Blue Colour of the Sky
Mr. Glaisher, in his Report of Scientific Balloon ascents made by him and Mr. Coxwell, in 1863, remarks that the Colour of the Sky in 1862 was of a deeper blue generally than in 1863. On the 31st of March the sky was of a deep Prussian blue, and on the 18th of April it was of a faint blue only, exhibiting another great contrast to the appearance of last year. Sir Isaac Newton considers this colour as a “blue of the first order, though very faint and little, for all vapours, when they begin to condense and coalesce into small parcels, become first of that bigness, whereby such an azure must be reflected.” Professor Clausius considers the vapours to be vesicles or bladders, and ascribes the blue colour of the first order to reflection from the thin pellicle of water. In reference to these opinions the following facts are important:—1. The azure colour of the sky, though resembling the blue of the first order when the sky is viewed from the earth’s surface, becomes, as observed by Mr. Glaisher in his balloon ascents, an exceedingly deep Prussian blue, as we ascend to the height of five or six miles, which is a deep blue of the second or third order. 2. The maximum polarizing angle of the atmosphere being 45 deg. is that of air, and not that of water, which is 55 deg. 3. At the greatest height to which Mr. Glaisher ascended—namely, at the height of five, six, and seven miles, where the blue is the brightest—“the air is almost deprived of moisture.”
Hence it follows that the exceedingly deep Prussian blue cannot be produced by vesicles of water, but must be caused by reflection from the molecules of air, whose polarizing angle is 45 deg. The faint blue which the sky exhibits at the earth’s surface is therefore not the blue of the first order, and is merely the blue of the second or third order, rendered paler by the light reflected from the aqueous vapour in the lower regions of the atmosphere.
Mr. Glaisher speaks of the curious changes in colour that he and Mr. Coxwell experienced in ascending, and remarked that they could now easily go a mile higher without turning quite so blue as before. In one descent they very nearly got into the sea, and only escaped that fate by coming down at the rate of four miles in two minutes.