Rainbows and Mists

Rainbows are produced by the collective effect of the refractive properties of molecules of water vapor in the atmosphere. Sunlight entering a water vapor droplet is refracted by differing angles depending on the wavelength. Consequently, the refraction produces a dispersive effect within the droplet, and after an internal reflection, the light leaves the droplet in that dispersed state. In principal, any luminous white light source can produce a rainbow, but the brightness, contrast, and scale of the phenomenon limit the sources to the Sun and the Moon.5 The center of the rainbow arc is opposite to the Sun in the sky and so, at any particular site, the effect is limited to particular ranges of the eastern or western horizon. The radius of the rainbow from the anti-solar point depends on wavelength. The violet has a radius of 40°; the red, 42°.

In addition to the primary rainbow, at a radius of 41° from the anti-solar point, a fainter secondary rainbow can sometimes be seen at a radius of 51° from this point. Whereas the primary rainbow is caused by light paths undergoing a single internal reflection inside a raindrop, the secondary bow is caused by paths undergoing two internal reflections. Figure 5.3 shows the light variation on either side of the rainbows. Note the reversal of the spectra in the secondary bow.

A much fainter tertiary rainbow, caused by three internal reflections, is located much closer to the Sun: 40°33 centered on the Sun, but is unlikely to be observed with the naked eye because it is so dim.

The same phenomenon can be seen in the mists surrounding waterfalls; the association of the rainbow with life-giving water is absolute. To an ancient society grateful for rain, the rainbow must often have been looked upon as a sign of divine beneficence; it is referred to as a sign of a divine covenant in Genesis 9:9-17. Rainbow deities are found in many ancient cultures, and the presence of the rainbow in mountainous areas where orogenic clouds are

5 Greenler (1980/1991) cites a photograph of a moonbow by William

Sager who reported that it appeared white to him when he took the photograph, but appears as a colored bow in the photograph. Such a phenomenon can be understood in terms of the lower color sensitivity of the eye in low illumination conditions.

Figure 5.3. Arcs of the same primary and secondary rainbows, seen from Calgary, summer 1997. Photos by E.F. Milone.

Figure 5.3. Arcs of the same primary and secondary rainbows, seen from Calgary, summer 1997. Photos by E.F. Milone.

common provides an association with sacred places (see §5.1.5).

Among the Inuit in Igloolik, NWT, a rainbow complete to the horizon is the kataujak ("entrance to an igloo"). In traditional Inuit cosmology, the Sun and the Moon "rest" on the rainbow (Spencer 1959, p. 258; cited in MacDonald 1998, p. 159). A myth widespread among west coast Indians and Inuit involve a sister and brother or wife and husband who rise to the sky along the rainbow and become the Sun and Moon, respectively (see §13).

Other striking atmospheric phenomena involving fog or mists are the Heiligenschein6 (Figure 5.4) and the "Spectre

6 A halo or aureole seen around shadows. It is due to total reflection in dew (best seen on certain types of grasses) or mist (Minnaert 1954, pp. 231-234). Figure 5.4 was obtained against a steaming fumarole in Yellowstone Park. The Heiligenschein is sometimes seen around balloon and even airplane shadows by passengers. The phenomenon must be viewed directly opposite the Sun for visibility.

of the Brocken".7 Both phenomena are clearly linked to the Sun. Ample illustrated discussions can be found in Minnaert (1954), Meinel and Meinel (1983), and Greenler (1980/ 1991).

In cold weather conditions, water droplets will usually freeze into ice crystals before rainbows can be observed (the

7 This phenomenon is the projected shadow of the viewer against a dense mist. Again, the Sun or other bright light must be directly behind the viewer (Minnaert 1954, p. 259). The shadow is greatly magnified, increasing with angle from the Sun-viewer line of sight. The resulting impression is that of a mountain, with the viewer's head as the peak! Black (1954) describes an example from the Grand Canyon in Arizona. The Fo kuang, "Buddha Light," reported in mountains in China (Franck 1925, p. 579, cited in Needham 1959, Vol. 3, p. 477;see also Needham, Figure 261 on p. 596), has been called an analogous phenomenon.

exception may be near waterfalls or unusual temperature gradient conditions). The presence of upper atmosphere ice crystals produces some interesting phenomena that are associated with the Sun or Moon. Halo phenomena are familiar

Figure 5.4. "Heiligenschein" around the shadow of the photographer amidst steam from a fumarole, Yellowstone, Wyoming. Photo by E.F. Milone.

sights at high latitudes. Among them are parhelia or sundogs, so called because they follow the Sun across the sky. If the crystals, found in high cirrus clouds, for example, are sufficiently widespread and randomly oriented, a complete circle can be seen around the Sun. The radius of the circle, caused by the prismatic effect of rays of sunlight passing through two sides of the hexagonal faces of elongated ice crystals, is ~22°. A fainter circle can be seen at 46°, caused by rays of sunlight passing through two faces 90° apart, at the ends of the elongated crystals or in flattened "plate" crystals (see Greenler 1980). The dispersive effects cause a rainbow-like effect, but with lower color contrast because of the proximity to the Sun. Because the phenomenon involves local atmospheric conditions, it affects primarily high-latitude, winter sites (see Figure 5.5). Similar effects can be seen around the Moon (paraselenae), especially the full Moon (Figure 5.6). Sometimes, convex and concave arcs may be seen adjoining or radiating from the parhelia circles, which depend on special orientations of the ice crystals. A related effect is the vertical ice column that is sometimes seen above lights viewed through ice fog (Figure 5.5 shows this line as well as other aspects of "sundog" phenomena).

Inuit art sometimes depicts the phenomenon of multiple Suns, usually in a vertical line. The vertical column is

Figure 5.5. (a) A vertical Sun column. Photo By T.A. Clark. (b) Sundog phenomena, including a vertical column of "multiple" Suns, due to the refractive effects of ice crystals in the air. Photo by E.F. Milone.

Figure 5.5. (a) A vertical Sun column. Photo By T.A. Clark. (b) Sundog phenomena, including a vertical column of "multiple" Suns, due to the refractive effects of ice crystals in the air. Photo by E.F. Milone.

Figure 5.6. Lunar halo. Photographed in Calgary by E.F. Milone.

referred to as the "Sun's walking stick" among Inuit in Alaska (MacDonald 1998, p. 158), and a (horizontal) pair of mock Suns have been described as "crutches," to support the Sun when a gale is approaching (Jenness 1922, p. 179; cited in MacDonald). A ring around the Sun is called the "drum of the Sun" (such as is used in the "festival house" for ceremonies). There is a possible instance of the vertical Sun being used as a winter solstice marker in Mesoamerica at Teotihuacan (see §12.22).

In the manuscript "Essays on Astronomical and Meteorological Presages" (Thien Yuan Ye Li Hsiang I Fu) by the Ming emperor Chu Kao-Chih in 1425 a.d., solar halos are declared to signify important matters of state: the defeat of an army or a conspiracy of ministers (Needham and Ronan 1981, p. 229). Illustrations from the manuscript depict the "conspiring ministers" as sundogs.

In Polynesia, there is evidence for a conceptual linkage between the 22° and 46° halos and the maximum elongations of Mercury and Venus, respectively, although Mercury's elongation varies between 10 and ~28° (see §11.4).

Finally, Stephenson (1990) uses the irregular distribution of solar halo and other reports in Chinese and Korean records from 1 a.d. to 1649 a.d. to cast doubt on the reliability of auroral and sunspot reports to indicate solar and geomagnetic activity over this internal.

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