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An original point of view

Optics, Weather

Light scattering in the Earth’s atmosphere part 3 – clouds, haze and surface

Cumulonimbus cloud Odrzykoń

Cumulonimbus cloud sweeping above Odrzykoń as a perfect example of non selective light scattering.

 

Previously I have described the light scattering mechanism on the smallest particles of the Earth’s atmosphere, where mainly the Rayleigh Scattering plays a role. Now is time to round off this whole article and say something more about a different group of elements, that also contributes to the light scattering process. There are bigger particles that build fog and clouds. As an output of this whole light scattering process in the Earth’s atmosphere, we can see the interaction of these all small particles with the Earth’s surface – a ground response.

  1. MIE SCATTERING

Mie scattering takes place when droplets and light waves are of similar size or these particles are bigger than the light wavelengths. This kind of scattering does not differentiate individual wavelength colors and therefore scatters all wavelength colors the same.
The result is equally scattered initial light coming from the illumination source. In bigger droplets size the scattered light coming from inside cancel each other and what remains is interpreted as reflected and refracted waves. In this case, the spectral composition of the scattered light depends on the scattering angle (Pic. 1).

Mie and Rayleigh scattering difference

Pic. 1 Angular distribution of scattered light comparison between Rayleigh and Mie scattering, whereas the direction of incident light remains stable (http://hyperphysics.phy-astr.gsu.edu).

Rayleigh and Mie scattering pattern

Pic. 2 The Rayleigh and Mie scattering mechanism (http://hyperphysics.phy-astr.gsu.edu).

The common particles causing the Mie scattering are: dust, pollen, smoke, and microscopic water droplets, that form clouds. Mie scattering occurs mostly in the troposphere, especially in the atmospheric boundary layer, where often haze is observed.

Odrzykoń, Mie scattering

Pic. 3 The Mie scattering for particles with similar size to light wavelengths (high level of haze concentration). It results in poor visibility; the hills in the picture on the background are located 10 – 30 km away are barely visible, whereas further mountain ranges 50 km away are invisible. Odrzykoń, Poland

Odrzykoń, mgła

Pic. 4 The Mie scattering for particles with size bigger than light wavelengths (fog). As a result the visibility range is restricted up to less than 1km. Odrzykoń, Poland.

This kind of scattering is very important in meteorological optics with respect to many problems regarding haze and cloud scattering. Mie scattering can change a hue of the sky. Whereas the sky in near-Rayleigh conditions is deep blue, in Mie scattering conditions this color can be washed out depends on the concentration of the big particles in the atmosphere (Pic. 5 – 7).

Mie scattering and impact on sky colour

Pic. 5 The impact of Mie scattering on the sky color (atmo.arizona.edu).

Mie scattering sky colour, St Michael Moud Cornwall

Pic. 6 The sky colour difference depends on the concentration of the big particles in the atmosphere: left – in hazy conditions, right – in near-Rayleigh conditions. St. Michel’s Mound, UK (left); Zadar, Croatia (right).

Mie Scattering sky extremely haze level Łęki Strzyżowskie

Pic. 7 Extremely high haze concentration in the lower atmosphere makes sky white-bluish, especially when looking near solar direction. Łęki Strzyżowskie, Poland.

2. TYNDALL EFFECT

Tyndal effect is similar both to Rayleigh Scattering and Mie Scattering. The similarity of the Tyndall effect to Rayleigh Scattering lies in the same law, where the intensity of scattered light depends on the fourth power of the frequency. As a result, blue light is scattered much more strongly than red light. This situation refers to particles in a colloid or very fine suspension. Tyndall scattering is more intensive than Rayleigh scattering due to bigger size particles involved. If the colloid particles are spheroid then the Tyndall effect can be analyzed in terms of the Mie scattering, which admits particle sizes in the rough vicinity of the wavelength of light. In Earth’s atmosphere, it refers to weather conditions, where Tyndall Effect can be noticed in foggy or dusty places, which is a good colloid. Sunlight passing through this colloid encounters the individual suspended particles, which scatter and reflect light. These individual suspended particles like water droplets make the headlight beam visible. The amount of scattering depends on the frequency of the light and density of the particles. When are no small particles in the air then it is not possible to see a path of light, but when there are enough particles in the air you can see the path of light, that is reflected by them.  We can observe it for example in mossy forest or dusty indoors (Pic. 8, 9).

