What causes change in color/intensity of sky?

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?

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.

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

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.

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?

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

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.

Curt said:

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

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Well the Aegean id Turquoise

Ian

Ian Grant said:

Well the Aegean id Turquoise

Ian

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

Edward_S said:

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

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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.

Actually, pollution plays a part as well, as particles can affect the color of light in sunsets. Interesting story: the famous painting "The Scream" has a strange set of background colors that look imaginary. But the artist painted it in northern Europe some months after Krakatoa blew and threw tons of dust into the world's atmosphere. This resulted in strangely-colored skies and sunrises/sunsets in many locations around the world, including in Europe.

*putting on my atmospheric physicist hat* The previous respondents have all given you bits and pieces of what's happening so, let's summarize for clarity.

Photons obey Newton's first law just like anything else and travel in a straight line unless forced to do otherwise. However, even when looking straight at the sun, the chance that any photon reaching your eye (or other photosensitive medium) has gone straight from the sun to your eye without scattering off something is incredibly small. After sunset, that chance is zero.

Describing what happens between the sun and our eyes falls under the study of radiative transfer which ends up being all about the subtleties of what stuff is between the sun and you and how it interacts with electromagnetic radiation. A lot of that subtlety has to do with how large the stuff doing the scattering is compared to the wavelength of light that is being scattered. Light scattering off gas molecules (scatterer size about same as wavelength of light) gives us the blue daytime sky: short wavelengths (blue) scatter more than longer (red) so the red stays in the straight line path away from the sun and the blue gets scattered out of the beam and into our eyes (from virtually all angles away from the sun). At sunrise/sunset, we are looking at the sun so this same effect scatters blue out of the beam and away from our eyes leaving enhanced red color to reach our eyes. Blue daylight and red sunsets are the same phenomenon just viewed from different angles.

Light scattering off larger particles (aerosols, dust and water droplets) gives rise to scattering that is generally less wavelength dependent. Notable exceptions are fine dust which tends to help redden sunsets and large water droplets (rain) which produces rainbows. Most of what changes here is how the intensity of light is distributed angularly in a beam of light coming from the sun. Large aerosols tend to scatter photons more forward in the beam and less out of the beam (you can think of this like focusing the beam, although that is not really what is happening). This is the source for the bright, white aureole around the sun during the daytime: dust and aerosol in the atmosphere forward scatter the entire spectrum making a white-ish, bright area around the sun. Compare the area of sky near the sun on several different sunny days with different humidity and different air quality forecasts. Cleaner, drier days should show much less whitening near the sun.

Anyway, this merely scratches the surface. Like I said, it all becomes about the subtleties and that is why we should all spend more time admiring sunrise and sunset.

snallan said:

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

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Edward_S said:

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

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Well some of the ancient Greek glass-ware in the museums is pale green-blue, so maybe not far off the colour of the Aegean when filled with white wine U usually drink the local Red wine though.

The sky's often Zone V here, rather deep Blue

Ian

Edward_S said:

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

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There is a strong bias toward red in this household, and within that range it varies. It is probably generally darker than the wine Homer drank, though I am not sure whether in 1000 BCE - or whenever he lived; I don't think anybody's really sure - the Greeks practiced watering their wine. They did so during the classical period, when to drink your wine "neat" was apparently considered vulgar. Seems to me I got this from Thucydides. It has been something like 50 years since I studied it.

Homer also thought (at least as translated by Richard Lattimore) that dawn has rosy fingers.

Curt said:

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

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While water has a very slight blue-green color (you'd need a lot of it to really notice it) the main influence on the color of the ocean is the sky, which is reflected by the surface. So, one might ask, further, why is it always darker than the sky? Light from the sky is not polarized, but what's reflected by the surface is. So, the ocean surface is a little less than 1/2 as bright (like your polarizer's filter factor) as the sky.

Variations occur that may make the surface appear to be markedly different from the reflected sky above it. One reason (I suspect it is the main influence) is that the surface of the ocean is not just like the mirror in your bathroom. It has ever-changing facets, each of which (millions, billions) is reflecting a different location in the sky. To complicate matters further, each of these facets reflects light that is polarized in it's own peculiar plane, so the polarization is, in the aggregate, an extremely complex dance. It could appear that there is a near complete compliment of planes, but that isn't at all the same as the unpolarized light coming directly from the sky.

Isn't stuff like this fun? Thanks sbuczkowski for the very informative contribution!

Larry Bullis
Anacortes, WA: where there's LOTS of water reflecting lots of sky,
especially covered with clouds much of the time, but when it's sunny
there's an amazing amount of really gorgeous blue.

Trask said:

Interesting story: the famous painting "The Scream" has a strange set of background colors that look imaginary. But the artist painted it in northern Europe some months after Krakatoa blew and threw tons of dust into the world's atmosphere. This resulted in strangely-colored skies and sunrises/sunsets in many locations around the world, including in Europe.

Click to expand...

Thanks for reminding me of this! If anyone is interested further, you can read the Sky & Telescope magazine article here:

http://ecommons.txstate.edu/cgi/viewcontent.cgi?article=1023&context=physfacp

Ian Grant said:

Well some of the ancient Greek glass-ware in the museums is pale green-blue, so maybe not far off the colour of the Aegean when filled with white wine

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Hmmm... interesting thought!

bowzart said:

Isn't stuff like this fun? Thanks sbuczkowski for the very informative contribution!

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Yes it's fun; I quite enjoy this kind of thing.

Edward.

bowzart said:

There is a strong bias toward red in this household, and within that range it varies. It is probably generally darker than the wine Homer drank, though I am not sure whether in 1000 BCE - or whenever he lived; I don't think anybody's really sure - the Greeks practiced watering their wine. They did so during the classical period, when to drink your wine "neat" was apparently considered vulgar. Seems to me I got this from Thucydides. It has been something like 50 years since I studied it...

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Homer meant that the sea was dark as the (red) wine. The fine detail is that there were two different words for two related things: οίνος - κράσος (oinos - krasos). Oinos, the word used in Odyssey, is the word for undiluted wine. Krasos (krasi in moder greek) is the word for the diluted.