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Atmospheric light phenomena
Rev. 9 — page content was last changed September 24, 2011
Except for white-out conditions, the atmospheric light phenomena described below have no effect on aircraft navigation but certainly provide some wondrous visual effects for the alert observer.
12.1.1 Rayleigh and Mie scatterSome of the visible light radiation from the sun, passing through the atmosphere bounces off atoms, molecules and other particles, and is scattered in all directions without losing energy or altering frequency. Gas molecules, being very much smaller than the wavelength of visible light (0.4 to 0.8 microns, see section 1.8 Electromagnetic wave spectrum), scatter the shorter violet and blue wavelengths much more strongly than the longer yellow and red wavelengths. But as the human eye is not very sensitive to violet light, the skyglow appears blue.
Atmospheric dust and smoke particles are considerably larger than the gas molecules. But they may still be smaller than the wavelengths of visible light and thus also selectively scatter the blue end of the spectrum, but more strongly than the gas molecules. This phenomenon is termed selective scatter or Rayleigh scatter.
Cloud droplets and small ice crystals are some 50 times larger than the light wavelengths and scatter all equally. Thus the light scattered from clouds retains the white light spectra, Mie scatter, and even though the droplets are colourless and transparent, the clouds appear white. Thicker clouds have darker bases because most of the light is scattered out the top and sides.
When the sun is directly overhead, the direct parallel rays that reach the eye from the sun's disc travel only a short distance through the atmosphere, so the sun's disc appears white. As the sun lowers, the distance travelled through the atmosphere increases, as does the scattering of the blue end. The depleted unscattered light that reaches the eye makes the disc appear yellow to orange to red, depending on the number and size of non-gaseous particles in the air. If there is a lot of dust or smoke haze in the path, only the red end of the sun's rays will remain unscattered — even the scattered light becomes reddish.
The amount of the visible light spectrum scattered is dependent on line-of-sight distance through the atmosphere. The sky near the horizon appears less blue, or whiter, at midday than the sky overhead; thus if the atmosphere were thicker, the sky would be whiter.
Similarly, when looking horizontally at a series of mountain ranges they appear bluer at a distance, until a point where the far ranges start to appear whiter than those in the middle distance. The trees on the ranges emit terpenes or essential oils — hydrocarbon molecules about 0.2 micron diameter, which combine with ozone infiltrating from the stratosphere. These molecules selectively scatter blue light — hence the blue haze on warm days.
Air molecules selectively scatter sunlight forward and backward equally, and at about twice the intensity of the light scattered at right angles to the beam. For particles larger than the wavelengths of light, back-scattered light is less intense than that for gas molecules but forward scatter is much more intense. Thus, in an atmosphere containing many large particles, the sky is less bright than blue sky when looking 'down sun' and much brighter when looking in the azimuth of the sun.
White-out conditions can occur when the surface has a complete snow or ice cover, matched with an extensive cloud cover. The brightness of the cloud cover is increased by light that is successively scattered many times between surface and cloud, with little absorption. The light travels in all directions and at all angles. In such conditions there can be no shadows, the horizon line disappears and the form of the landscape is no longer discernible. This leads to spatial disorientation. Partial white-out or flat light is a less severe condition where a pilot's ability to judge ground references for distance, height and attitude are detrimentally affected.
Last light is the end of evening civil twilight; and the official end of daylight in VFR air navigation regulations.
First light is the beginning of morning civil twilight and the official start of daylight in the regulations. It is not the time at which a line of light appears on the eastern horizon — if you take-off in those conditions you will be night flying.
Evening nautical twilight ends when the sun is 12° below the horizon. During this period the western horizon is still clearly defined, weather permitting, and the brighter stars are visible — thus providing good conditions for ocean navigators to take star sights; hence nautical. Noctilucent clouds may be seen in higher latitudes. Evening astronomical twilight ends when the sun is 18° below the horizon, after which all scattered sunlight disappears from the upper atmosphere and the stargazers have good viewing conditions. The morning twilight periods are reversed, of course.