Forest fog sun Tyndall effect

Pic. 8 An example of the Tyndall effect is to be seen in the forest when Sun shines through a thin fog (weggi.ch).

Watykan crepuscular rays Tyndall effect

Pic. 9 An example of Tyndall Effect in indoor, where is caused by dust (Wikimedia.org, Pinterest.org).

3. NON-SELECTIVE SCATTERING

Non-selective scattering occurs in the lower portion of the atmosphere when the particles are much larger than light wavelengths. The non-selective scattering creates a white appearance of the sky and plays the main role in cloud formation. The presence of clouds on the sky creates the cloud cover, that says roughly how much the sky is affected by any kind of cloud (Pic.10). In dense clouds, this dependence is averaged out by varying drop sizes and multiple scattering and the clouds appear white or grey (Pic. 11). Cloud droplets scatter all wavelengths of visible light creating the appearance of a white cloud. The main difference between non-selective scattering and others is the ability to reflect the light. The water droplets are much bigger from light wavelengths. Due to this non-selective scattering enables to reflect the light also unlike to Mie scattering and Rayleigh Scattering.

Sunlight sky cloud non-elective scattering

Pic. 10 A polarity between non-selective scattering (white cloud) and Rayleigh scattering (blue sky). Big puffy clouds scatter light effectively making the sky around them much brighter (atmo.arizona.edu).

White clouds cool Earth by reflecting some sunlight up into outer space and they warm it bouncing some sunlight down to the surface (Kassinov et all., 2011).

Non selective light scattering cumulus humilis cloud

Pic. 11 Cumulus humilis cloud on the blue sky is a good example of non-selective scattering in conjunction with Rayleigh scattering. Most of the cumulus clouds are dense, so they appear white and grey at once.

Non selective light scattering altocumulus and cumulus cloud

Pic. 12 White altocumulus clouds under fair weather conditions, whereas cumulus below, shaded by altocumulus appears to be grey, even darker than grey altocumulus base.

Cloud light scattering graph

Pic. 13 The light scattering by clouds – scheme. White light is scattered in all directions, some light penetrates to cloud base (weather.gov.hk).

Non selective light scattering cumulonimbus cloud

Pic. 14 Especially in vertically developed clouds we can see a very dark base. As the cloud grows thicker the more sunlight is reflected from it and less light can penetrate through it. Małe Pieniny Mts, Poland.

Light scattering vs cloud depth

Pic. 15 Relation of light scattering with cloud depth (weather.gov.hk).

The cloud cover affects the diffuse sky radiation. The flux of light is not wavelength-dependent because of the fact, that cloud droplets are larger than the light’s wavelength and scatter all colors approximately equally (Pic. 16).

Diffuse sky radiation under overcasted sky

Pic. 16 Under an overcast sky, diffuse sky radiation plays the main role in light scattering. Odrzykoń, Poland.

The intensity of direct sunlight is various depends on cloud thickness. It ranges from 1/6 for relatively thin clouds down to 1/1000 under the extreme thick storm clouds (Pic. 17).

Cloud thickness vs intensity of direct sunlight

Pic. 17 The intensity of direct sunlight is various depends on cloud thickness. Odrzykoń, Poland.

Light scattering in multicellular thunderstorm cloud

Pic. 18 An example of a multicellular thunderstorm, where young cumulonimbus with dark base is growing under older capillatus, making the sky in grey appearance. The cumulonimbus elements, that could look white in solar beams appears to look much darker, because the low portion of light goes through them.

Often the clouds cover the sky partially only. There are different sky radiation circumstances. The clear section of the sky will still scatter the light in Rayleigh conditions whereas the cloudy sections radiation is different. Clouds can bounce a lot of light around. It is noticeable especially in the case of vertical-developed clouds. These clouds are big puffy white objects, that reflect the sunlight effectively making the sky around them look brighter (Pic. 10). Brighter is also the shadows produced by other clouds located in the vicinity.

