g4 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



The most freqiieiit of the miinerous phenomena caused l)y tlie pass- 

 a^-e of light tliroiio-li ice crystals, of ^vhich the halo of 2"2° is an ex- 

 ample, are occasioned by prismatic refrac-tion between the sides of the 

 hexagonal spicules, forming angles of 60° Avitli each other. The less 

 numerous phenomena, among which is the halo of 46°, are caused 

 by refraction between the sides and bases of spicules, forming angles 

 of 90° with each other. 



P 



m 



P 



Q! 



/yor/zon 



Figure 29. — Perspective view of the sky, showing the observer (O) : his horizon, and his 

 meridian (O S zenith, n) ; the parheiic circle <m) ; ordinary paranthelia of 120° (p) ; 

 tlie paranthelion of 90° (q') ; the oblique arcs of the anthelion (r, r') ; and the 

 anthelion (n). 



Parhelia of 22° . — ^^Vhenever the air through any depth or at any 

 level contains innumerable hexagonal snow crystals wdth their sides 

 vertical (the position about which relatively broad crystals oscillate), 

 two colored bright spots, known as parhelia, or sun dogs, appear at 

 22°, or more, from the sun, one to the right, the other to the left. 

 Each bright spot is in the direction of maximum light or minimum 

 refraction, and has the same altitude as the sun. 



Halo of 22 °. — When the refracting edges of the ice crystals are 

 vertical, as they tend to be in the case of relatively thin snowflakes 

 falling through still air, parhelia are produced, as just explained. 

 But, in general, these edges lie in all directions, especially at the 

 windy cirrus level and when the crystals are of the short columnar 

 type; and as refracted light reaches an observer in every plane 

 through his eye and the sun. (or moon), to which the refracting edges 

 are approximately normal, it follows that the effect produced by for- 

 tuitously directed snow crystals must be more or less symmetrically 

 distributed on all sides of the exciting luminary. There may, how- 

 ever, be a maximum brightness both directly above and directly 

 below the sun, since ice needles tend to settle with their refracting 

 edges horizontal. 



This condition gives rise to the halo of 22°, the most frequent and 

 best known of the halo family. Its inner portion is red, because light 



