Advertisements
Advertisements

An original point of view

Astronomy, Optics

Light scattering in the Earth’s atmosphere part 2 – why is the sky blue and how the sky colour change?

Daylight sky colour Pico del Teide, Tenerife

A blue sky colour seen from the Pico del Teide at the Tenerife.

 

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?

  1. 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 basic mechanism

Pic. 1 Basic mechanism of scattering the light on the molecules in the atmosphere. Incoming sunlight as white light is next scattered mainly on blue wavelengths, with a small presence of green and violet wavelengths (atmo.arizona.edu).

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.

Rayleigh_sunlight_scattering visible wavelengths

Pic. 2 The visible wavelength scale against the scattering of direct sunlight (wikimedia.org).

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).

A sun on the blue sky

Pic. 2 The direct sunlight is scattered in the Earth’s atmosphere causing diffuse sky radiation, what we can see during a typical sunny day. Sun appears to look bright whereas the sky is blue playing the role of medium, where the direct solar beam is scattered and subsequently reaches the Earth’s surface. When Sun is overcast by a thick cloud layer and another part of the sky clear at once then we can experience how scattered solar beam reaches the Earth’s surface from the clear section of the sky.

Typical blueness of the sky

Pic. 3 The typical color of the clear sky. The sky appears to be darker when closer to the zenith. It is caused by Earth’s atmosphere thickness. Once we are looking up the atmosphere is the thinnest. As our sight goes down the thickness of the Earth’s atmosphere increases up to 38x times near the horizon. Hence lower parts of the sky are always much brighter than zenith sky.

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.

Hunstanton cliff with daylight sky blue colour

Pic. 4 Daylight sky seen from mean sea level. Hunstanton, United Kingdom.

Bernes Alps at Grimsel Pass with daylight sky beyond

Pic. 5 Daylight sky seen from Grimsel Pass, 2164 m.a.s.l. Bernese Alps, Switzerland.

Pico del Teide Tenerife daylight sky

Pic. 6 Daylight sky seen from Pico del Teide 3718 m.a.s.l. Tenerife, Spain.

LOT Airlines Tbilisi - Warsaw and upper troposphere sky colour

Pic. 7 Daylight sky, as is seen from the plane at the cruising altitude around 10000 m.a.s.l. LO 724 TBS – WAW.

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).

Sky polarization pattern at the sunrise and sunset

Pic. 8 The sky polarization pattern at the sunrise or sunset, where the highest polarization occurs at the south-zenith-north line alongside the local meridian (wikimedia.org).

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).

Sunset Phuket Thailand

Pic. 9 The Sun is the most reddish when shining on the horizon. Phuket, Thailand.

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).

Reddish sky at the sunset, Gorce in Carpathian Mountains

Pic. 10 The reddish sky appears to be seen close to the solar disk only. The intensity of reddiness depends on the degree of aerosols concentration. The sky is more blue with increasing distance from the Sun. Czorsztyn, Poland.

Reddish twilight sky above Odrzykoń

Pic. 11 Twilight sky in solar direction remains reddish near the horizon and more blue with increasing distance from the horizon towards the zenith. Odrzykoń, Poland.

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).

Solar beam scattering in the atmosphere pattern

Pic. 12 The pattern of solar beam scattering in Earth’s atmosphere throughout the day: A – during the sunrise, B – at the noon, C – around sunset. Blue line – upper troposphere, faint blue – upper Earth’s atmosphere. An observer located in B point see only short wavelengths coming through, the solar disk appears to look yellowish then; Observers located both in point A and C see the reddish Sun, because the light beam passes longer way through the atmosphere (pattern 3). When taking into account patterns 2 and 1 at the moment of sunrise or sunset we can understand, that solar beam passes through a thinner part of the atmosphere reaching only upper troposphere (pattern 2) or only upper atmosphere (pattern 1). For pattern, the 2nd sky will appear yellow and reddish during the sunset, for pattern1 sky remains still blue, because the short wavelength is not absorbed in upper atmosphere conditions. Thus Sun from aircraft cruising altitude looks more orange and yellow, at the moment of sunset (Pattern 2nd) and still yellowish on the upper atmosphere (Pattern 1st).