This is the answer to why clouds are grey. Sometimes we can see grey clouds shaded by another white cloud floating on a higher level. Then less light is scattered, therefore cloud appears to be grey. Similar or even more noticeable effect we can see when a cloud grows thicker. Then more sunlight is reflected from it and less light can penetrate through it (Pic. 17-20). Since little sunlight reaches the underside of the cloud, less light is scattered and the cloud base appears grey. Moreover (especially in cumulus clouds case) water droplets near the cloud base grow larger being in effect more effective absorber and less effective scatterer more sunlight is either reflected or absorbed before reaching the cloud base. For an observer on the ground below the cloud, the base can look dark grey before the rain begins to fall.

Light scattering in multicellular thunderstorm cloud graph

Pic. 19 The light scattering mechanism in multicellular thunderstorm cloud, where: 1 – direct solar beam, 2 – white appearance of the external parts of the sunlit clouds, 3 – light firstly scattered by cloud surface headed sunwards, 4 – grey appearance of the clouds, shaded by another clouds, 5 – light secondly scattered by shaded clouds or their parts, 6 – light portion penetrating the cloud, 7 – the smallest light portion, that penetrates to cloud base, making it the most darker. Look also on the edges of these clouds, that are to be seen brighter than the middle due to brightest shadows produced by other clouds in the vicinity.

Light scattering inside cumulonimbus cloud

Pic. 20 Cumulonimbus cloud reflecting the light scattered by the cloud’s top, down to the darkest base. Odrzykoń, Poland.

The spectral character of the radiation emanating from the bottom of the cloud also depends on the spectral character of the radiation illuminating the cloud, both at the bottom from the irradiance of the ground and the top from the direct attenuated solar radiation and the Rayleigh scattered skylight. Its intensity and spectral character, therefore, will depend on the surface albedo, the cloud thickness and type, and the solar zenith angle.

In the low light conditions like moonlight or twilight, the mechanism remains the same. the clouds are the brightest when haded towards the incident light and the darkest beyond (Pic. 21).

Light scattering by cumulus clouds in twilight conditions

Pic. 21 Cumulus clouds seen in twilight conditions with the brightest parts headed toward twilight glow and dark base. Odrzykoń, Poland.

4. THE INTERACTION OF LIGHT WITH SMALL PARTICLES AND SURFACE

White light is a mix of wavelengths. We can see the interaction of light both with the surface and the medium, which is our atmosphere. Obviously, the Earth’s atmosphere consists of a few layers, separate mediums with different density. We are focusing mainly on the lower layer – troposphere. When the light ray hit the surface we have a reflection of this ray of light. As we know most of the surfaces are coarse, which causes the scattering of this simple ray of light. At the outcome, this ray of light reverberates in a few different directions. Moreover, the colour of the surface can decide  (in case of white light) or modify the color of scattered light.

 

Interaction of light with white surface

Pic. 22 The interaction of white light with the surface. Considerating a white light (the easiest example) in interaction with the surface can produce an optical effects as follows: a) specular reflection, b) lustrous reflection by paper with a slightly rough surface, c) diffuse reflection from whitewashed wall, where no absorption d) diffuse reflection with absorption of the shorter wavelengths at a painted yellow wall (Flammer et al., 2013).

The situation is straightforward for sunlight, which appears to be white. When reflected from for instance orange surface give us orange light scattering, etc (Pic. 20). The surface colour depends on the chemical elements, which are built-up. These chemical elements run the process where light meets an electron it is “shaken” at the frequency of the light. As a result, the electron sends out light with the same frequency in any direction. Thus light scattering takes place  (Flammer et al, 2013). According to quantum theory: when an atom absorbs a UV photon or a photon of visible light, the energy of that photon can excite one of the atom’s electron to a higher energy level. This moment is known as transition. In order for a transition to occur, the energy of the photon absorbed must be greater than or equal to the difference in energy between the 2 energy levels. Once the electron is in the excited, higher energy level, it is in a more unstable position than it was when it was in its relaxed, ground state. As such the electron will quickly fall back down to the lower energy level and it doing so, it emits a photon with an energy equal to the difference in energy levels.
The simplified scheme of the transition process we can see in the previous article.

Zilina Slovakia nice spring 2012

Pic. 23 Each object is built in different chemical element, that incurs an individual transition process. Finally, we can see the surfaces in different colours. Zilina, Slovakia.

5. OPTICAL EFFECTS RELATED TO LIGHT SCATTERING

There are at least a few optical effects related to light scattering. I will not describe all of them perfectly today, because I am going to prepare a separate article about it.