The twilight wedge, or curve, divides the Earth's shadow from that part of the sky lit by direct sunlight. It appears on clear days as a blue-grey arc next to the eastern horizon as the sun disappears, highest at the antisolar point and curving down to the horizon. Initially there is a fairly sharp boundary bordered by a reddish band, the counterglow, then becoming diffuse as it rises. An airborne observer should see a sharp boundary above the horizon. Similar shadowing occurs at sunrise on the western horizon.
Usually after sunset the sky above that point is pale yellow with a blue-white arch above, the twilight arch, with yellow above and orange sky to either side. As twilight progresses, the arch above the sunset point becomes pink with yellow and orange below. These areas gradually flatten as the sky above changes from blue-grey through to dark blue. The final glimmers on the horizon are possibly greenish-yellow. Very rarely, and mostly when viewed over water when the air is free from any form of haze, a green flash is seen on the top of the sun's disc just before it disappears.
Zodiacal light is a faint, luminous glow in the night sky, easily seen in low to mid-latitudes at twilight in moonless conditions. It is caused by sunlight scattered by dust particles in interplanetary space. Zodiacal light extends over the entire sky but is brightest in the zodiacal band, and at about 30° angular distance from the sun, where the intensity is about three times that of the brightest part of the Milky Way. It is best seen when the ecliptic is close to vertical; i.e. autumn evenings and spring mornings. Brightness decreases with angular distance from the sun, being lowest at 120° then gradually increasing to the 180° antisolar point. The enhanced brightness near the solar point, and covering an area 6° by 10°, is the Gegenschein or counter-glow.
Airglow is visible infrared [IR] and ultraviolet [UV] emissions from the atoms and molecules in the ionisation layers caused by absorption of much of the solar UV radiation and of cosmic radiation. Daytime airglow, dayglow, may be seen from the surface at twilight when the blue skyglow is sufficiently weak. Dayglow is caused mainly by the dissociation of atoms, whereas nightglow emissions are due to recombination. The sum of all visible nightglow emissions, together with zodiacal light and scattered starlight, can be seen as the faint light between stars.
Crepuscular (twilight) rays are alternate light and dark bands that appear to diverge fan-like from the sun's position when it is hidden behind a cloud bank or the topography, in a humid or hazy atmosphere. The rays pass through gaps, like light beams shining through high windows. The divergence is due to perspective, if the rays pass overhead they then appear to converge on the antisolar point — anticrepuscular rays.
There are three types of crepuscular rays:
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12.2.1 Electromagnetic wave refraction, reflection and diffractionWhen a light ray passes obliquely from one transparent medium to another, or between layers of different density within the same medium, part of the ray is returned back at the boundary. The remainder, passing through, is deviated from its original course; i.e. its direction changes. The deviation is dependent on angle of incidence; the wave lengths of the light beam, or radio wave; and the refractive index for that medium. The refractive index is the ratio of the speed of electro-magnetic radiation in free space to the speed of radiation in that medium; in air it is effectively 1.0, and in water it is 1.33.
Refraction has two components — deviation and dispersion. As the components of sunlight have different wavelengths, in the atmosphere the deviated light ray is dispersed into its component colours but the red light deviates less than the blue light when passing from air through ice crystals or water droplets.
Radio waves in the High Frequency [HF] bands are refracted by the ionisation layers in the atmosphere. The downward bending of the wave is sufficient to redirect the wave back to the Earth's surface but at a distance from the transmission point. If there is sufficient energy, the wave may then be reflected back to the ionosphere. Thus a high-energy HF transmission is able to 'skip', between the surface and the ionosphere, for a considerable distance around the world.
Reflection is the bounce back of all, or part, of a light ray when it encounters the boundary of the two media, and the angle of reflection equals the angle of incidence. The amount of light reflected depends on the ratio of the refractive indices for the two media.
Diffraction is the bending of a light beam (or radio wave) into the region of the geometric shadow of an obstacle, or the spreading of light waves around obstacles. This produces a series of light and dark bands or rings or coloured spectra, from the inter-ray interference; constructive interference results in light bands, while destructive interference results in dark bands. The degree of diffraction depends on wavelength — red light is diffracted more than blue — and particle size.
