BRIGHT SUNSETS

Hokitika Guardian, 31 May 1932, Page 8

BRIGHT SUNSETS

PECULIARITY IN ATMOSPHERE

EXPLANATION BY DR FARR

CHRISTCHURCH, May 28.

Volcanic dust drifting 4500 mile»' through the upper air, from the Chhetn volcanoes to New Zealand, is stated by Dr -C. C. Farr, Professor of Physics at Canterbury Coi'lege, to be a likely cause of the brilliance of' the sunsets observed in th 6 past few weeks. There -.scorned to be no limit to the distance such fine wbiiild carry, a notable- example being that "of the dust sent up by the eruption of Krakatoa, in the East Indies, 'fifty years ago, when sunsets in Europe" 'were made brilliant for weeks. ' Eight seemingly extinct volcanoes on H the Chilean side of the Andes a hundred miles south of .the Trans-Andine railway—the. chief of them being T’nquiririca—became active early in April. They emitted a dense volume of ashes which was 'blown across th e -South American Continent to fail on Buenos Aires and Montevideo, : a thousand miles from its source, as well as on the narrow Chilean coastland. The possibility is that part of this- dust has been carried westward and is now, through the brilliance of the sunset glow, showing its presence in the New Zealand atmosphere. \

CAUSE OF ALL SUNSET COLOUR

The redness of sunsets at any time, said Dr,Farr, was due to the presence in the, atmosphere of -small particles 01 dust. The fact was that ordinary white fight, which was experienced under normal conditions, was compounded of the colours of the spectrum—red, orange, ye-flow, green, blue, indigo, and violet-. If white light was passed through - air or water which had particles of dust or other material -'in suspension, the component rays, were affected in. different manners according to their wave-length. The red rays, which had the longer! wave-length, got through without interference, but the others were deflected in varying degrees.

When there were insufficient fine particles, the- white light came through unaltered, but at sunset, when the sun was low in the sky, its rays, coming at a low angle along the earth’s surface, traversed a greater distance in the eqrth’s atmosphere, and consequently the presence of any fine- particles was shown in the- red glow. The sky irnmediately above, an observer remained bhie, because, although it also contained the particles, it was being regarded at .an agle- to the source of the light (the sun), so that the blu e rays, deflected by the interruption of the particles, predominated.

SIMILAR EFFECT ON WATER

Any form of particles could cause the -sunset effect, as when the smoke of a biv-'hfire caused the sun to glow red at midday, or a volcanic eruption loaded the atmosphere with fine dust, or a sandstorm ’sent particles of sand into the air. The more dust there was in the atmosphere the redder was the sunset. Water vapour particles also could give the same effect.

The blueness of lakes and riven - was also an illustration of the effect of the scattering of the components of white light by particles held in suspension—this time in water. If the particles in suspension were too large, or if there Were too many of them, the water looked merely muddy, but in gen-ral there was only sufficient to give a blue tone—this again resulting from the far t that the observer looked at the water at an angle from the direct line of the sun’s rays. If he looked through a sufficient quantity of the water d’rectly at the 'sun red would be the predominant colour.

Dr Farr demonstrated this point by an experiment.. Through a jar containing water with fine particles, of resin in suspension .he passed a strong wh't" light from a projector. From the side the, water was a pale blue, but .viewed from, the direct line of the •light it appeared red, and cast a red image on the wall.