question: The sky is blue because the blue wavelengths get scattered - but isn't that just how colours work?
comment: So air is most definitely colourless, not blue. Gaseous Oxygen has a very pale blue tinge to it, but only makes up ~20% of the atmosphere. The remaining content is mostly Nitrogen, which is colourless and dilutes the already faint blue of Oxygen to essentially no colour at all. Oxygen being blue does not explain the colour of the sky. The blue you see from the sky is in fact the sun's colour, not the sky's. The air scatters the sunlight in all directions (which is why the sky is everywhere), and it scatters the blue photons further (which is why the sky is specifically blue). This is different to the sky being blue like oxygen is blue - ie: absorbing all wavelengths but blue, which it doesn't because air is colourless.
passages: Section::::Background.
   Light from the sky is a result of the Rayleigh scattering of sunlight, which results in a blue color perceived by the human eye. On a sunny day, Rayleigh scattering gives the sky a blue gradient, where it is darkest around the zenith and bright near the horizon. Light rays incoming from overhead encounters of the air mass that those coming along a horizontal path encounter. Hence, fewer particles scatter the zenithal sunbeam, and thus the light remains a darker blue. The blueness is at the horizon because the blue light coming from great distances is also preferentially scattered. This results in a red shift of the distant light sources that is compensated by the blue hue of the scattered light in the line of sight. In other words, the red light scatters also; if it does so at a point a great distance from the observer it has a much higher chance of reaching the observer than blue light. At distances nearing infinity, the scattered light is therefore white. Distant clouds or snowy mountaintops will seem yellow for that reason; that effect is not obvious on clear days, but very pronounced when clouds are covering the line of sight reducing the blue hue from scattered sunlight.
   Except for light that comes directly from the sun, most of the light in the day sky is caused by scattering, which is dominated by a small-particle limit called Rayleigh Scattering. The scattering due to molecule sized particles (as in air) is greater in the direction toward and away from the source of light, than it is in directions perpendicular to the arrival path. Scattering is significant for light at all visible wavelengths but is stronger at the shorter (bluer) end of the visible spectrum, meaning that the scattered light is bluer than its source, the sun. The remaining sunlight, having lost some of its short wavelength components, appears slightly less blue.
   After the sun has also set for these altitudes at the end of nautical twilight, the intensity of light emanating from earlier mentioned lines decreases, until the oxygen-green remains as the dominant source.
  
  When astronomical darkness has set in, the green 557.7 nm oxygen line is dominant, and atmospheric scattering of starlight occurs.
  
  Differential refraction causes different parts of the spectrum to dominate, producing a golden hour and a blue hour.
  
  Section::::Relative contributions.
   The sun is not the only object that may appear less blue in the atmosphere. Far away clouds or snowy mountaintops may appear yellowish. The effect is not very obvious on clear days but is very pronounced when clouds cover the line of sight, reducing the blue hue from scattered sunlight. At higher altitudes, the sky tends toward darker colors since scattering is reduced due to lower air density; an extreme example is the moon, where there is no atmosphere and no scattering, making the sky on the moon black even when the sun is visible.
   The still commonly presented incorrect explanation claims that the Earth’s post-sunset and pre-sunrise atmosphere solely receives and disperses the sun’s shorter blue wavelengths and scatters the longer, reddish wavelengths to explain why the hue of this hour is so blue. In fact, the blue hour occurs when the Sun is far enough below the horizon so that the sunlight's blue wavelengths dominate due to the Chappuis absorption caused by ozone. Even some websites dedicated to information about ozone still propagate the incorrect claim that ozone does not contribute to the blue color of the sky, though this effect is much weaker during the rest of the day, between morning and evening twilight.
   Sky blue is the name of a colour that resembles the colour of the unclouded sky at noon (azure) reflecting off a metallic surface. The entry for "sky-blue" in Murray's "New English Dictionary" (1919) reports a first sighting of the term in the article on "silver" in Ephraim Chambers's "Cyclopaedia" of 1728. However, many writers had used the term "sky blue" to name a colour before Chambers. For example, we find "sky blue" in "A Collection of Voyages and Travels" (London: Awnsham and John Churchill, 1704), vol. 2, p. 322, where John Nieuhoff describes certain flowers: "they are of a lovely sky blue colour, and yellow in the middle". 
   Different light sources produce differing amounts of visual sky glow. The dominant effect arises from the Purkinje shift, and not as commonly claimed from Rayleigh scattering of short wavelengths (see ). When observing the night sky, even from moderately light polluted areas, the eye becomes nearly or completely dark-adapted or scotopic. The scotopic eye is much more sensitive to blue and green light, and much less sensitive to yellow and red light, than the light-adapted or
   Airglow is the collective name of the various processes in the upper atmosphere that result in the emission of photons, with the driving force being primarily UV radiation from the Sun. Several emission lines are dominant: a green line from oxygen at 557.7 nm, a yellow doublet from sodium at 589.0 and 589.6 nm, and red lines from oxygen at 630.0 and 636.4 nm.
   Van Gogh described to his brother Theo how he composed a sky: "The dark blue sky is spotted with clouds of an even darker blue than the fundamental blue of intense cobalt, and others of a lighter blue, like the bluish white of the Milky Way ... the sea was very dark ultramarine, the shore a sort of violet and of light red as I see it, and on the dunes, a few bushes of prussian blue."
  
