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In pictures taken from the summit of Mount Everest (such as this one), the colour of the sky is a very dark blue or even black in some pictures. I remember from my own experiences of hiking in the Himalayas that the sky appeared to be darker blue as I climbed higher. In shots looking outward from the summit there's a very interesting effect of having a black sky above with a blue sky lower down near the horizon. (one example here) I believed that this happens because there is less atmosphere above the climbers to scatter the blue wavelengths.

However, this same black sky effect does not occur when looking out the window of a plane. As the plane climbs, the sky does not appear to "change colour" at all as the plane ascends. The cruising altitude of commercial airliners on an international flight is slightly higher than Everest, so one would expect the sky to appear quite similar.

What explains these two observations?

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@user22834 - The explanation for these observations is that your second paragraph is incorrect. –  Rory Alsop Apr 6 '13 at 12:30
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I've often noticed that the sky does get darker as an aircraft ascends. I think it's harder to notice because you really have to crane your neck to look up out of an aircraft window, whereas when hiking you're surrounded by sky, so it's a lot more obvious in that case. –  Nathaniel Apr 6 '13 at 12:30
    
Your explanation for the phenomenon is correct, of course. –  Nathaniel Apr 6 '13 at 12:31

3 Answers 3

up vote 14 down vote accepted

However, this same black sky effect does not occur when looking out the window of a plane.

It doesn't?

enter image description here

(image credit: http://www.123rf.com/photo_10994787_view-of-jet-plane-wing.html)

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+1 ... it does this on every high flying aeroplane, and I have always enjoyed that deep blue above the curve of the horizon. –  Rory Alsop Apr 6 '13 at 12:29
    
well, i had never seen that type of horizon on when looking out the window of a plane, and i have been on a few transpacific flights. the sky was always just a very light blue colour. i'll try to look at multiple points during the flight next time. thanks –  dcorks Apr 15 '13 at 2:51

Don't forget that the bulk of the atmosphere's mass is in the troposphere and it is not uniform in thickness around the globe. In fact the troposphere is almost twice as thick around the equator as it is at the poles. This is due to differing air density resulting from substantially different average air temperature close to the surface.

Consequently, when flying in an airliner at say 10km altitude in equatorial or mid latitudes, your aircraft is still well and truly in the troposphere with plenty of atmosphere still above you. However flying at latitudes nearer the north or south pole at the same 10km altitude, you will be flying very close to the top of, or even above the troposphere.

Therefore despite being at the same altitude, the sky will look significantly darker for a passenger flying in the middle of the day at high latitudes than someone at mid or equatorial latitudes.

Hope this helps resolve some of the confusion regarding peoples differing opinions on how dark the sky looks from an airliner.

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Intriguing! I would have expected the pressure profiles of the troposphere to follow the same geoid shape as the earth's surface and ocean, so that its density vs. altitude is roughly constant; I'm surprised by your claim that there's a factor of two difference in the altitude of the troposphere-stratosphere boundary. Do you have a citation? –  rob Jul 30 at 17:49
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As a pilot I can vouch for this fact from personal experience. However, a quick Google search will back it up. Here's a quote from the NOAA website: 'The height of the troposphere varies from the equator to the poles. At the equator it is around 11-12 miles (18-20 km) high, at 50°N and 50°S, 5½ miles and at the poles just under four miles high'. You can check it out at srh.noaa.gov/jetstream/atmos/layers.htm –  Turiontoga Jul 30 at 21:35

The color of sky is no different than the color of grass, milk, trees etc.

Two basic kinds of light we perceive: direct light and reflected light. Direct light is only seen when it is perceived directly by our eyes. All other light is reflected from something. In the case of grass, milk, trees, it's obvious.

In the case of the sky, we perceive the light after it has been reflected from things that are present in the atmosphere such as gases, vapor, dust ... and, of course, the air itself.

And all of those things serve to diffuse the light as well. Nothing has a completely clean edge, it's always got a blurry boundary, if you look closely enough at it.

Where there is nothing to reflect from, there is no color. No light. No blurry edges.

In the case of sky, our atmosphere does not suddenly stop as if it has hit a brick wall, it becomes thinner and thinner and eventually fades away and becomes space.

If you look at images from the space station or other vehicles out there in space, you'll not be able to see the sky. That's because, for the purposes of our seeing light, it's essentially a vacuum. There's nothing for the light to be reflected from in the same way that it is in the atmosphere.

And every image has a sharp edge, because there's no atmosphere to diffuse the light gradually.

Similarly, if there aren't many things for the light to reflect from, or too few to be registered as reflections by our eyes, we don't see so much light.

When we see the sky, we must be somewhere. Usually (and always for people on Earth) the person doing the seeing is located in the atmosphere. So some light will always be reflected by the atmosphere that surrounds the person.

The amount of pure black, or "no-reflections", that you see depends upon your proximity to the vacuum of space. Unless you are really close to the vacuum, you will always see evidence of light being reflected in some degree from the atmosphere.

We see fewer and fewer reflections depending upon the amount of atmosphere between us and space in the direction we look. The darkest part will usually be directly above us, partly because that's usually where the atmosphere is thinnest, and mostly because the reflected light is reflected back up and away from us.

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