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 the 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.
- 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, the scattered light coming from inside cancels 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).
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.
This kind of scattering is very important in meteorological optics with respect to many problems regarding haze and cloud scattering. Mie scattering can change the hue of the sky. Whereas the sky in near-Rayleigh conditions is deep blue, in Mie scattering conditions this color can be washed out depending on the concentration of the big particles in the atmosphere (Pic. 5 – 7).
2. TYNDALL EFFECT
The Tyndall 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 the bigger size of 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 the 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 the density of the particles. When there 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 forests or dusty indoors (Pic. 8, 9).
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 in the sky creates the cloud cover, which 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 light. The water droplets are much bigger from light wavelengths. Due to this non-selective scattering enables the reflection of the light unlike Mie scattering and Rayleigh Scattering.
White clouds cool Earth by reflecting some sunlight up into outer space and they warm it by bouncing some sunlight down to the surface (Kassinov et al., 2011).
The cloud cover affects the diffuse sky radiation. The flux of light is not wavelength-dependent because the fact, cloud droplets are larger than the light’s wavelength and scatter all colors approximately equally (Pic. 16).
The intensity of direct sunlight varies depending on cloud thickness. It ranges from 1/6 for relatively thin clouds down to 1/1000 under extremely thick storm clouds (Pic. 17).
Often the clouds cover the sky partially. There are different sky radiation circumstances. The clear section of the sky will still scatter the light in Rayleigh conditions whereas the cloudy section’s 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 are 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, and therefore cloud appears to be grey. A similar or even more noticeable effect is seen 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 the cumulus clouds case) water droplets near the cloud base grow larger being in effect a 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.
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 low-light conditions like moonlight or twilight, the mechanism remains the same. the clouds are the brightest when headed towards the incident light and the darkest beyond (Pic. 21).
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 and separate mediums with different densities. We are focusing mainly on the lower layer – the troposphere. When the light ray hits 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. In the outcome, this ray of light reverberates in a few different directions. Moreover, the color of the surface can decide (in the case of white light) or modify the color of scattered light.
The situation is straightforward for sunlight, which appears to be white. When reflected from for instance orange surface gives us orange light scattering, etc (Pic. 20). The surface color depends on the chemical elements, that 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 electrons 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 in doing so, it emits a photon with an energy equal to the difference in energy levels.
The simplified scheme of the transition process can be seen in the previous article.
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 them.
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 create the diffraction phenomenon, which is iridescence for clouds built by tiny water droplets and halo for clouds built by small ice crystals. Then we can see the halo phenomenon.
Common is also the corona phenomenon when the cloud is built by small water droplets (Pic. 26).
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 water droplets bigger than light wavelength they can reflect the light from the illumination source depending on their color. 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).
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 in 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.
Sometimes under the overcast sky, when clouds are not thick enough we can notice the sunrise or sunset moment as clouds are turning yellowish.
Behind the cloud-level terminator line, only scattered light affects the cloud color, which is mainly grey-reddish (Pic. 32, 33). Clouds being located beyond the terminator will reflect light scattered from sunlit clouds, reflecting long light wavelengths. I will describe it more in the future.
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).
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.
Upper parts of clouds or clouds floated on higher levels appear to be dark grey or black, without any light features unless different conditions occur (moonlight or deep twilight), which 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 crystal clouds. Then halos and sundogs can be visible.
In thunderstorm cases (shelf cloud) when the cloud is much thicker and cloud-based at a much lower altitude observer can spot the light reflection from the adjacent ground being still illuminated by a skylight or light scattered by neighboring tiny clouds (green clouds) (Pic. 37, 38). Basically quite often during thunderstorms, before starts raining a light from another cloud is reflected (Pic. 37, 39 – 42).
Optical effects with haze
Haze is represented by large aerosols with a size similar to light wavelength so it does not scatter a blue color preferentially like small molecules do. Hazy conditions in the atmosphere correspond 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 – the 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 the 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, is 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 toward the antisolar direction. All these atmospheric optics are produced by the presence of clouds. Mainly there are 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-shadow regions (Pic. 43). These rays are in fact near-parallel shafts of sunlight.
Anticrepuscular rays are more frequently visible around sunrise and sunset. Anticrepuscular rays are also near parallel, but appear to converge in an 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.
Both crepuscular and anticrepuscular rays are similar to dawn’s warm glow, which is often called solar or antisolar rays. This kind of ray occurs when the Sun is about to rise/set and mainly during the civil twilight. These rays are mainly produced in a “free atmosphere” where aerosol 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 favorable (e.g. at the top of high mountains) (Pic. 45).
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 mountains 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).
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 condition ranging from very fair days with near-Rayleigh scattering conditions down to foggy weather. Atmosphere behavior affects the object’s appearance seen by the observer from a distance. As the distance between the object and the observer increases, the contrast between the object and background decreases (Pic. 51). More about it I will develop in further writing.
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 the Sun or Moon (Pic. 53).
In the finish is worth mentioning about morning/evening sky color is affected by haze concentration. As I have shown in the previous article the color of the 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 reddish because longer wavelengths are scattered on aerosol particles. Nevertheless, even in the worst-case scenario, the main color of the evening zenith sky remains blue (Pic. 54).
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 the things included here need to be described separately as I want to describe much more about them. 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
- Ahrens C.D., 2003, Meteorology Today: An Introduction to Weather, Climate, and the Environment, vol. 1, Nature, London.
- Bohren, C.F., 1987, Clouds in a glass of beer, Dover Publications Inc., New York
- 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
- Kassinov E., et all, 2011, Shortwave spectral radiative forcing of cumulus clouds from surface observations (in:) Geophysical Research Letters, vol. 38.
- Van de Hulst H. C., 1957, Light scattering by small particles, Dover Publications Inc., New York.
- Mie plot – Mie scattering computer simulation.
- Gsp.humboldt.edu: Atmospheric scattering
- Corpuscular rays in St. Peter Basilica
- Cowley: Antisolar rays above Tibet
- Antisolar rays
- Antisolar rays – explanation mechanism
- Mountain shadow
- Perspective effect – mountain shadow formation
- Sciencedaily.com: Effect of cloud-scattered sunlight on Earth’s energy balance depends on the wavelength of light
- Why does the sky turn green before a tornado?
- Bishop’s ring.htm
- Pollen corona