Optical effects with clouds:

Cloud presence can produce a phenomenon related to light scattering. First of all when clouds are close to the Sun or Moon can produce the diffraction phenomenon, which is iridescence for clouds built by small water droplets and halo for clouds built by small ice crystals. Then we can see e.g halo phenomenon.

The halo seen on cirrostratus clouds

Pic. 24 The halo phenomenon caused by cirrostratus clouds.

Circumhorizontal arc seen through cumulus clouds

Pic. 25 Circumhorizontal arc (wikimedia.org).

Common is also the corona phenomenon when the cloud is built by small water droplets (Pic. 26).

Lunar corona seen through altocumulus clouds

Pic. 26 Lunar coronae (wikimedia.org).

These phenomena are the effect of the spectral dependence of the angular distribution of scattered light. The intense scattering, that occurs in clouds can quickly extinguish any beam and create in its place a very diffuse radiation flux. I have described this process previously.

Because clouds include the water droplets bigger than light wavelength they can reflect the light from the illumination source depends on its colour. When for example white midday sunlight strikes a cloud, white light is scattered and reflected (Pic.11, 12, 15). This is why clouds are white (with some shades of grey mixed in if the cloud is thick. In this case, when you look up at a cloud you see a white cloud  (sunlight being scattered by cloud droplets) surrounded by blue sky  (sunlight being scattered by air molecules).

Clouds white and grey scale appearance

Pic. 27 Clouds throughout the day has a white or greyscale appearance due to almost white sunlight scattering and reflecting. Odrzykoń, Poland.

In the evening we will face a similar situation with the reddish light described above. As twilight progresses, the color of clouds, depending on altitude level and location on the sky will change as per the picture below (Pic. 28). This is because Earth  (both with the atmosphere) is spherical. Thus is very easy to distinguish the cloud layers near sunrise or sunset conditions.

Cloud colours around sunrise and sunset

Pic. 28 Color of clouds around sunrise/sunset and during the twilight is dependant on altitude level and position on the sky (angular distance from the solar point) (weather.gov).

sunrise_colors altocumulus cloud and cirrus cloud above

Pic. 29 At the sunset altocumulus looks (and reflect) orange whereas cirrus yellow (weather.gov)

Sunset and light reflection on stratocumulus cloud

Pic. 30 Morning or evening clouds reflects a long light wavelengths from the Sun (reddish and orange). Shaded parts remain in greyscale, sometimes slightly mixed with direct sunlight color. Szczawnica, Poland.

Sometimes under the overcast sky, when clouds are not thick enough we can notice the sunrise or sunset moment as clouds are turning to yellowish.

Light scattering on clouds and angular distance to the Sun

Pic. 31 When we see only 1 layer of cloud at the sunrise or sunset the important role plays an angular distance from the solar position. See the cirrus clouds just before sunrise, when Sun is around 2 deg below the horizon clouds are yellow near the solar point and become more orange when closer to zenith point. Zręcin, Poland.

Behind the cloud-level terminator line only scattered light affects the cloud colour, which is mainly grey-reddish (Pic. 32, 33). Clouds being located beyond the terminator will reflect light scattered from sunlit clouds, reflecting a long light wavelengths. I will describe it more in the future.

Clouds after sunset or before sunrise

Pic. 32 The clouds after sunset appearance (weather.gov.hk).

Light scattering on cirrus clouds when Sun about 8 deg below horizon

Pic. 33 Cirrus clouds, when Sun around 8 deg below the horizon. You can spot, that some of them appear to look reddish due to reflection of red light scattered by other clouds far far away from the observer. Zręcin, Poland.

Looking at the clouds in the sky we can spot only grey color there. Another situation can occur when watching the mountain shadow. Clouds outside the shadow are sunlit, whereas clouds inside the shadow reflect only the light scattered in the atmosphere, which gives them a grey-bluish appearance (Pic. 34).

Pico del Teide mountain shadow

Pic. 34 The Pico del Teide shadow is seen near the Teide Observatory. Yellow arrows show the solar direct beam, that makes clouds yellowish appearance. Each cloud shows, that solar point is located behind the observer. Blue arrows indicate the clouds being in the shadow, whereas only light scattered in the atmosphere is reflected. Unlike illuminated clouds, these clouds are the brightest on the top, because basically from the top light hit them. Moreover, these clouds look grey-bluish reflecting Rayleigh-scattered light in the atmosphere. Tenerife, Spain.

