SKY COLOURS

New Zealand Herald, Volume LXXII, Issue 22166, 20 July 1935, Page 5 (Supplement)

SKY COLOURS

SECRET OF THE SUNSET We may regard dust, whether it be in the home or in the air, as unhealthy and unpleasant, but it is to this same dust that we owe the beautiful colours of the sunset sky. Without the dust in the atmosphere the amount of rain and snow which falls would be reduced enormously, for each raindrop is formed around a tiny dust particle and so the rain clouds are formed when the moisture condenses around a dust particle. It is these dust particles in the air and the air molecules which account for the blueness of 1 the sky, which varies considerably. At the zenith tlie blue is generally deeper and it gradually brightens toward the horizon, so much so that thei sky just above the horizon line may appear to be practically white. If we go high up into the air it will be founcl that the blue gets darker and darker as the air gets rarer. This question of sky colour has been investigated by a number of scientists and a special device known as a cyanometer was produced over a century ago by H. B. de Saussure. The cyanonieter consists essentially of a disc containing a complete range of 53 shades of sky colours. Number ona was a pure white, the next had an almost imperceptible trace of blue and then the blue shade became gradually darker and darker until number 53 waa a very deep shade of indigo, practically black. By the use of this scale, colour intensities at various heights and times can be classified and identified. On one observation it was found that the colour of the sky at the zenith was number 23, whereas at the horizon it was only the equivalent of number four on the cyanomcter. The red and yellow shades of sunset skies are also produced by the dust particles in the air. The ravs of light from the sun consist of white light; when they strike the dust particles, however, the white light is split up into iis spectrum colours. Whether the dust particle is surrounded by moisture or not some or the rays are absorbed, others arc scattered and deflected, and in this way the colours, chiefly the red, orange and yellow, which have long ware lengths, that reach the eye, go to make up the beautiful sunset array.