Outback Dust Storm
imaged by Helen S of Sydney, Australia at 6:15 am on September 23, '09.
�Helen S, shown with permission.

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Fierce winds from inland New South Wales lofted dust from the ground and carried it as an enormous cloud some 2 km high to Sydney (more images & reports). The sky colour was fading when this picture was taken but earlier it was an opaque blood red. As the sun climbed the colour slowly faded. By mid-day the atmosphere was still opaque but the sky was a dull gray.

How was the red colour produced? Red sky colours as at sunset are mostly produced by Rayleigh scattering. Air molecules, very fine dust and other particles all much smaller than the wavelength of light (lambda/15) scatter blue light more strongly than red and so the sun seen in the remaining unscattered light appears red. The same scattered light makes the sky blue. Rayleigh scattering is unlikely to have produced the Sydney sky because most of the the dust cloud particles were too large and also the cloud was so optically thick - notice how the background trees are partially obscured. Light is multiply scattered by thick clouds and any transmitted light is less dependent on the wavelength discrimination of a single scattering event.

More likely, part (not necessarily all) of the sky colouration came from the colour of the particles themselves. The cloud consisted of red coloured particles typical of outback soil and larger than the wavelength of light. The overall red sky colouration is then partly a consequence of simple selective absorption by the dust itself.

There is another effect. Clues to it are that the sky changed to gray later on and that other photos taken when the cloud was thin enough to reveal the sun show the latter with a bluish hue. This suggests that some of the dust in suspension (particularly towards the end of the event) was of smaller particle size. Micron sized particles act as Mie scatterers with little wavelength dependence - hence the gray or dilute sky colour. The same particles, if all of similar size, sometimes selectively produce the fabled rare 'blue moon' or in this case a bluish sun - the scattered light is reddened. There was a lot happening the morning of the 23rd!