INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS §5 



of that color is least refracted. Other colors follow, with increase of 

 distance, in the regular spectral sequence, but with decrease of wave 

 length they so rapidly fade that even green is indistinct and blue 

 seldom detected, 



Wlien the sun is within 10° of the horizon, the halo of 22° and 

 the parhelia of 22° are practically superimposed. At greater alti- 

 tudes they become distinctly separated. 



Tangent arcs of the halo of 22° are the well-known and fairly 

 common arcs that occur above and below the circular halo. These 

 arcs change with the elevation of the sun. Portions below the natural 

 horizon can only be seen from sufficient height. The horizontal tan- 

 gent arcs are produced by crystals whose principal axes are horizon- 

 tal, which is the position in which ice spicules or needles tend to float. 



A7'cs of Loivitz. — On rare occasions oblique extensions of the par- 

 helia of 22°, concave toward the sun and w4th red inner borders, are 

 seen. These are known as the arcs of Lowitz, after the astronomer 

 who described them as seen in the famous Petersburg halo complex 

 of July 18, 1794. 



Produced by crystals whose principal axes oscillate in that par- 

 ticular vertical plane that passes through the sun, they are nearly 

 always too faint to be seen, because, in part, this unique attitude can 

 only rarely be assumed by any considerable proportion of the crystals 

 present. 



Parhelia of 46°. — Similar to the parhelia of 22°, but due to refrac- 

 tion between the sides and ends of snow crystals, having an angle of 

 intersection of 90°, when the intersection is vertical. 



Halo of Ii.6° and 90°. — The image of the sun produced in the prin- 

 cipal plane of a 90° refracting angle of an ice crystal, as seen by the 

 observer, is 45°44' from the sun itself. Hence it follows that when 

 such crystals are very abundant and set at random in all directions 

 the innumerable images so produced must together assume the shape 

 of a ring about the sun of radius 45° 44'. This is the w^ell-known, 

 though not very common, halo of 46°. 



Occasionally a faint white halo, sometimes called the halo of 

 Hevelius, is seen at 90° from the sun. 



C ircuinzemtlial and circumsorigontal arcs. — Occasionally, an arc of, 

 perhaps, 90°, having its center at the zenith, and, therefore, known as 

 the circumzenithal arc, is seen some 46°, or a little more, above the 

 sun. It generally lasts only a few minutes, about five on the average, 

 but during that time often is so brilliantly colored, especially along 

 that portion nearest the sun — red on the outside, to violet inclusive — 

 as to be mistaken by persons unfamiliar with it for an exceptionally 

 bright rainbow. It occurs most frequently when the altitude of the 

 sun is about 20° and at times Avhen the parhelia of 22° are conspicu- 

 ous; presumably, therefore, when the principal axes of a large por- 

 tion of the crystals are practically vertical. 



Kern's arc is so-called from the name of the first person to report it. 

 It occurs exactly opposite the corresponding circumzenithal arc and 

 on the same circle. It might, therefore, also be called the anticircum- 

 zenithal arc. 



A colored arc, red on the upper side, of perhaps 90° in extent, 

 occasionally seen parallel to the horizon and about 46°, or a little 

 more, below the sun is called a circumhorizontal arc. 



