WATER IN THE ATMOSPHERE. 217 



parhelia to the right and left of the sun when it is near the horizon. 

 The halo at 22 from the sun or moon is the most frequent and con- 

 spicuous effect, and has value as a weather sign. Very often after clear 

 weather, the arrival of a cyclone and rain is preceded by a damp wind in 

 the upper air, so cold that ice crystals are formed in it. These are in 

 the form of regular hexagonal prisms with angles of 120. Alternate 

 faces therefore make angles of 60 with each other, and through prisms 

 of 60 the minimum deviation for ice with refractive index 1*31 is 22. 

 At this minimum deviation there is a concentration of the rays, and so 

 the rays from the sun striking a crowd of such 60 prisms will be some- 

 what concentrated in a direction making 22 with the original direction. 

 When an observer looks through the crowd towards the sun and then 

 outwards from it, he will receive no light refracted through the GO" 

 prisms till he has reached a distance of 22, when there will be a concen- 

 tration of refracted light. Of course the radius of the bright halo will 

 differ slightly for different colours, but partly through the appreciable 

 diameter of the sun and the consequent overlapping of halos due to 

 different parts of his disc, and partly 

 due to the scattering of other light in 

 all directions, the colours are hardly 

 evident, the inner edge of the halo at 

 the best only showing a red tinge. 



The ends of the ice prisms are 

 planes perpendicular to the faces, 

 and so form a second series of prisms 

 with angle 90 and minimum deviation 

 46. These form a second halo, which, 

 though rarely observed in our latitude, FIG. 127. Cumulus on Horizontal 

 is sometimes seen farther north. Base of Cloud. 



When the sun is near the horizon, 



and the air is in such a condition that a large proportion of the hexagonal 

 prisms have their axes in a vertical position, the refraction is much 

 greater in the horizontal direction through the sun to right and left, and 

 sometimes quite brilliant patches of light are seen with well-developed 

 colour, at 22 on each side of the sun just above the horizon. If at the 

 same time a large proportion of the prisms have their axes horizontal, 

 a patch is also seen above the sun at the same distance, though this is 

 much rarer. These patches are termed parhelia. Sometimes the parhelia 

 lie on a circular halo. The phenomena are more frequent and much 

 more marked in northern latitudes. Other circles, not all yet explained, 

 have been occasionally observed passing through the parhelia.* 



Coronas. The coronas of colour seen within a few degrees of the 

 moon when shining through the edge of fleecy clouds, and round the sun 

 (seen best when reflected in a pool so that the excessive light is dimin- 

 ished), are due to diffraction by equal sized water drops. The explanation 

 is given in the volume on Light. 



Rate Of Fall Of Cloud Drops. The drops of water in a cloud are 

 subject, of course, to gravity, and they therefore begin to fall directly they 

 are formed. But the viscosity of the air through which they pass 

 introduces a resisting force which increases with their velocity, and is 



* Tait, Light, p. 132. 