The situation is completely different during the nighttime conditions in light-polluted areas. During the night in light-polluted areas, clouds reflect a yellowish artificial light that originated from street lamps. The phenomena are more obvious when the cloud is lower or denser.

Illuminated clouds by artifical light, Kuala Lumpur

Pic. 35 Illuminated clouds above light-polluted area during the night. Kuala Lumpur, Malysia.

Upper parts of cloud or clouds floated on higher level appear to be dark grey or black, without any light features unless different conditions occur  (moonlight or deep twilight), that I will describe in the future.
Another situation is when even a thin cloud looks grey rather than white. We can explain the perception of our eyes. A light gray cloud on a bright white background will look much darker than the same cloud on a dark or black background, in which case it might look white and bright (Pic. 36). On the other hand, a cloud can look dark or gray because it is partially transparent and the blue sky behind it can be seen through the cloud. This will happen in light wispy clouds with little water content and more often in ice crystals clouds. Then halos and sundogs can be visible.

Cloud optic illusiion and polarization, cumulus humilis

Pic. 36 A thin wisp of clouds is often seen darker than the sky near the horizon, which is caused by human eye perception. On the right is the same picture with the polarizing filter, which roughly explains this phenomenon (wikiwand.com).

In thunderstorm cases  (shelf cloud) when the cloud is much thicker and cloud base on much lower altitude observer can spot the light reflection from the adjacent ground being still illuminated by a skylight or light scattered by neighbouring tiny clouds (green clouds) (Pic. 37, 38). Basically quite often during thunderstorms before start raining a light from another cloud is reflected (Pic. 37, 39 – 42).

The green clouds appearance graph

Pic. 37 The green cloud effect mechanism is shown with cloud light reflection process: 1 – direct solar beam, 2 – light scattered and reflected by puffy cumulus or cumulonimbus cloud, 3 – light secondly reflected by another cloud, 4 – light reflected from the ground both from the sun and from the cloud. 5 – green cloud appearance at the bottom of the cumulonimbus cloud.

Green clouds and tornado

Pic. 38 Often the green clouds effect is identified with the forthcoming tornado, although in general refers to an extremely low suspended shelf cloud, which is able to reflect light from the ground. This green is not perfect as seen on grass, but clouds are slightly greenish in its greyscale. Adequately to the conditions we can observe a blue cloud (above sea) or yellow cloud (above desert) effect depends on the surface (Matt Denis, box72.com).

Cumulonimbus cloud Odrzykoń

cumulonimbus cloud light reflection after thunderstorm

Pic. 39, 40 An example of light reflection before or after thunderstorm outside the precipitation area, where: 1 – direct sunlight reflection and scattering, 2 – scattered sunlight reflection. In human eye perspective this part of cloud looks even darker than indicated in point 3, because this shaded portion of cumulonimbus cloud is drowned by much brighter portion from point 1, 3 – scattered sunlight reflection, 4 – secondary scattered light, coming from another cloud or part the cloud being in the vicinity (yellowish edges).

Cumulonimbus cloud sweeping above Odrzykoń

Cumulonimbus cloud secondary scattered light example

Pic. 41, 42 Another example of secondary scattered light coming from the different clouds, group of clouds or the atmosphere. Blue arrows show discernable brighter cloud edges, which reflects the scattered light coming from the brighter section of the sky seen on the left.