The magnitude of the deviation angle depends on the orientation of the crystal. For a 60° ice prism the minimum deviation angle for all orientations is 22°; and for small rotations of the crystal, at the minimum deviation angle, the variation from 22° is insignificant. Thus in an atmosphere of randomly oriented crystals there will be a concentration of rays deviated by 22°. The deviation of light from its original path, through many hexagonal crystals, brings sunlight or moonlight to the observer's eye from different directions and in varying intensities. However, the concentration of refracted rays around 22° produces a solar or lunar halo whose inner, red edge has an angular radius of 22° from the observer's eye. The red edge merges into a yellow band then all the colours overlap in an outer white band. Halos are minimum deviation effects; each colour has a concentration at its minimum deviation angle, but also has a significant amount of light refracted at greater angles and overlaps other colours. Only the red, with the lowest deviation, cannot be overlapped.
Light passing through one side and an end of a hexagonal crystal is refracted in the same way as in a 90° prism and, in this case, there will be a concentration of rays at a 46° deviation angle. In suitable conditions a very large solar or lunar halo with an angular radius of 46° may appear, but it will be much less intense than the 22° halo and will rarely be complete. The 22° halo is the most frequently observed of all the ice crystal displays; the 46° halo is rather rare.
As cloud crystals grow during fall (flat plates perhaps 50 microns thick and several millimetres across, columns perhaps 100 microns across and several millimetres long), the drag creates lee eddies and the crystals tend to orient with their longest dimension near horizontal. They oscillate randomly as they fall in a spiral path, producing complicated optical effects through reflection, refraction and diffraction.
Sun pillars are vertical columns of light that appear above or below the sun, or both, when the sun is near the horizon. They are caused by reflection of sunlight from the near-horizontal surfaces of ice crystals and are similar to the glitter path of sunlight reflected on water. Light pillars are also associated with the moon. A subsun is a particular form of sun pillar seen from an aircraft when the sun is high — becoming a reflected, elongated image of the sun in nearly horizontal ice crystals in lower clouds. The image appears as far below the horizon as the sun is above. Sun pillars may be associated with AC.
The parhelic circle is a reflection from the vertical surfaces of horizontally oriented flat plate or columnar crystals when very small ice crystals, diamond dust , fall through the air. The crystals reflect the light in all directions of the azimuth but always downward at the same elevation as the sun. Thus if the sun's elevation is 25° an observer would see the parhelic circle 360° around the horizon by looking up 25°, but usually only part of the faint white circle is seen. The parhelic circle and a sun pillar may form a cross in the sky, centred on the sun.
If falling plate crystals maintain a horizontal position, with the sun low in the sky, they have the possibility to refract light to the observer from the sides of the 22° halo, but not from other positions in the halo. The result is a spot of increased light intensity and colour separation — red towards the sun — in the 22° halo each side of the sun, where the halo would intersect the parhelic circle; sometimes it appears with a white tail pointing away from the sun. As the sun elevation increases, the spots move further from the sun and outside the halo, disappearing at sun elevations greater than 60°. These intensified light spots are called parhelia, sundogs or mock suns and are the most common ice crystal phenomenon after 22° halos; they are often associated with CI, CS and possibly AC. Similar effects associated with the moon are paraselena or mock moons.
Refraction through the edges of plate crystals with nearly horizontal bases may produce a circumzenithal arc. This is part of a circle, possibly one third, centred directly above the observer's head and above the sun, just outside the 46° halo position. The halo may also be visible. The circumzenithal arc cannot occur when the sun's elevation exceeds 32°. Colour separation occurs with red on the outer rim, blue on the inner. The arc may be associated with CI and CS.
An anthelion is a concentration of back reflected light at the anthelic point, 180° from the sun and at the same elevation. The anthelic point may be the centrepoint for various reflection / refraction phenomena — the anthelic arcs.