  Section::::History.:Blue suit.
   Although no images from within Jupiter's atmosphere have ever been taken, artistic representations typically assume that the planet's sky is blue, though dimmer than Earth's, because the sunlight there is on average 27 times fainter, at least in the upper reaches of the atmosphere. The planet's narrow rings might be faintly visible from latitudes above the equator. Further down into the atmosphere, the Sun would be obscured by clouds and haze of various colors, most commonly blue, brown, and red. Although theories abound on the cause of the colors, there is currently no unambiguous answer.
   Section::::Atmospheric extinction.
  
  Atmospheric extinction gives the rising or setting Sun an orange hue and varies with location and altitude. Astronomical observatories generally are able to characterise the local extinction curve very accurately, to allow observations to be corrected for the effect. Nevertheless, the atmosphere is completely opaque to many wavelengths requiring the use of satellites to make observations.
   Section::::History.
   Section::::Day lighting.
  
  As the Sun crosses the sky, it may appear to be red, orange, yellow or white depending on its position. The changing color of the Sun over the course of the day is mainly a result of scattering of light and is not due to changes in black-body radiation. The blue color of the sky is caused by Rayleigh scattering of the sunlight from the atmosphere, which tends to scatter blue light more than red light.
   Section::::Environmental pollution.
  
  A clean atmosphere is indicated by the fact that the Milky Way is visible. Comparing the zenith with the horizon, shows how the "blue quality" is degraded depending on the amount of air pollution and dust. The twinkling of a star is an indication of the turbulence of the air. This is of importance in meteorology and for the "seeing" of astronomy.
  
  Light pollution is a significant problem for amateur astronomers but becomes less late at night when many lights are shut off. Air dust can be seen even far away from a city by its "light dome".
   The blue hour (from French ) is the period of twilight in the morning or evening (during the civil twilight and nautical twilight), when the Sun is at a significant depth below the horizon and residual, indirect sunlight takes on a predominantly blue shade that is different from the blue shade visible during most of the day (on clear days), which is caused by Rayleigh scattering. The blue hour occurs when the Sun is far enough below the horizon so that the sunlight's blue wavelengths dominate due to the Chappuis absorption caused by ozone. It is a colloquial term and doesn't have an official definition similar to dawn, dusk, and the 3 phases of twilight. Rather, it refers to a state of natural lighting that usually occurs at daybreak in the morning and during the last stages of twilight in the evening.
   Near sunrise and sunset, most of the light we see comes in nearly tangent to the Earth's surface, so that the light's path through the atmosphere is so long that much of the blue and even green light is scattered out, leaving the sun rays and the clouds it illuminates red. Therefore, when looking at the sunset and sunrise, the colour red is more perceptible than any of the other colours.
   In detail, the effects are complex, depending both on the distance from the source as well as the viewing direction in the night sky. But the basic results of recent research are unambiguous: assuming equal luminous flux (that is, equal amounts of visible light), and matched optical characteristics of the fixtures (particularly the amount of light allowed to radiate directly upward), white sources rich in shorter (blue and green) wavelengths produce dramatically greater sky glow than sources with little blue and green. The effect of Rayleigh scattering on skyglow impacts of differing light source spectra is very small.
   Air Force blue
  
  Air Force blue colours are a variety of colours that are mostly various tones of the colour azure, the purest tones of which are identified as being the colour of the sky on a clear day.
  
