What Causes A Blue Sun?

Question: What can cause the sun the appear blue?

Answer:Volcanic dust, as well as smoke from forest fires, desert dust and heavy industrial pollution, can make the sun appear blue by scattering out the longer wavelengths of visible light. The phenomenon only occurs when the scattering particles are in a certain size range.

A blue color to the sun results when the sky background is a neutral grey coloured cloud composed of small suspended particles. Particles must be less than 0.85 micrometres (0.03 thousandths of an inch) to produce a blue sun. If they were slightly larger (around 1.1 micrometres), the scattering would produce a green sun instead.

The effect here is due to what is known as Mie scattering of light, named after G. Mie who in 1908 showed mathematically the effect on light rays of small particles in the air.

According to Aden and Majorie Meinel in their book Sunsets, Twilights and Evening Skies, reports from the Royal Society of London after the Krakatoa eruption of 1883 mention blue and green suns in various tropical countries.

They also report the observations of Peter Fraken of a blue sun in Beijing, China in 1978 that he attributed to dust storms from the deserts of Mongolia.

Learn More From These Books Chosen by The Weather Doctor

• Lynch, David K. and Livingston, William: Color and Light in Nature, 2nd edition, 2001, Cambridge University Press; ISBN: 0521775043 (PB).
• Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.
• Dunlop, Storm: The Weather Identification Handbook: The Ultimate Guide for Weather Watchers,, 2003, The Lyons Press, ISBN 1585748579. (pb).

The Weather Doctor's Weather Almanac What Causes A Blue Sun?
©2000, Keith C. Heidorn, PhD. All Rights Reserved.
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