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).

Zeith sky colour difference at different daylight conditions

Pic. 13 The zenith sky color difference throughout the day: left – deep blue as seen throughout the day, right – deep blue mixed with greenish and a little bit reddish tint, as we can see during the morning or evening time.

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.

twilight wedge

Pic. 14 The twilight wedge (Belt of Venus) simplified pattern depends on the Sun’s altitude below the horizon, where: A – upper atmosphere, O – ozonosphere, B – solar beam. The absorption of short wavelengths has been omitted. The image shows the difference of the Belt of Venus appearance throughout the civil twilight for sunset or sunrise (S), Sun position around 1-2 degrees below the horizon (-1), 4 degrees below the horizon (-4) and at the civil twilight border (-6).

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.

Sztokholm 21h15m twilight wedge, webcam

Pic. 15 Twilight wedge (the Belt of Venus) becoming visible at the beginning of the dusk, just after sunset in Stockholm (sheratonstockholm.com). It is easy to spot the reddish belt, which stretches alongside the horizon.

Sztokholm 21h35m twilight wedge webcam

Pic. 16 Twilight wedge (the Belt of Venus) visible above Stockholm when Sun is around 2 degrees below the horizon. Comparing to the previous picture it looks less reddish (sheratonstockholm.com).

Sztokholm 21h45m twilight wedge webcam

Stockholm twilight wedge seen in webcam and enhanced in Photoshop Express

Pic. 17, 18 Twilight wedge (the Belt of Venus) above Stockholm, when Sun is around 3 deg below the horizon. Normally it has more orange and yellow hues instead of reddish at the sunset: 16 – normal image, 17 – modified in Photoshop Express (sheratonstockholm.com).

Cień Ziemi i Księżyc, Jedlicze - Długie

Pic. 19 Twilight wedge (the Belt of Venus) seen above Długie in Poland. You can easily point a greenish-yellowish line. The color of the Belt of Venus can depend on the aerosols concentration in the lower atmosphere also. Długie, Poland.

Sztokholm 21h55m twilight wedge webcam

Sztokholm 21h55m twilight wedge webcam enhanced in Photoshop Express

Pic. 20, 21 Twilight wedge (the Belt of Venus) seen above Stockholm when Sun was around 4 deg below the horizon. A faint orange hue, making sky greenish is visible on the emphasized image. Just below an Earth’s shadow is visible as a dark bluish area of sky: 19 – normal image, 20 – modified in Photoshop Express (sheratonstockholm.com).

Zmierzch cywilny w Podniebylu z widokiem w kierunku Łubna-Opace

Pic. 22 When the civil dusk end or civil dawn begins, then the twilight wedge (the Belt of Venus) fades out on around 90 deg from the Sun’s location. Podniebyle, Poland; view towards SSE.

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.

Mariusz Krukar

References:

  1. Bohren F. C., 2003, Optics, atmospheric ,(in:) Digital Encyclopedia of Applied Physics
  2. Kondratyev K. YA, 1969, 6 diffuse radiation of the atmosphere, (in:) International Geophysics, vol. 12, p. 363 – 410.
  3. 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
  4. West W., 2014, Absorption of electromagnetic radiation, Eastman Kodak Company, New York

Links:

  1. Blue sky
  2. Why is the sky blue?
  3. Why is the sky blue? – scheme
  4. Why the sky is blue instead of purple?
  5. Why the sky is not purple?
  6. Why the sky is never green?
  7. Why the sky is not violet?
  8. Earth’s shadow and the Belt of Venus
  9. Light scaterring – information
  10. Atmospheric scattering
  11. Scattering of light – graphs
  12. Why is Earth’s shadow blue?
  13. How does the Belt of Venus work?

Wiki:

  1. Polarizability
  2. Diffuse_sky_radiation
  3. Polarization
  4. Rayleigh_sky_model
  5. Degree_of_polarization

Read also:

1. What is the colour of the Martian sky?

 

 

 

 

 

 

 

 

 

Advertisements
Advertisements
Follow

Get the latest posts delivered to your mailbox:

%d bloggers like this: