MNM 



Mr. H. C. Sorby on the Colour of the Clouds and Sky. 357 



light passed through a hundred miles or more of air in which a 

 comparatively small amount of fog was disseminated. By these 

 two suppositions nearly all the facts of the case may be explained 

 in the following manner : — The blue colour of the sky is due to 

 the absorption of a considerable amount of the red light by the 

 vapour of water present as transparent gas in the clear, pure at- 

 mosphere, far from the surface of the earth. If, however, mi- 

 nute particles of liquid water are present in the form of thin 

 mist, the depth of this blue colour will be diminished ; and hence 

 in winter, and in cold countries, we have not the clear deep- 

 blue sky of summer or of subtropical districts. The blue colour 

 of distant mountains may also be explained in the same manner, 

 the effect being due to the influence of the vapour of water con- 

 tained in the air between the mountain and the observer, and 

 also to some extent to the parts shaded from the direct white 

 rays of the sun being often chiefly illuminated by the blue light 

 of the sky. If the air be much charged with dry transparent 

 vapour, the blue colour will be deeper ; whereas if there be any 

 mist or fog it will be obscured ; and hence the blue colour is a 

 sign of the air being loaded with vapour, which indicates rain. At 

 sunrise and sunset the light of the sun would have to pass through 

 about 200 miles of atmosphere within a mile of the surface of 

 the earth, in order to illuminate a cloud a mile from the ground. 

 In passing through this great thickness the blue rays are ab- 

 sorbed to a far greater extent than the red, and much of the yel- 

 low is also removed by a number of absorption-lines, probably 

 due to the vapour of water. Hence clouds thus illuminated are 

 red; but when the sun rises higher, the yellow light passes 

 more readily, and the clouds become orange, then yellow, and 

 finally white. Clouds in different parts of the sky, or at differ- 

 ent elevations, might show these various colours at the same 

 time. They would perhaps be detached, and the blue sky be 

 seen between them; but this sky would also have its colour 

 somewhat modified by the presence of very finely divided mist, 

 which would reflect more or less red or yellow light ; and hence 

 the sky itself would be blue overhead, passing towards the sun 

 into a peculiar green, and then into yellow or red. We can 

 thus easily explain all the glorious phenomena of red and yellow 

 clouds, lying, as it were, on a blue, greenish, or orange sky 

 mixed with the dark and almost black clouds, which are either 

 not illuminated by the sun, or are too thick to permit the light 

 to pass through them to the observer. In order that these phe- 

 nomena may occur, it is requisite that no great masses of clouds 

 should block out the light of the sun; and therefore when we 

 see a fine red sunrise, we may conclude that there are few clouds 

 within a hundred miles or more towards the east ; whilst, when 



