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ASTRO 101

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Rainbows, Sundogs, and Glories and other optical effects
rainbows   moonbows   halos   sundogs   glories   pillars   fogbows   noct. clouds   brocken   heiligen   crep. rays   green flash
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"The Double Alaskan Rainbow" © 2007 photo by Eric Rolph

"Why are there so many songs about rainbows?" ~ Kermit the Frog

Rainbows
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"Anatomy of a Rainbow" René Descartes 1637
A rainbow is a continuous spectrum of color that appears when light shines onto droplets of moisture in the air. It forms a circle around the point directly opposite of the observer from the light source. Usually the light source is the Sun, so the center of the circle is the anti-solar point (the observer's shadow). Also typical is that only a portion of the circle is above the ground so that the visible part is an arc, with red on the outside edge and violet on the inside edge. From the vantage of an airplane one oftens sees the full circle around the shadow of the plane.

Although the spectrum is continous through millions of colors, it can be simplified as having seven colors in this order: Red-Orange-Yellow-Green-Blue-Indigo-Violet. The common mnemonic for the first letters of these colors in order is "ROY G BIV."

When light goes through water droplets it is redirected at every surface. First it is bent at an angle, or "refracted" as it enters the front surface. