When parts of clouds are thin and have similar size droplets, diffraction can make them shine with colours like a corona. In fact, the colours are essentially corona fragments. The effect is called cloud iridescence or irisation, terms derived from Iris the Greek personification of the rainbow. The usually delicate colours can be in almost random patches or bands at cloud edges. They are only organised into coronal rings when the droplet size is uniform right across the cloud. The bands and colours change or come and go as the cloud evolves. They occur most often in altocumulus, cirrocumulus and especially in lenticular clouds. Iridescence is seen mostly when part of a cloud is forming because then all the droplets have a similar history and consequently have a similar size. Sometimes iridescence can be seen far from the sun but is most frequent near to it. As for coronas, search safely by hiding the sun behind a building and, even better, also viewing the reflection of the sky in water.

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The Jet d’Eau, or water-jet, is a large fountain in Geneva, Switzerland, and is one of the city’s most famous landmarks, being featured on the city’s official tourism web site and on the official logo for Geneva’s hosting of the 2008 UEFA Championships.  It is also one of the largest fountains in the world.  Situated at the point where Lake Geneva empties into the Rhone River, it is visible throughout the city and from the air, even when flying over Geneva at an altitude of 10 km (33,000 ft).

Five-hundred litres (132 gallons) of water per second are jetted to an altitude of 140 metres (459 feet) by two 500 kW pumps, operating at 2,400 V, consuming over one megawatt of electricity.  The water leaves the nozzle at a speed of 200 km/h (124 mph). When it is in operation, at any given moment there are about 7,000 litres (1849 gallons) of water in the air. Unsuspecting visitors to the fountain—which can be reached via a stone jetty from the left bank of the Lake—may be surprised to find themselves drenched after a slight change in wind direction.

A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth’s atmosphere. They take the form of a multicoloured arc, with red on the outer part of the arch and violet on the inner section of the arch. A rainbow spans a continuous spectrum of colours; the discrete bands are an artefact of human colour vision. The most commonly cited and remembered sequence, in English, is Newton’s sevenfold red, orange, yellow, green, blue, indigo and violet . Rainbows can be caused by other forms of water than rain, including mist, spray, and dew.

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Construction began in 1963 and was completed in 1967. It surpassed the Empire State Building to become the tallest free-standing structure in the world. It held this record for nine years until the CN Tower was completed in Toronto, Canada in 1976, which surpassed its height by 13 metres (43 ft). The Ostankino Tower remained the second-tallest freestanding structure in the world for another 31 years until the Burj Dubai surpassed both it and the CN Tower in height in 2007.

The tower created a desire for other large cities in the Soviet dominated countries to build high towers. Towers taller that 300 metres were built in Kiev, Tashkent, Almaty, Riga, Berlin, Vilnius, Tallinn, Jerevan, St Petersburg and Baku. No one is taller than the Ostankino Tower however.

The Ostankino Tower has remained the tallest free-standing structure in Europe for 42 years. The Russia Tower, a proposed 612-metre (2,010 ft) mixed-use skyscraper planned for the Moscow International Business Centre, was originally expected to exceed the Ostankino Tower’s height when completed. However, the project has been suspended due to financial difficulties and it remains unclear if construction will resume.

A 1994 plan to increase the tower’s height to 561 meters by adding an antenna was not implemented for lack of funding.

Noctilucent clouds, are tenuous cloud-like phenomena that are the “ragged-edge” of a much brighter and pervasive polar cloud layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly night shining in Latin. They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator.

They are the highest clouds in the Earth’s atmosphere, located in the mesosphere at altitudes of around 76 to 85 kilometers (47 to 53 mi). They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth’s shadow. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon; there is no evidence that they were observed before 1885.

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We’ve all observed the worrying proliferation of condensation trails, or ‘contrails’, criss-crossing our skies in the wake of high-altitude aircraft. They look like man-made scars across the blue. But how many have noticed the much less common but related cloud effect, known as a ‘distrail’?

Short for ‘dissipation trail’, this is not so much a cloud, as a gap in cloud cover. It can appear when an aircraft happens to pass through a fairly thin cloud layer composed of ’supercooled’ droplets. These are in unfrozen, liquid form, even though temperatures are well below 0degC. Water droplets can stubbornly refuse to freeze when there is a lack of air-borne particles to act as nuclei onto which the ice crystals can start forming.

As an aircraft climbs or descends through one of these supercooled clouds, the turbulence of its wake and the many minute particles contained in its exhaust encourage the cloud’s droplets to freeze. This happens when some of the particles act as the nuclei onto which the droplets can start freezing. As the crystals form, they grow in size and fall below. Left behind, is a just gap in the cloud – the distrail.

