When we ask ourselves why the sky is blue we can think about the most important type of light scattering in the Earth’s atmosphere. This is Rayleigh scattering. I was writing about this type of light scattering on some occasions, like watching the solar eclipse through the webcams or wondering about the Martian sky. Today I would like to develop much more this topic and bring a rough answer for one of the most basic questions from geography or physics: why the sky is blue?
- THE RAYLEIGH SCATTERING & WHY IS THE SKY BLUE DURING THE DAY
Rayleigh scattering is the scattering of light by particles much smaller than the wavelength of the radiation. Rayleigh scattering works the best in a gaseous environment. The particles may be individual atoms or molecules. The intensity of scattered light significantly depends on the scattered light wavelength – increasing with the decrease of wavelength in the majority of cases. Sunlight reaches Earth’s atmosphere and is scattered in all directions by all gases and molecules in the air. We see the blue sky for most of the time because blue is scattered more strongly than other colours in the atmosphere. It happens, because Rayleigh scattering is inversely proportional to the fourth power of wavelength so that shorter wavelength violet and blue light will scatter more than longer wavelengths (Pic. 2). Why don’t we see the violet in the sky then? Violet basically is almost gone because of three factors: firstly Sun likewise other stars has its own spectrum of light emission, which is not constant and falls away in the violet. Secondly, the shortest, violet wavelengths are absorbed by oxygen particles of the atmosphere. Thirdly human eye is less sensitive to violet. At the finish of this consideration I can add up the other factor, which is a presence other light wavelengths scattered in the atmosphere, that finally makes the output sky color like pale blue (a mixture of all the scattered colours, mainly green and blue)(Pic. 1). This output can be explained as a dominant wavelength, which for Rayleigh scattering is about 475nm, which lies solidly in the blue if we take this to mean light with wavelengths between 450 and 490nm (Bohren, 2003). Conversely glancing towards the Sun directly the colors are not scattered away. The longer wavelengths such a red or yellow light are visible directly then, giving the Sun itself a slightly yellowish hue throughout the day and accordingly yellow, orange and red when near the horizon (Pic. 2, 8).
Rayleigh scattering results in the electric polarizability of the particles. The oscillating electric field of a light wave acts on the charges within the particle causing them to move at the same frequency. The particle, therefore, becomes a small radiating dipole, whose radiation we see as scattered light.
The Rayleigh scattering of sunlight in the atmosphere causes diffuse sky radiation. The diffuse sky radiation is solar radiation reaching the Earth’s surface after having been scattered from the direct solar beam by molecules or particulates in the atmosphere (Kondratyev, 1969).
2. VARIATION OF SKY COLOUR AND BRIGHTNESS
Skylight is not only pure blue. As per the image above we can see different blue hues across the vault of the sky. The best blues is at zenith (omitting the situation, when Sun is at zenith). Near the astronomical zenith, the sky is brighter than overhead but of considerably lower purity (Bohren, 2003). The variation of sky brightness becomes better visible when an observer is located higher above mean sea level (Pic. 4 – 7). This effect is best visible for us when flying at cruising altitude. At 10000 m.a.s.l the sky is inherently the most nonuniform in colour for us comparing with the view from the ground, especially from sea level in lower latitudes. To better understand this phenomenon we must invoke a multiple scattering, that plays here a major role. A solar beam being scattered in Earth’s atmosphere when entered into the medium has been scattered again on atmosphere molecules affected by the earlier single scattered solar beam. It causes a multiple scattering, which tends to wash out the strong wavelength-dependence. That’s why during the bright day the blue color of the sky is much less saturated near the horizon than higher up.
The difference between sky color variations arises out of a few factors:
– altitude – comparing the sea level and aircraft cruising altitude there is a huge difference in atmosphere density. Basically, at the upper troposphere, we have around twice molecules less than at sea level. Because of this, the sky appears much darker, that we used to see from the ground. This effect can be also noticeable in high mountains when altitude is higher than 2000 m.a.s.l. (Pic. 5, 6).
– latitude & climate – basically tropopause lies higher above lower latitudes. The color sky variation will be slightly less noticeable on the equator than in polar areas.
– waver vapor – will strongly influence the sky color variation, however, in conjunction with the altitude will bring the same effect eventually. Basically in the “free atmosphere,” it will affect sky colour variations marginally. I will bring more details in forthcoming articles.
3. POLARIZATION OF THE SKY
The light coming from the Sun is unpolarized, but the light of the sky during the day is polarized, as per the video below:
Looking at the sky through a polarizer, which preferentially blocks the polarization in one direction and allows the perpendicular direction to pass, one sees that light in the sky possesses some amount of polarization.
This amount of polarization is dependent on the daily position of the Sun. While all scattered light is polarized to some extent, light is highly polarized at a scattering angle 90deg from the light source. This light source is mainly Sun, but also a Moon. The degree of polarization first increases with increasing distance from the light source, and then decreases away from the light source. Thus the maximum degree of polarization occurs in a circular band 90deg from the light source.
