and to the north of the center, as humilis indicate unstable stratification. Their slope indicates suggested by the stratus in Fig. 6. The cirrus elements above toward the east (right) with height through the flight level may have been remnants from cumulus building cloud layer. There is a concentration of cloud remnants near the northeast shore of the bay and advected westward and/or just below the level of the top of the into the field of view from behind the camera. stratus. This implies a discontinuity of thermodynamic In summary, stereo pairs have been used in this article to properties there and a discontinuity in infrared absorptivity draw attention to the slender structure of cumulus clouds and heating rates. The stereo-pair brings out the existence of generally observed during MONEX; the visual appearance this contrast. of "hot towers"; the prevalence of thin stratus layers, includ- Taken within a band of cloud oriented east-west south of ing the existence of discontinuities in the vertical through Sumatra, the stereo-pair in Fig. 4 again shows a layer of stra- them and the growth of altocumulus from them; and the deli- tus just below flight level. Stereo viewing shows that the stra- cate structure and considerable depth of elements of cirrus. tus is very thin. Congestus clouds extend well above it, and many cumulus are moistening the lower layer. The view also illustrates growth above such a layer of stratus; an element of Acknowledgments. I thank Joanne and Robert Simpson, partly to altocumulus can be seen in the foreground. (Its sharp outline whom I owe my participation in WMONEX, for their support and shows that it is growing.) There appears to be a connection encouragement; and Joachim Kuettner, for an invitation to partici- downward with a tower further away, but the air imme- pate in SMONEX. Omar Lucero and Kris Macoskey gave helpful diately beneath the new growth is clear—this is a common comments. This material is based upon work supported by the Divi- sion of Atmospheric Sciences, National Science Foundation, under phenomenon over the tropical oceans. It appears that cloudy Grants ATM-7900233 and ATM-8012214. ascent occurs by stages: first, moistening occurs below a sta- ble layer; then, from cumulus reaching it, stratus develops at the stable layer; (radiative) destabilization follows across the top of the stratus layer; and finally altocumulus or cumulus References pass through it. The Summer Monsoon Experiment (SMONEX) has been Fein, J. S., and J. P. Kuettner, 1980: Report on the Summer MONEX field phase. Bull. Amer. Meteor. Soc., 61, 461-474. described by Fein and Kuettner (1980). The author photo- Fraser, A. B., 1968: Stereoscopic cloud photography. Weather, 23, graphed 1000 views of clouds during eight missions over the 505-512. Bay of Bengal. Figures 5 and 6 present two stereo-pairs from Greenfield, R. S., and T. N. Krishnamurti, 1979: The Winter Mon- reconnaissance of a monsoon depression, passing westward soon Experiment—Report of the December 1978 field phase. Bull. over the Bay of Bengal on 5 July 1979. The stereo-pair in Fig. Amer. Meteor. Soc., 60, 439-444. 5 illustrates a striking phenomenon seen over the bay during Hasler, A. F., M. desJardins, and A.J. Negri, 1981: Artificial stereo SMONEX, namely, haze in the lower . This is a presentation of meteorological data fields. Bull. Amer. Meteor. view looking down into the westerly How 8° south of the cen- Soc., 62, 970-973. Medical ter of the depression in nearly clear skies as seen by satellite. Poggio, G. F., 1980: Central neural mechanisms in vision. In Physiology, Vol. 1, 14th ed., edited by V. B. Mountcastle. C. V. The visibility of clouds in the haze layer below is poor; but Mosby, St. Louis, pp. 544-585. the visibility of distant elements of stratus above the haze layer Scorer, R. S., and H. Wexler, 1963: A Colour Guide to Clouds. Per- is good. A dropwindsonde released at 90°E, 11.25°N—close gamon Press, Oxford, 63 pp. by—at 0918 GMT showed a subsidence inversion based at Warner, C., 1981: Photogrammetry from aircraft side camera movies: 470 hPa (1 hPa = 1 mb) with high humidity in the lower Winter MONEX. J. Appl. Meteor., 20, 1516-1526. troposphere and instability near the surface. Again, it seems , 1983: Stereo-pair photographs of cumulus clouds. Weather, important that meteorologists treat the radiative properties 38, 178-182. •

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