INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS §3 



rain progresses. Additional complications occasionally resnlt from 

 the reflection of bows and from bows produced by reflected images 

 of the sun, but though unusual and thus likely to excite wonder and 

 comment, such phenomena are easily explained. 



Rather narrow bands of color, essentially red, or red and green, 

 often appear parallel to both the primary and the secondary bows, 

 along the inner side of the first and outer of the second. These also 

 differ greatly in purity and color, number visible, width, etc., not only 

 between individual bows but also between the several parts of the 

 same bow. No such colored arcs, however, occur between the prin- 

 cipal bows. 



REFRACTION BY ICE CRYSTALS 



The cirrus clouds and others formed at temperatures considerably 

 below 0° C. usually consist of small but relatively thick snowflakes 

 with flat bases, or ice spicules with flat or, rarely, pyramidal bases, 

 always hexagonal in pattern and detail. 



Figure 28. — Perspective view of tlie sIvy, sliowing the sun (S) ; ordinary Iialo of 22° (a) ; 

 great halo of 46° (b) ; upper tangent arc of the halo of 22° (c) ; lower tangent arc of 

 halo of 22° (d) ; ordinary parhelia of 22° (e, e') ; Lowitz arcs (f, f) ; parhelia of 

 46° (g, g') ; circumzenithal arc (h) ; infralateral tangent arcs of the halo of 46° (i) ; 

 the parhelic circle (m) ; a paranthelion of 90° (q) ; plane of the horizon ; the 

 observer (O). 



Light from the sun, for instance, obviously takes many paths 

 through such crystals and produces in each case a corresponding and 

 peculiar optical phenomenon. Several of these phenomena, the halo 

 of 22° radius, the halo of 46° radius, the circumzenithal arc, parhelia, 

 etc., are quite familiar and their explanations definitely known. 

 Others, however, have so rarely been seen and measured that the 

 theories "of their formation are still somewdiat in doubt. Finally, 

 many phenomena, theoretically possible, as results of refraction by 

 ice crystals, appear so far to have escaped notice. The more common 

 forms of halos as well as some phenomena less frequently observed, 

 are illustrated in fio-ures 28 and 29. 



