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Is the color of the sky at noon (local time) in, say, NY, Buenos Aires, London, Nairobi, Sydney, New Delhi and Tokyo the same? I choose the specific time of noon to exclude the twilight colors of the sky and I choose cities that aren't above the northern or southern pole to make it easier.

Also can the color of the sky be the same or not depending of the season? For instance, the color of the sky of Buenos Aires on January 1st would be the color of the sky in NY on June 1st.

Also, is the color of the sky of one region on earth the same all year round and all day long, once again, without taking in account twilight?

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  • $\begingroup$ Any specific season? $\endgroup$ – user889 Dec 16 '14 at 10:38
  • $\begingroup$ Thanks for helping me making my question more specific! My answer is no for the second question because it's for one region all year round, but "could be" for the first one (the sentence in parentheses). $\endgroup$ – MagTun Dec 16 '14 at 10:45
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    $\begingroup$ To measure this you'd want to look at solar noon, not local noon. Solar noon is the time when the sun is most directly overhead and will vary from local noon depending on the time of year, where in the time zone you are (east-west), and whether your time zone observes DST. $\endgroup$ – casey Dec 16 '14 at 16:00
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    $\begingroup$ If you are interested in the effects of local air pollution on the color of the sky, please make that clear. You have named a lot of cities that do have significant pollution. $\endgroup$ – farrenthorpe Dec 16 '14 at 16:54
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    $\begingroup$ Also - please note that "the color of the sky" is not the same at any two points of the sky, even at noon. I assume you mean "at some fixed angle to the sun". See hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c5 $\endgroup$ – Floris Dec 16 '14 at 19:47
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The blue color of the sky is due to Rayleigh scattering. As light from the sun shines down it scatters off of (mostly) nitrogen molecules in the atmosphere. Without this process, the sky would be dark during the day except for the bright sun, moon, and stars.

Rayleigh scattering is more efficient for shorter wavelengths. So, even though the Sun provides a full spectrum, blue is scattered more efficiently than the longer wavelengths of light. Thus blue is the dominant color you see. See Rayleigh scattering (Wikipedia) for more info on Rayleigh Scattering.

So, to answer your question, the color of the sky is the same for most places on the Earth at noon because the sunlight is relatively constant and the atmosphere is relatively uniform in composition (nitrogen and oxygen). The exceptions are air pollution, high latitudes, and high altitudes. Air pollution and high latitudes have similar effects in that more attenuation of the light occurs as it travels through more atmosphere, and therefore colors change like you see at twilight. Air pollution in particular is a highly variable source of scattering and absorption that gets quite extreme when there are high concentrations of particles. High latitudes change the geometry, similar to twilight. In contrast, high altitudes have less atmosphere, so if you go high enough (e.g. in a hot-air balloon) there just won't be much scattered light and it will get dark due to the lack of atmosphere.

From the wikipedia article:

In addition the oxygen in the Earth's atmosphere absorbs wavelengths at the edge of the ultra-violet region of the spectrum. The resulting color, which appears like a pale blue, actually is a mixture of all the scattered colors, mainly blue and green.

and

The reddening of sunlight is intensified when the sun is near the horizon, because the volume of air through which sunlight must pass is significantly greater than when the sun is high in the sky. The Rayleigh scattering effect is thus increased, removing virtually all blue light from the direct path to the observer. The remaining unscattered light is mostly of a longer wavelength, and therefore appears to be orange.

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    $\begingroup$ the atmosphere is relatively uniform in composition - I disagree. See my answer for details. $\endgroup$ – Floris Dec 16 '14 at 16:37
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    $\begingroup$ The atmosphere is mostly nitrogen and oxygen... a relative uniformity. Yes air quality affects scattering and absorption but this is similar to twilight colors and not the point of the question. $\endgroup$ – farrenthorpe Dec 16 '14 at 16:52
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    $\begingroup$ I disagree. I was recently in Australia and was struck by the quality of the light. Their blue sky looks quite different than our blue sky. I will try to find a more authoritative reference than my eyes. $\endgroup$ – Floris Dec 16 '14 at 16:58
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    $\begingroup$ See for example en.wikipedia.org/wiki/Cyanometer which includes "De Saussure concluded, correctly, that the color of the sky was dependent on the amount of suspended particles in the atmosphere". See also hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c5 for a simple example of changing blueness with position in the sky. $\endgroup$ – Floris Dec 16 '14 at 18:05
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    $\begingroup$ @Floris Yes i understand that color depends on scattering and that changes depending on atmospheric composition. However, it was apparent to me that the poster's intent is to solicit answers about the nominal color of the sky across large regions... not about the effects of air pollution on scattering. I've asked the poster to clarify if that is his intent. $\endgroup$ – farrenthorpe Dec 16 '14 at 21:57
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Absolutely not. While the answers by casey and farrenthorpe correctly state that the blue color is due to Rayleigh scattering, the composition of the atmosphere varies considerably from place to place - and with different composition come different degrees of scattering, and different color / intensity. Typically regions of greater industrial activity will carry larger densities of particles which, in the presence of moisture in the air, will become scatterers. The wavelength dependence is a function of the size of the particles - so as the size changes, so will the color.