Or in meteorological terms: dissipation trail—(Or distrail.) A clearly delineated limpid lane forming behind an aircraft flying in a thin cloud layer; the opposite of a condensation trail.

The heat of combustion of the aircraft fuel, released into the swept path by the exhaust stacks of the aircraft, can, under certain conditions, evaporate existing clouds (if not too dense) and yield a distrail. Clouds of low liquid water content and relatively high temperature are susceptible to distrail formation but the phenomenon is comparatively rare.

Pictures taken on March 5th (optics with circumscribed halo, the first 9 pictures), on March 7th (distrails…, pictures 10 to 15) and March 18th (picture 16 to 45…morning Sundogs, Shanti… our own dog, a twisted contrail part, some faint iridescence, early frozen grass, a small part of the world through a drop of water and some afternoon sundogs with a would be blue like parhelic attachment…)

Unfortunately I had to “work” this afternoon which probably led to missing an opportunity in photographing some nice displays in the sky.

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Below a 400 mm zoom into the seawater bow that was visible in the bow wave of the ship. Due to high speed the water splashed high up to land back up onto the flat surface of the sea where it splattered into millions of drops… and millions of directions…

Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind a person at a low altitude angle (on the ground). The most spectacular rainbow displays happen when half of the sky is still dark with draining clouds and the observer is at a spot with clear sky in the direction of the Sun. The result is a luminous rainbow that contrasts with the darkened background.

The rainbow effect is also commonly seen near waterfalls or fountains. The effect can also be artificially created by dispersing water droplets into the air during a sunny day. Rarely, a moonbow, lunar rainbow or night-time rainbow, can be seen on strongly moonlit nights. As human visual perception for colour is poor in low light, moonbows are often perceived to be white. It is difficult to photograph the complete arc of a rainbow, as this would require an angle of view of 84°. For a 35 mm camera, a lens with a focal length of 19 mm or less wide-angle lens would be required. From an aeroplane, one has the opportunity to see the whole circle of the rainbow, with the plane’s shadow in the centre. This phenomenon can be confused with the glory, but a glory is usually much smaller, covering only 5°–20°.

The light is first refracted as it enters the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back over a wide range of angles, with the most intense light at an angle of 40°–42°. The angle is independent of the size of the drop, but does depend on its refractive index. Seawater has a higher refractive index than rain water, so the radius of a ‘rainbow’ in sea spray is smaller than a true rainbow. This is visible to the naked eye by a misalignment of these bows.

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Coronae have an intensely bright central aureole which is almost white and fringed with yellows and reds. Sometimes that is all to be seen but the better coronas have one or more successively fainter and gently coloured soft rings surrounding the aureole. The first ring is bluish on the inside grading through greens and yellows to red outermost. The colours are subtle mixtures rather than the more direct hues of the rainbow. The corona can be 15º or so in diameter and often it shrinks and swells as different clouds scud across the moon.

The coronae is much smaller than the 22° halo which can also ring the sun and moon. The corona also has nothing to do with the Sun’s outer atmosphere visible during a total eclipse and confusingly given the same name. Coronae are produced by the diffraction of light by tiny cloud droplets or sometimes small ice crystals.

In meteorology, a corona is produced by the diffraction of light from either the Sun or the Moon by individual small water droplets (and sometimes tiny ice crystals) of a cloud.

The corona consists of small number of concentric colored rings around the celestial object and a central bright aureole. The angular size of the corona depends on the diameters of the cloud droplets – small droplets produce bright coronae.  Coronae differ from haloes in that the latter are formed by refraction from comparatively large rather than small ice crystals. Reddish colours always occupy the outer part of a corona’s ring. More info on Atmospheric optics on the site of Les Cowley…

The corona shown on the left  should not be confounded with the suns corona which is of a totally different order… the outermost layer of the solar atmosphere. The corona consists of a highly rarefied gas with a temperature greater than one million kelvin. It is visible to the naked eye during a solar eclipse. More info on the sun:

SOHO, the Solar & Heliospheric Observatory, is a project of international collaboration between ESA and NASA to study the Sun from its deep core to the outer corona and the solar wind.

