What causes change in color/intensity of sky?

[apconan]

Hi,
Right now, it's 9:50PM, official sunset time is 9:05. The sky is still blue, and there's a beautiful, intense dark orange coming from the horizon.
On other days, the sky could be much darker by now, and the sunset is a more pink/light orange.
Both days were clear and cloudless.
What causes these changes, if both nights had no clouds?

[Vaughn]

Moisture in the air (dry vs humid) will affect light passing through. Temperature differences. There could easily be colorless contaminates in the air, while can not be seen, could alter the light. Time of year -- angle of light.

[mike c]

As the sky approaches the horizon you are looking through more air while looking up there is less atmosphere or air to see through.

[Worker 11811]

Think of the earth's atmosphere as if it was a gigantic prism. As the light passes through the prism, some of the colors are refracted in different directions, just like in the classic schoolroom experiments we saw in science class as kids.

As Mike C. says, the closer the sun gets to the horizon, the more air... the more of the prisim... the sunlight has to go through and the more the light gets refracted.

Violet, blue and green light is refracted more than yellow, orange and red light.
Therefore, as the sun sets, more of its light has to go through the "atmospheric prism" and more of the violet, blue and green light gets refracted away from your view and the redder the light becomes.

Clouds can interact with the light coming from the sun and cause all sorts of colors to appear. That's why you'll see blue and purple clouds in a red and orange sky. The clouds are "catching" the different colors and reflecting it back down to us.

We don't necessarily need clouds to reflect light back down to the ground. Water vapor, dust, smoke and smog can do the same thing. It doesn't need to be visible to the naked eye, either. All it takes is a very slight haze to change a red and yellow sky into a purple and blue sky.

[bowzart]

Also, blue light scatters as it passes through minute particles in the atmosphere. (I'm not a scientist, so I'm sure someone will argue with the way I'm stating this). As wavelengths get longer, they scatter less. This is why the sky is blue. Some of the blue is taken out of the full spectrum, remaining in the sky. So as the atmospheric prism transmits light going toward the red, we begin to see that gorgeous reddish glow. I've always suggested to folks, when they see a beautiful sunset, turn around and enjoy what it's lighting.

Long ago, back in the 60's, I read all of AA's Basic Photo series. Some things really stuck. One of those was where he wrote about using colored filters to control atmospheric perspective. If you want planes to recede, you know, like "Purple mountain majesty [...] amber waves of grain" you'd use a blue filter, which records the blue light due to moisture in the air. The greater the distance, the more you'd see of the haze, and things far away could be even invisible. Using a hard red filter, like a #25 or #29 (Wratten) the atmospheric haze is minimized; the red light cuts right through. You can see this effect most clearly by using infrared film with a filter that cuts out all but the infrared. It is quite amazing; I've used this to describe the edge where the clear cut logging abuts the old growth forest in the Olympic National Park in Washington State. The air is very moist over there (rain forest) so haze is the normal state. Infrared cut right through it showing no haze at all. In the image, each tree is clearly visible.

Randy's statement above: "as the sun sets, more of its light has to go through the "atmospheric prism" and more of the violet, blue and green light gets refracted away from your view and the redder the light becomes" is interesting in that were it NOT true that reddish light penetrates haze, we would expect that with the added depth of the atmosphere as seen looking toward the horizon, we would experience greater obscuring due to the haze. We don't, though. If you look out at the ocean on a sunny midday, the horizon is far less clear than it would be at sunset.

One thing I've always wondered but never (yet!) tested: If one were to set up two cameras, one with pan film and the other with infrared looking toward the horizon and with both, make the exposure at the exact moment that the sun goes down, would the infrared film show the image of part of the sun where the pan would not?

[Curt]

The question that follows this one would be: "What color is the ocean?"

[Edward_S]

You might find the answer to your question in "Light and Color in the Outdoors" by Marcel Minnaert. There may be more modern treatments of the subject available now, but it's a beautiful book that I recommend to anybody. It's full of interesting observations and contains many simple experiments that you will inevitably want to try yourself! He devotes an entire chapter to 'Light and Color of the Sky'. It's also available on Google books if you want to take a peek. I was given a copy as a present a few years ago and couldn't put it down.

Edward.

[Ian Grant]

The question that follows this one would be: "What color is the ocean?"

Well the Aegean id Turquoise

Ian

[Edward_S]

Well the Aegean id Turquoise

Ian

Homer (and I don't mean Homer Simpson) thought the Aegean was 'wine dark'. What colour is your wine, just out of interest?

[snallan]

Homer (and I don't mean Homer Simpson) thought the Aegean was 'wine dark'. What colour is your wine, just out of interest?

It seems the ancient Greeks did not have a great need for naming colours, indeed they often referred to the sky as being "bronze".

As bowzart says, the reason for the colouration of the sky is light scattering due to dust/water vapour/crap in the atmosphere. Short wavelengths (blue) are scattered most, which is why the sky appears blue (and is more blue when you look at the area of sky opposite to the area containing the sun. Long wavelengths (red) are scattered less, which gives sunrises/sets their gorgeous array of colours.