  Some "air force blue" colours, notably the air force blue colour used by the United States Air Force and the colour used by the US Air Force Academy, may look like they are tones of blue instead of azure. However, they are actually dark tones of azure, not blue.
   Blue Sky
  
  Blue Sky, BlueSky or Bluesky may refer to:
  
  Section::::Science.
  
  BULLET::::- Blue skies research (also called blue sky science), scientific research in domains where "real-world" applications are not immediately apparent
  
  BULLET::::- Diffuse sky radiation, solar radiation reaching the Earth's surface after Rayleigh scattering
  
  BULLET::::- Rayleigh scattering, which causes the sky to appear blue
  
  Section::::Places.
  
  BULLET::::- Bluesky, Alberta
  
  BULLET::::- Bluesky Formation, a stratigraphic unit of Lower Cretaceous age in the Western Canada Sedimentary Basin
  
  BULLET::::- Blue Sky, Colorado
  
  Section::::People.
  
  BULLET::::- Blue Sky (artist) (born 1938), American painter and sculptor formerly known as Warren Edward Johnson
  
  Section::::Art, entertainment, and media.
   This is why the sky appears blue and the sun yellow — more of the higher-frequency blue light arrives at the observer via indirect scattered paths; and less blue light follows the direct path, giving the sun a yellow tinge.
   In most photometric systems filters (passbands) are used from which readings of magnitude of light may take account of latitude and humidity among terrestrial factors. Interstellar reddening equates to the "color excess", defined as the difference between an object's observed color index and its intrinsic color index (sometimes referred to as its normal color index). The latter is the theoretical value which it would have if unaffected by extinction. In the first system, the UBV photometric system devised in the 1950s and its most closely related successors, the object's color excess formula_1 is related to the object's B−V color (calibrated blue minus calibrated visible) by:
   Displayed at right is the color sky magenta. The color "sky magenta" is a representation of the color of the sky near the sun during the brief period during twilight when the pink of sunset transitions into the blue of early evening. This color was one of the colors in the set of "Venus Paradise" colored pencils, a popular brand of colored pencils in the 1950s. 
  
  This color is also called "medium lavender pink".
  
  A photograph of the sky displaying the color "sky magenta" in its natural context by photographer Dave Horne  is displayed here: 
   When the sky is clear, the blue hour can be a colorful spectacle, with the indirect sunlight tinting the sky yellow, orange, red, and blue. This effect is caused by the relative diffusibility of shorter wavelengths (bluer rays) of visible light versus the longer wavelengths (redder rays). During the blue "hour", red light passes through space while blue light is scattered in the atmosphere, and thus reaches Earth's surface. Blue hour usually lasts about 20-30 minutes right after sunset and right before sunrise. For instance, if the sun sets at 6:30 p.m., blue hour would occur from 6:40 p.m. to 7 p.m.. If the sun were to rise at 7:30 a.m., blue hour would occur from 7 a.m. to 7:20 a.m.. Time of year, location, and air quality all have an impact on the exact timing of blue hour. 
   Bleu celeste
  
  Bleu celeste (, "sky blue") is a rarely occurring and non-standard tincture in heraldry (not being one of the seven main colours or metals or the three "staynard colours"). This tincture is sometimes also called ciel or simply celeste. It is depicted in a lighter shade than the range of shades of the more traditional tincture azure, which is the standard blue used in heraldry.
  
presuppositions: The sky is blue. The sky is blue.
corrections: Air is colourless. The sun is blue, rather than the sky.
raw_presuppositions: The sky is blue. The sky is blue.
raw_corrections: Air is colourless. The sun is blue, rather than the sky.