29 September 2009, jd2020 @ 1:59 am

Nature created this eerie lighting in Australia on September 23,2009.

Through this dust storm, Australia’s worst in some 70 years, the skies exchanged colors from orange, pink, red and yellow from morning to night. Why?
Because tiny particles can scatter sunlight. Light interacts differently with minuscule particles than with objects in the macroscopic world. If the sky were filled with dust particles that were each significantly wider than the largest wavelength of visible light—i.e., if each one were much more than 750 nanometers wide—then the atmosphere would appear to be approximately the same color as the particles themselves. (In this case, orange.) But many of the dust particles hanging over Sydney were probably less than 750 nanometers. Sunlight scatters when it hits such small particles; its various color components are redirected in a complicated pattern, and only limited wavelengths of light pass through to the observer. In such situations, which physicists still don’t understand perfectly, the atmosphere can take on any number of colors, from blue to deep red, and can even look different depending on where the observer is standing.

Why is the dust orange in the first place? Because there’s so little vegetation. Southeastern Australian soil is composed of weathered ferric rocks. The iron makes the resulting clay minerals—like nontronite, saponite, and volkonsokite—orange-ish. This process is certainly not unique to the land Down Under. Many regions started out orange but eventually transitioned to brown or black as vegetation sprang up in the fertile clay and composted into dark organic matter. The climate around Sydney is too arid for trees and shrubs to proliferate, so the area retains its original hue. The lack of vegetation also explains the frequent dust storms. Clay is flaky, and there aren’t many trees or roots to prevent it from sweeping across the plains.
Read more @ Slate.com

Very nice photo collection of the freak dust storm @ Boston.com

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