Various other light intensifications are associated chiefly with refraction and may appear in ice crystal displays in Antarctic conditions. Among them are:
• Parry arcs
• circumhorizontal arcs
• supralateral arcs
• infralateral arcs
• contact arcs
• upper and lower tangents to the 22° halo.
Cloud irisation or iridescence ( Iris = the Greek rainbow goddess ) appears when a cloud element or streak, usually AC or CC and sometimes lenticularis, is evaporating around its edges so that the droplet size changes quickly over a short angular distance. Also the entire element or small cloud is contained in roughly the same angular distance from the sun. The diffraction pattern traces blue light around the edge of the cloud where the droplets are smallest, and red light where the drops are uniformly larger. The result is iridescent bands — predominantly pinks and blues or greens with pastel shades — appearing along the thinner edges of individual cloud elements. Cloud iridescence is common but the cloud must be within 20° of the sun and thus not readily noticeable. It can occur in thin SC or AS, and also in nacreous clouds.
The corona is the diffraction pattern seen in cloud droplets when looking towards the sun. The glory is the diffraction pattern seen in cloud or fog droplets when looking toward the antisolar point. (A glory is the circle of light or aureole around the depiction of the head of a saint, etc.) When flying in sunlight over a cloud layer, the coloured rings of glory may be seen around the antisolar point; i.e. around the aircraft shadow if it is not diffused. The antisolar point is that of the observer, so the luminous coloured halos are centred on the position of the observer's head shadow. As in other diffraction rings, the blue halo is on the inside and the red on the outside.
The 'silver lining' that may be seen around the outer edges of heavier clouds, containing larger droplets, is a diffraction effect.
12.2.4 RainbowsAs a light ray from the sun strikes a small spherical raindrop (drops less than 150 microns diameter are held as a sphere by surface tension, while larger raindrops are distorted by drag into a flattened sphere) some light is reflected by the outer surface. Some light passes through and reaches the opposite inner surface, where a fraction of the light is reflected internally and the rest passes out of the drop. A ray may be reflected only once inside a drop, or many times, but each reflection is accompanied by light leaving the drop, so each internal reflection diminishes the reflected ray.
Each spherical raindrop reflects and refracts, in all directions, the light rays that are striking it. However, due to the spherical surface there is a concentration of first reflection rays reflected back towards the sun, around a maximum angle of about 42° to the axis line joining the raindrop and the sun. The red light is refracted less than the other colours and has a concentration at about 42°. The blue light is concentrated at 40° with the other colours in between.
The observer will see this concentration of reflected light rays as an intensified coloured light band. This band consists of the first reflection rays from all the raindrops that lie on the surface of a cone, subtended at the observer's eye, with an angular radius of 42° from an axis line drawn from the sun (directly behind the observer) through the observer's head and extended down-sun to the antisolar point; i.e. below the horizon where the shadow of the observer's head might be. This primary rainbow will have the red band on the outer edge. An observer on the Earth's surface sees only an arc of the rainbow circle. When the sun is 40° above the horizon, just the top of the bow can be seen. The rainbow will rise as the sun lowers, until much of the circle can be seen. The lower ends may appear very close to the observer. An airborne observer could possibly see the full circle.
Light that is reflected twice within the raindrops has a deviation angle of 51° and produces the weaker secondary rainbow — concentric with and outside the primary, but with the red band on the inner edge. Thus the observer is seeing the concentration of twice reflected rays from all the raindrops that lie on the surface of a 51° cone, at the same time they are seeing the first reflections from the raindrops on the 42° cone. Third and fourth reflection rays would also form rainbows with angular radii of 40° and 46° respectively. These are so weak, and would also form up-sun, so that they are most unlikely to be seen except against a dark cloud.
As the first reflection rays from spherical raindrops have a maximum deviation angle of 42°, it follows that all the low-angle reflections coming back to the observer's eye, from all the raindrops enclosed within the 42° cone, will increase the brightness of the sky within the primary bow. Similarly the sky is also brighter outside the secondary bow.
The rainbow ends are frequently brighter than the rest of the bow, particularly when the sun is low. This comes from the approximate straight back reflection / refraction in the larger, flatter raindrops added to the reflection / refraction of the smaller, spherical drops.