Optical effects with haze

Haze is represented by large aerosols with a size similar to light wavelength so it not scatter a blue color preferentially like small molecules do. Hazy conditions in the atmosphere are corresponding to Mie scattering. The atmosphere is not clear enough to give a perfect blue sky and clear horizon but is more transparent than a cloudy or foggy medium. Mainly Mie scattering is typical for the lowest part of the troposphere – atmospheric boundary layer, where the concentration of haze is the biggest. Everywhere above – in the “free atmosphere” zone the haze density is low and the effects are somewhere between Rayleigh and Mie scattering. The haze phenomena will be described in the following article, however here I will list all atmospheric optic phenomena related to hazy conditions. Firstly I would like to say a bit about macroscale phenomena created by haze presence in the atmosphere. They’re crepuscular rays, anticrepuscular rays, dawn’s warm glow (solar and antisolar rays) and mountain shadow. These atmospheric optics are similar to each other and often visible in more or less hazy conditions. Both crepuscular rays and dawn’s warm glow are rays of sunlight, that appear to radiate from the point of the sky, where Sun is located. The difference between crepuscular rays and dawn’s warm glow is the period of daytime, when they are to be observed. Crepuscular rays can be seen throughout the day, whereas a dawn’s warm glow, are to be seen around sunrise and sunset only. The dawn’s warm glow is the kind of both crepuscular and anticrepuscular rays. Anticrepuscular rays are to be seen when looking towards the antisolar direction. All these atmospheric optics are produced by the presence of clouds. Mainly there are a cumulus, stratocumulus or cumulonimbus clouds, with sharp borders. The solar rays stream through the cloud gaps and appear as columns of sunlit air separated by darker cloud-shadows region (Pic. 43). These rays are in fact near-parallel shafts of sunlight.

Crepuscular rays produced by stratocumulus

Pic. 43 Crepuscular rays produced by stratocumulus clouds above Southampton coast (wikimedia.org).

Anticrepuscular rays are more frequently visible around sunrise and sunset. Anticrepuscular rays are also near parallel, but appears to converge in antisolar point (Pic. 44). This convergence is actually an optical illusion, because their apparent convergence is toward a vanishing point, which is an infinite distance away from the viewer.

Anticrepuscular rays

Pic. 44 Anticrepuscular rays (wikimedia.org).

Both crepuscular and antirepuscular rays are similar to dawn’s warm glow, which is often called as solar or antisolar rays. This kind of rays occurs when the Sun is about to rise/set and mainly during the civil twilight. These rays are mainly produced in “free atmosphere” where aerosols concentration is low, thus they are not dependent on Mie scattering only, because Rayleigh Scattering is also important to factor there.  Dawn’s warm glow is visible as a group of extended reddish rays or darker elongated shadows from solar points throughout the sky sphere towards an antisolar point or civil wedge. Basically, the dawn’s warm glow can be visible from deep nautical twilight when conditions are favourable (e.g at the top of high mountains) (Pic. 45).

Dawn's warm glow Tenerife before sunrise

Pic. 45 Dawn’s warm glow (solar rays) seen from Tenerife at 2400 m.a.s.l near Teide Observatory, when Sun was 9 degrees below the horizon (middle of nautical dawn). Tenerife, Spain.

Anticrepuscular rays touching the twilight wedge

Pic. 46 Dawn’s warm glow (antisolar rays) can touch the twilight wedge (the Belt of Venus) (Jeff Dai, atoptics.co.uk).

The last phenomenon caused by Mie scattering is a mountain shadow. It is produced analogically to dawn’s warm glow, but in this case, solar beams are covered by mountain rather than clouds. Likewise in the case of solar and antisolar rays, Mie scattering is not the main factor here, because also Rayleigh scattering is important due to “free atmosphere” conditions. This view is typical for the area with freestanding high mountains above much lower terrain or sea surface. The mountain shadow is perfectly visible when standing at the top (Pic. 47). However, an observer can notice it is next to the mountain (Pic. 45, 46). When the freestanding mountain is very high (a few km above ground level) then its shadow can even merge with a twilight wedge (the Belt of Venus) (Pic. 46).

Mountain shadow Mauna Kea

Pic. 47 A typical example of mountain shadow seen from the top of the Mauna Kea (Greg Chavdarian, apod.nasa.gov)

Pico del Teide mountain shadow on clouds

Pico del Teide mountain shadow on clouds2

Pic. 48, 49 – Pico del Teide mountain shadow seen from the position, when an observer is located somewhere next to the mountain (near Teide Observatory). Aside from shadow, there are two other things to spot: 1 – glory seen on clouds, which shows a roughly current antisolar point. 2 –  the shadow column is seen in the air, which becomes more visible as goes closer to the clouds. It is related with increasing thickness of the shadow when closer to its end. Tenerife, Spain.

Pico del Teide shadow at the twilight wedge

Pic. 50 The Pico del Teide shadow merging with a rising twilight wedge (the Belt of Venus). Tenerife, Spain.

