358 Ml*. Hi C. Sorby on the Colour of the Clouds and Sky. 



we have a fine red sunset, we must conclude that there are few 

 clouds within a hundred miles or so towards the west. Since, 

 then, the prevailing winds are from the west, and since rain very 

 commonly depends on the advance of clouds from the west, red 

 clouds at sunrise indicate that clouds are advancing from the 

 west, and that therefore rain is probable ; whereas a red sun- 

 set shows that there are few or no clouds in the far west, which 

 by their advance could bring rain, and therefore it is a sign of 

 fine weather. These are the chief facts connected with this sub- 

 ject; but there are besides a number of a more complicated cha- 

 racter, not easily accounted for, unless we also take into account 

 a variety of circumstances the full explanation of which would 

 make this notice far too long." 



In such an abstract I could only give a very general account 

 of my views • and I purposed to publish a more complete paper 

 when I had collected more facts, and had had the opportunity 

 of reexamining some kinds of coloured clouds that I have not 

 since seen. In the paper read at the Meeting, but not published, 

 I contended that it is often mainly the minute particles of dust 

 and fog which occur in the atmosphere, comparatively near the 

 surface of the earth, that give rise to the red colour of the clouds 

 at sunrise and sunset, and that therefore these magnificent 

 colours, which we are apt to look on as so very celestial, are to 

 a great extent due to the atmosphere being contaminated with 

 terrestrial impurities. 



I must confess that I was quite unacquainted with LommePs 

 previous more mathematical paper in Grunert's Archiv (1861, 

 xxxvi.); but from what 1 have said it will be seen that previously 

 to his paper in PoggendorfFs Annalen I had been led to form very 

 similar conclusions ; and at the same time it appears to me that 

 what I have said with reference to the effect of the clear transpa- 

 rent vapour of water will serve to remove a number of difficulties 

 connected with the subject. I say this, notwithstanding the facts 

 described by Janssen (Brit. Assoc. Report, 1886, Sects, p. 11), 

 which I think admirably account for the blue colour of the sky, 

 and for the greatly increased intensity of the narrow black at- 

 mospheric lines in the red end of the spectrum of morning and 

 evening light, but do not explain why, independent of narrow 

 black lines, the intensity of the blue end is so greatly dimi- 

 nished, which I attribute, with Lommel, to the presence of mi- 

 nute solid or liquid particles. 