See my answer to the question Why is the sun brighter in Australia compared to parts of Asia. The same mechanism that makes the sun "brighter" also makes the sky "darker" (namely, less scatter).

This is true even on a nominally "clear" day - because there is always some particulate matter in the atmosphere. And the concentration of particulates really does vary with location on earth - see this image as an example (from NASA).

enter image description here

The other point worth keeping in mind is altitude: the sky looks darker when you are at the top of a mountain, even if the composition of the atmosphere is the same. That is because there is "less atmosphere" to scatter. And of course, some places on Earth are (always) at a different altitude...

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    $\begingroup$ I see your point but I don't think that air pollution and its effects on light scattering/absorption are what the question is asking about. Pollution varies constantly but the question is posed in terms of long-term trends (e.g. seasonal). $\endgroup$ – farrenthorpe Dec 16 '14 at 16:56
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    $\begingroup$ There is a lot of temporal and spatial variability in aerosols that is highly dependent on source and meteorology. You talk about particles on any given day but then show a long-term average of aerosol loading. I think it would be helpful if you added how long of an average (and when) it is. ... or perhaps show a panel of different days to illustrate the large variance. $\endgroup$ – farrenthorpe Dec 16 '14 at 22:05
  • $\begingroup$ @farrenthorpe - Fair point - but "on average" the distribution is not uniform across the globe. Lots of details at sedac.ciesin.columbia.edu/data/set/… , including maps with "annual average" which shows a very distinct non-uniform pattern. These things do really change how blue the sky is - especially in conjunction with humidity. There is no single "blue sky" color. Not even in the same place on earth - let alone as you move around. $\endgroup$ – Floris Dec 16 '14 at 22:12
  • $\begingroup$ I looked at this graph for why Italian sky looks much brighter than south Indian sky, was disappointed. $\endgroup$ – Jesvin Jose Dec 17 '14 at 13:53
  • $\begingroup$ @aitchnyu - pollution is only part of the story; relative humidity is the other part. Humidity in South India is higher, so there will be more scatter for the same amount of particulate matter. That's the best explanation I can give, sorry. $\endgroup$ – Floris Dec 17 '14 at 14:07
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To supplement farrenthorpe's answer, the color of the sky will be a function of the path length between the top of atmosphere and your eye along the path from the sun to you. Due to Rayleigh scattering, shorter wavelengths will scatter before longer ones. As the path length increases more of the longer wavelengths will be scattered. The color of the sky is the sum of all of the photons scattered out of the beam of photons from the sun and then re-scattered so they end up hitting your eye.

Our blue sky is a result of the depth of the atmosphere (small). If the atmosphere was less deep (shorter path length) the sky would tend toward darker blue/violet. If the atmosphere was deeper (longer path length) the sky would tend toward oranges, pinks and reds. We see the latter case during sunset.

Given observations taken at solar noon at latitudes between the 66 N/S there likely is some variance in sky color as there will be subtle path length differences at different latitudes, particularly between the hemispheres due to the earths tilt. However, you would likely need a sensitive instrument to notice these differences and could consider it a constant. The variance in sky color during the diurnal cycle from dawn till dusk is a far stronger effect than looking only at solar noon day-to-day.

Note that I am only considering scattering by a uniform atmosphere neglecting the effects of aerosols and water vapor. Foris's answer takes up this topic (and you can easily observe the difference in how the sky looks on a very dry day vs a very humid day).

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  • $\begingroup$ Thanks casey for this explanation about the effect of the depth of the atmosphere. $\endgroup$ – MagTun Dec 17 '14 at 16:52
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Look near the Sun on a humid summer noon and then look at it after a strong cold front clears and tell me the sky is the same everywhere on Earth. Make sure your eyeballs are in shadow when you do this.

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The color of the sky in Norway was greenish. In the Netherlands it was just blue. I saw this on my vacation to Norway around 1990. I supposed the sky in Norway was more greenish because of the large woods. So I suppose the color of the sky is also due to the reflection in it of what is on the ground. Presumably the sky in a large desert will cary the color reflection of the sand. Also the draining (partly) of the IJselmeer, a large Sea in the Netherlands caused the light in the sky supposedly to have become different. As I remember, less bright.

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