When the Sun is located at the zenith, the band of maximal polarization wraps around the horizon and light from the sky is polarized horizontally along the horizon. During twilight at either the vernal and autumn equinox, the band of maximal polarization is defined by the north-zenith-south plane or meridian. In particular, the polarization is vertical at the horizon in the north and south, where meridian meets the horizon (Pic. 8).
Because the polarization pattern is dependant on the Sun and Moon it changes not only throughout the day but also throughout the year.
4. MORNING, EVENING AND TWILIGHT SKY
When the Sun is on a lower altitude above the horizon, then it appears to look redder instead of yellowish. The reddening of the Sun increase when closer to the horizon, because the light being received directly from it must pass a long way through the atmosphere. When Sun is on the horizon the sunlight must pass through the greatest proportion of the atmosphere and looks the most reddish (Pic. 9).
It happens like this, because when solar beam must go through the longest portion of the atmosphere thus the longest wavelengths like red and orange are scattered much more than short wavelengths (like a blue). It said to be, that Sun is slightly more reddish during the set than during the rise because of the different level of aerosols concentration.
The long wavelengths scattering refers mainly to this part of the sky, which is the closest to the solar disk. When further from the Sun the sky appears to be bluer (Pic. 10, 11).
This is because of two basic reasons. Firstly when further from the Sun you see at least a bit upper atmosphere (Pic. 12). Being around 10-20 km above the ground the solar beams has a shorter way to encounter in the atmosphere, hence scattering shorter wavelengths is more significant. Secondly, the ozone layer absorbs ultraviolet light (invisible for human eye very short wavelengths with frequency from 10 to 400 nm) and in the result keeps the sky blue for entire twilight (Pic 12).
The third factor lies in scattering the small particles. When closer to reddish Sun we have more front scattering so this section of the sky appears more reddish than the opposite one when backscattering occurs. The 3rd factor is more important in hazy conditions.
Even in near-Rayleigh conditions, a zenith sky color during the sunrise/sunset looks slightly different (Pic. 13). Whereas it’s a deep blue during the day when evening approaches some greenish and reddish tint appears to be visible on a still dark blue background (Pic. 13). It is effective for both aerosols concentration, which makes sky hue different according to Mie scattering (more in the next article) and general red wavelengths involvement, especially in the lower part of the atmosphere (Pic. 12).
The reddish effect is visible sometimes in the opposite part of the sky during the sunset. The purple sky is to be seen alongside the twilight wedge in a particular moment just after the sunset or before the sunrise. This is a secondary response of the long wavelengths that comes from the troposphere (this is caused by back-scattering sunlight bouncing off dust particles suspended high in the atmosphere) and it depends on the atmospheric conditions.
When observer see the sunset (Pic. 14) already can see rising reddish civil wedge at the antisolar point (Pic. 15). When Sun is 1-2 degrees below the horizon (Pic. 13) the civil wedge is clearly visible as a reddish belt, that appears to be fainter as Sun plunge deeper under the horizon. The reddiness of the Belt of Venus arises out of a long way of scattered light between the sunset at the Ozonosphere layer and the observer. When Sun is around 4 degrees below the horizon (Pic. 14) then the way of single scattered light is shorter to an observer, thus the belt of Venus looks less reddish and includes more orange and yellow hue. Finally at the end/beginning of the civil twilight, when the civil wedge covers local meridian the way of single scattered light to an observer is the shortest. In this case, the border between sunlit and shaded Earth’s atmosphere is marked only by blue hue, because again only short wavelengths are secondarily scattered towards the observer. Eventually, at this moment the Belt of Venus merges with the twilight sky.
This is caused by long wavelengths scattering in the stratosphere and also the long distance to observe, that this reflected and scattered light has to encounter through the atmosphere. As twilight proceeds, the twilight wedge rises up and the purple sky effect disappears (Pic. 15 – 22). Next, a whole belt of Venus merges in the twilight sky (Pic. 22).
Beside the belt of Venus, we can see the Earth’s shadow (Pic. 20, 21), which looks blue. This is because the upper atmosphere is the only part of the atmosphere that remains sunlit above the eastern horizon after sunset. Thus the sunlight hitting the upper atmosphere is Rayleigh scattered. Similarly in case of late twilight when nearly whole sky plunge in the Earth’s shadow the sky remains bluish until total darkness due to Rayleigh secondary scattering, when you are looking in anti-solar direction.
- Bohren F. C., 2003, Optics, atmospheric ,(in:) Digital Encyclopedia of Applied Physics
- Kondratyev K. YA, 1969, 6 diffuse radiation of the atmosphere, (in:) International Geophysics, vol. 12, p. 363 – 410.
- Thekaekara M.P., Drummond J., 1971, Standard values of the solar constant and its spectral components,(in:) Nature Physical Science, vol. 229, p.6-9
- West W., 2014, Absorption of electromagnetic radiation, Eastman Kodak Company, New York
- Blue sky
- Why is the sky blue?
- Why is the sky blue? – scheme
- Why the sky is blue instead of purple?
- Why the sky is not purple?
- Why the sky is never green?
- Why the sky is not violet?
- Earth’s shadow and the Belt of Venus
- Light scaterring – information
- Atmospheric scattering
- Scattering of light – graphs
- Why is Earth’s shadow blue?
- How does the Belt of Venus work?