Diffraction interference of light rays ( the waves are out of synchronisation ) produces changes in light intensity, which may appear as a series of light / dark bands within, and close to, the primary rainbow.
When rainbow rays pass through very small water droplets (e.g. cloud or fog droplets) they are spread by diffraction, and each colour band is broadened and overlaps adjoining bands. Where all the colours overlap, the result is a white rainbow, cloud bow or fog bow; this is often seen from an aircraft flying over a smooth, extensive cloud layer. Near sunset, a white rainbow may appear as a red rainbow in a low cloud bank. A full moon can produce a rainbow that appears to be white in the low light conditions but, when photographed, is revealed as a normal rainbow.
12.2.5 Atmospheric density layer effectsWhen a light ray passes through the atmosphere, where the density changes gradually, the light ray changes direction in a curved path rather than abruptly as when passing through an ice crystal. With changes in atmospheric density, the deviation path curves toward the denser air. Thus when a star is low in the sky, the change in atmospheric density with height, particularly with a cold surface layer under an inversion, causes refraction to bend the light rays so that the star's apparent position is higher than actual and the dispersion may produce a multi-colour image — upper part blue, middle white and lower part red. This gives the impression of an aircraft's lights, and is often reported as strange, moving lights in the sky, as the atmospheric effects make the object appear to jiggle. At sunset or sunrise, refraction can cause the sun's image to appear above the horizon when it is actually below.
Small-scale atmospheric temperature and density variations in the line of sight between the observer and a star, or other light-emitting object, produce the twinkling effect scintillation, and the shimmering of distant landscape. Parcels of cooler or warmer air can act as lenses, reducing or increasing the apparent brightness or size of the object.
Mirages are optical phenomena produced by refraction of light rays through air layers with large temperature gradients. An inferior mirage (i.e. it appears below its actual position) occurs when the temperature initially decreases rapidly with height. For example, the heat flux from a hot surface, such as tarmac or sand, greatly increases the temperature of the adjacent shallow air layer and consequently the density of that layer decreases (see equation of state). The result is a layer of less dense air underlying denser air, the reverse of the normal lapse rate. Light rays from the sky moving through the layers will be refracted upward in the less dense air (i.e. bent toward the denser air), giving the appearance of a layer of water.
When seen from the ground or water, a superior mirage (i.e. it appears above its actual position) occurs when there is a pronounced inversion near the surface, and normally over the sea or a large body of water. A distant object within the inversion layer, even something below the horizon, will appear in the sky above its actual position — possibly totally upside down or the upper portion upside down, but certainly distorted and wavering. For more information google the phrase "superior mirage".
An inversion layer of cooler air, with warmer air above and below, acts as a wave guide for light rays introduced into the layer at a small angle to the horizontal. Unless there is a discontinuity in the layer, the trapped rays cannot escape and may be confined within the wave guide for very long distances, following the curvature of the Earth. In such circumstances, a spectacular superior mirage might be seen from an aircraft flying over land within that wave guide. Whit Landvater is a Nevada balloonist who experienced such a display on November 27, 2003 and said "It was like "living inside a Photoshop document while someone was going crazy with the clone tool and filters!"
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The geometry of the sun–Earth–moon orbits gives rise to the eight commonly recognised moon phases and the associated moonrise/moonset periods.. The elapsed time from one full moon to the next is about 29.5 days.
|The next, and last, section of the Aviation Meteorology Guide covers aviation weather reports and forecasts|
Aviation meteorology guide modules
| Meteorology guide contents | The atmosphere and thermodynamics (part 1) | Thermodynamics (2) and dynamics |
| Effects of altitude — contained in the Flight Theory Guide module 2 & module 3 |
| Cloud, fog and precipitation | Planetary-scale tropospheric systems | Synoptic scale systems |
| Southern hemisphere winds | Mesoscale systems | Micrometeorology — atmospheric hazards |
| Airframe and engine icing | Atmospheric electricity | Atmospheric light phenomena |
| Aviation weather reports and forecasts |
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