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Old 07-13-2008, 07:33 PM
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ceptimus ceptimus is offline
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Join Date: Aug 2004
Location: UK
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Default Re: Atmospheric optical phenomena

I'll have a shot at an explanation, but it may be a bit long-winded. :shrug: Scroll down to the end for my guess as to what happened the day you and your friend witnessed it.

The reason we get red skies mornings and evenings is that the sunlight that reaches us then is passing through many miles of atmosphere.

The main part of the atmosphere, where the air is thick and almost all weather happens is only about ten miles deep. So if the sun is overhead, the light is passing through about ten miles of air to reach us.

But when the sun is low the light still has to do the ten vertical miles plus maybe fifty or more miles horizontally.

The atmosphere scatters the shorter wavelength (blue) light more than the longer wavelength (red). So the blue light gets smeared all over the sky (this is the simple explanation of why the sky is blue). This leaves the direct light from the sun missing some of its blue light, so the sun appears redder than it would if you viewed it from space. The more air the sunlight passes through, the redder it gets.

Now any dust, smoke, or other particles in the atmosphere enhance the effect - so we get spectacular sunsets for a few months after a big volcanic eruption due to the smoke from the volcano in the high atmosphere.

When the air is 'dusty' like this, most of the blue light has been scattered long before the light reaches us, and then the red part begins to scatter too, turning the whole sky red.

My guess is that on the day you witnessed the spectacular reddening, there was a layer of dirty air close to the ground, capped by upper clearer air. The dirty air could be caused by a fire - maybe a forest fire - or even just lots of pollen from trees, grasses or grain crops if it was hay fever season. The 'capped layer' of air is often caused when there is a temperature inversion in the atmosphere.

As the sun moved lower in the sky it would fairly quickly encounter a lot more of the dirty air in reaching you. And hence the sudden reddening. The effect would be most pronounced if the 'dirt' didn't exist immediately above you. If there were forest fires or pollen plumes in the direction of the sun this could happen.
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