A bright and expansive coronal mass ejection (CME) unfurled itself on January 24, 2007. As seen in SOHO’s LASCO C2 coronagraph, the bright front emerged in the shape of an arc from behind the occulting disk but soon expanded into a ragged, bulbous shape with lots of structural lines inside it. The source of this CME was an active region that had just began to rotate into view the next day. An EIT 304 Angstrom image of the Sun taken at nearly the same time was enlarged and superimposed on the occulting disk. (Images copyright SOHO (left) and ESO (right)). Click on the image for more…

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To see a rainbow we need sunshine and falling rain. Rainbows are rarer than might be thought. In any one place in rainy England there are fewer than ten bright ones in a year. Halos occur much more frequently.
Early morning and late afternoon are the best times to see them because the sun must not be too high. Rainbows are always opposite the sun and their centres are below the horizon at the the antisolar point. The lower the sun the higher is the bow.
Red is always outermost in the primary bow with orange, yellow, green and blue within. Occasionally, when the raindrops are small, fainter supernumerary arcs of electric greens, pinks and purples lie just inside the main bow.
A rainbow is not just a set of coloured rings. The sky inside is bright because raindrops direct light there too. The primary bow is a shining disk brightening very strongly towards its rim. (Text Les Cowley)

More pictures of rainbows: Belfast Rainbows… and On top of Brussels…

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Fogbows form in the same way as rainbows. A small fraction of the light entering droplets is internally reflected once and emerges to form a large circle opposite the sun.
But… …beyond that there are major differences. Rainbows are formed by raindrops which are so large that rays passing through them follow well defined ‘geometrical optics’ paths. Fogbows are formed by much smaller cloud and fog droplets which diffract light extensively.
The emergent light is mostly deviated 135 to 150° from its incident direction to produce the main fogbow of 30 - 45° radius centered on the antisolar point. The deviation corresponds roughly to the geometric optics angle of minimum deviation of ~138° for the 42° radius rainbow.
Fogbows are almost white with faint reds on the outside and blues inside. The colours are so washed out because the bow in each colour is very broad and the colours overlap.

Glories are always directly opposite the sun, centered at the antisolar point and therefore below the horizon except at sunrise and sunset.
Look for them whenever mist or cloud is beneath you and the sun breaks through to shine on it.
Glories can be seen on mountains and hillsides, from aircraft and in sea fog and even indoors.
They are formed when light is scattered backwards by individual water droplets.
They have a bright centre but not nearly as bright as the corona’s aureole. Their rings are delicately coloured like those of the corona’s, blue on the inside changing through greens to red and purple outside. The ring intensities fall off much more slowly than those of the corona and sometimes three or even four rings are visible.
Shadows converge on the antisolar point and so glories are nearly always accompanied by your shadow or that of the aircraft you are in. When the shadow is grotesquely distorted by perspective it is called a “Brocken spectre”.
There are other glows at the antisolar point, the heiligenschein and the opposition effect but these do not have the glory’s shimmering rings.

The Brocken Spectre appears when a low sun is behind a climber who is looking downwards into mist from a ridge or peak. The “spectre” is the shadowy figure – the glow and rings are of course a glory centered directly opposite the sun at the antisolar point. But how is the ghostly figure produced? It is no more than the shadow of the climber projected forward through the mist. All shadows converge towards the antisolar point where the glory also shines. The sometimes odd triangular shape is a perspective effect. The Brocken Spectre is a similar effect to anti-crepuscular rays and cloud shadows. (Text Les Cowley)

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A second rainbow 2 hours later…

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Crepuscular rays appear to converge on the sun, anticrepuscular rays converge in the opposite direction and you must have your back to the sun or sunset point to see them. They appear to converge towards the antisolar point, the point on the sky sphere directly opposite the sun. Like crepuscular rays they are parallel shafts of sunlight from holes in the clouds and their apparently odd directions are a perspective effect. Think of a long straight road, it converges towards the horizon but turn around and it also converges to the opposite horizon. Crepuscular and anticrepuscular rays behave in the same way. Anticrepuscular rays are not rare but they must be sought carefully. When ordinary crepuscular rays are visible, turn around and search for their opposite numbers. More rarely, sunrays pass along great circles right across the sky.(Text Copyright Les Cowley)

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And an example taken at La Réunion last year…

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The big eye in the sky…

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A contrail shadow…

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22º halos often encircle the moon. More rarely, because the moon is relatively dim, it is possible to see moondogs and other halos. Colours are faint or non existent because their light is barely strong enough to excite the colour sensors of our eyes.

Much smaller coloured rings sometimes surround the moon. These are not a halo but a corona produced by the diffraction of light by the water droplets of clouds. And of course moonlight creates a rainbow although to the unaided eye it is usually a wan creature devoid of colour.(Credits: Atmospheric optics, Les Cowley)

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First recorded in 1820 by Parry during his search for the North West Passage, the arcs are rare. The column crystals with their long axes nearly horizontal which form tangent arcs are even further constrained in the Parry orientation. Parry oriented crystals have their long axes and their upper and lower prism side faces nearly horizontal. The high degree of orientation produces many different and rare halos. Parry arcs proper result from ray paths through prism side faces inclined at 60°. There are four possibilities and the Parry arcs are called ‘suncave’ or ‘sunvex’ depending whether they are concave or convex with respect to the sun.

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