Other interesting phenomena, which occur mainly in the lowest part of the troposphere are: aerial perspective, Bishop’s ring, and pollen corona.
Aerial perspective is a very common phenomenon present in the Earth’s atmosphere and refers to every weather conditions ranging from very fair days with near-Rayleigh scattering conditions down to foggy weather. Atmosphere behaviour affects on the object’s appearance seen by the observer from the distance. As the distance between object and observer increases, the contrast between the object and background decreases (Pic. 51). More about it I will develop in further writing.

Panorama okolic Przeł. Beskid 784m ze szlaku prowadzącego przez polanę aerial perspective

Pic. 51 The aerial perspective seen in Bieszczady Mts. Every subsequent range from the observer looks fainter and fainter due to intensive light scattering on haze particles. Wołosate, Bieszczady, Poland. Wołosate, Bieszczady, Poland.

Bishop’s ring is produced by the scattering of sunlight by ash and hydrate sulfate droplets. This phenomenon is often present after volcano eruptions.

Pollen corona is caused by high pollen concentration in the lowest part of the atmosphere when stable high-pressure conditions. The pollen aerosols are quite big and produce small coronae around Sun or Moon (Pic. 53).

Bishop's ring around Moon

Pic. 52 The Bishop’s ring phenomena (Bob Harrington, bobgat.com, atoptics.co.uk).

Pollen corona around the Moon

Pic. 53 Pollen corona around Moon (skyandtelescope.com)

At the finish is worth to mention about morning/evening sky colour affected by haze concentration. As I have shown in the previous article the color of zenith sky varies throughout the day. The sky color near sunrise/sunset depends on the haze concentration likewise throughout the day (Pic. 5).  As haze presence increases sky looks more reddish, because more long wavelengths are scattered on aerosols particles. Nevertheless, even in the worst-case scenario the main colour of evening zenith sky remains blue (Pic. 54).

Evening sky comparison in terms of haze concentration

Pic. 54 Evening zenith sky is blue with some greenish, orange and reddish tint, which appears to be more significant, when haze concentration is bigger (on the right).

Well, it’s time to finish this extremely long writing. Light scattering in the atmosphere is a very wide topic, that goes far beyond this article. Many of thigs included here heeds to be described separately due to bring much more learns about. Anyway, you can treat this whole article as a good indicator of places, where you can find more on the web. Hopefully, I have encouraged you to observe these phenomena and broaden your knowledge about them

Mariusz Krukar

References:

  1. Ahrens C.D., 2003, Meteorology Today: An Introduction to Weather, Climate, and the Environment, vol. 1, Nature, London.
  2. Bohren, C.F., 1987, Clouds in a glass of beer, Dover Publications Inc., New York
  3. Flammer J., Mozzafarieh M., Bebie H., 2013, The interaction between light and matter (in:) Basic sciences in Opthamology; Physics and Chemistry, Oxford University Press, Oxford
  4. Kassinov E., et all, 2011, Shortwave spectral radiative forcing of cumulus clouds from surface observations (in:) Geophysical Research Letters, vol. 38.
  5. Van de Hulst H. C., 1957, Light scattering by small particles, Dover  Publications Inc., New York.

 

Links:

  1. Mie plot – Mie scattering computer simulation.
  2. Gsp.humboldt.edu: Atmospheric scattering
  3. Corpuscular rays in St. Peter Basilica
  4. Cowley: Antisolar rays above Tibet
  5. Antisolar rays
  6. Antisolar rays – explanation mechanism
  7. Mountain shadow
  8. Perspective effect – mountain shadow formation
  9. Sciencedaily.com: Effect of cloud-scattered sunlight on Earth’s energy balance depends on wavelength of light
  10. Green_clouds.shtml
  11. Why does the sky turn green before tornado?
  12. Bishop’s ring.htm
  13. Pollen corona

 

Wiki:

  1. Mie_scattering
  2. Tyndall_effect
  3. Colloid
  4. Crepuscular_rays
  5. Vanishing_point
  6. Anticrepuscular_rays
  7. Antisolar_point
  8. Multicellular_thunderstorm
  9. Diffuse_sky_radiation
  10. Halo_(optical_phenomenon)
  11. Aerial_perspective

Read also:

1. Tenerife and night sky

 

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