(6) Atmospheric Optics 2

Physics of the Atmosphere IIAtmo II 95a

Rainbows

Rainbows are caused by (double) refraction at the surface and reflection. On the backside of spherical raindrops. This can happen under different angles, but there is a maximum angle of 42° (Caspar David Friedrich, C. D. Ahrens).

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Rainbows

Snell's Law (nAir = 1)

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Rainbows

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Les Cowley

The refraction of violet light (n = 1.3435) is stronger than that of red light (n = 1.3318), leading to a color separation (photo: Steve Crowe, illustrations: wikimedia, C.D. Ahrens).

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The Colors of the Rainbow

For some unknown reason an alarmingly high number of teachers seems to believe that the sequence of colors in a rainbow is a matter of choice – it is not (UF).

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The Colors of the Rainbow?

A second rainbow appears (at an angle of ~ 51°), when the light is reflected twice inside the raindrop. Note the reversed order of the colors and Alexander’s dark band in between (credit: Kimberly Perez, wikimedia).

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Double Rainbow

It also works with other light sources – here we see a rare double moonbow over the Azores (credit: Antonio Araujo).

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Double Lunar Rainbow

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Higher Orders

As late as 2011 the first photographs of the third and fourth rainbow (at an angle of 40° and 45° around the sun) could be captured, thy are almost always outshined by the zero order glow (M. Großmann (l), M. Theusner, Schematic: Les Cowley).

There seem to be too many bows – “Supernumerary Rainbows”. They are caused by interference – an they provided a first hint that light can act like a wave (credit: Verena Tiessen) (small, same-sized raindrops).

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Supernumerary Rainbows

Additional rainbows caused by reflection (Terje O. Nordvik, Les Cowley).

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Rainbows for Advanced Students

Twinned rainbows are (most likely) caused by non-spherical, large raindrops (photo: Benjamin Kühne, simulation: Les Cowley).

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Rainbows for Advanced Students

From the right (elevated) observation point you may see a complete rainbow (credit: S.S. Matthiasson (l.), I. Parker, T. Gamache (inset)).

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Complete Rainbows

But where is the treasure now? (C. Leonhardt)

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Complete Rainbows

Fogbows

Fogbows are formed in a similar way – but at (much smaller) fog (cloud) droplets. They are lacking the brilliant colors, but in the center we see a new phenomenon (credit: Mila Zinkova).

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Brocken Spectre

From above a cloud you may see your own shadow (at the anti-solar point) as “Brocken Spectre“, surrounded by a glory (credit: Hannes Pichler). For the explanation we (would) need Mie scattering and wave optics (diffraction and interference).

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The phenomenon is named after Brocken, the highest elevation in the Harz mountain range, where it can be frequently observed. Glories and “Brocken Spectre“ are often observed from aircrafts (credit: Rick Stanciewicz, Franz Kerschbaum). [These are not rainbows (google 360° rainbow images)]

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Brocken Spectre and Glory

Each observer will see a glory around its own head – and no glories around those of others (photo: Neil Adams). The formation of glories is still not entirely understood. 180° retro-reclection can only be explained with surface waves (Les Cowley).

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Glory

Ice Halos

Halos (here with the sun almost in zenith) are caused by refraction in hexagonal ice crystals, where the ice crystals act like prisms (UF).

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Ice Halos

The 22° Halo can be frequently observed (in the presences of cirrostratus clouds) (photo: UF, illustration: C. Donald Ahrens).

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Solar and Lunar Halos

The size of the 22° Halo does not depend on the nature or distance of the light source (credit: G. Strand).

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Halos and Parhelia

Hexagonal plate shaped ice crystals tend to be horizontally oriented, they produce parhelia („Nebensonnen“), which are aslo known as sundogs (photo: Andrea Steiner, illustration: C. Donald Ahrens).

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Halos and Parhelia

Horizontally oriented hexagons produce parhelia, poorly oriented hexagons the rest of the 22° halo. 22° is the minimum deviation angle (credit: Les Cowley).

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Parhelia

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Jerry Walter

Halos and more

Here we can see many phenomena, which can be caused by refraction and reflection at/in ice crystals (credit: Jay Brazell).

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Frequent Halos

Les Cowley

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Parhelic Circle

Parhelic circles are caused by (multiple) internal reflection on (near) vertical faces of ice crystals (credit: Koby Harati).

Moondogs

While sun dogs can be quite often observed (left, David Wigglesworth), moondogs (paraselenae) are very elusive (David Cartier).

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Halo, moondogs and tangent arcs during the polar night by Fridtjof Nansen (thanks to Georg Buchner).

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Moondogs

Sun pillars are created by reflection on tilted ice crystals (photos: J. Kirkpatrick, P. Sears, illustr.: Les Cowley) (see also right photo on slide 159).

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Pillars

Diamond dust (here at the South Pole) creates particularly beautiful halos – even in anti-solar direction, where we can see the rare Anthelion (photos: Marko Riikonen).

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Multiple Halos

Antihelic Point

Halos produced by poorly oriented crystals (left) and by oriented plates crystals (right, photos: Marko Riikonen, illustration: Les Cowley).

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Multiple Halos

Halos produced by oriented columnar crystals (left) and by (even) parry-oriented ones – upper and lower prism sides are horizontal (right, photos: Marko Riikonen, illustrations: Les Cowley).

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Multiple Halos

Halos over New Mexico (credit: Joshua Thomas). In the unlikely event that you cannot identify all of them, the explanation is on the next slide.

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Multiple Halos

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Multiple Halos

Around noon on January 24, 1630, seven suns seemed to shine over Rome (of course being regarded as celestial omen). The record by the Jesuit scholar Christoph Scheiner helped to inspire Christiaan Huygens to develop the first theories of how such 'halo effects' are formed (credit: Herzog August Bibliothek Wolfenbüttel).

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Seven Suns?

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Halos on other Worlds

Les Cowley

On Mars, Jupiter and Saturn we could observe other phenomena. Octahedral crystals can cause four sundogs (simulations: Les Cowley).

Corona

A corona is caused through diffraction by small particles – usually by cloud droplets (above, credit: Martin Dietzel), but some-times by (opaque !) pollen grains (right, P.-M. Heden) – “reflecting” even their shape.

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Northern (southern) lights are certainly beyond the realm of meteorologywww.northern-lights.no

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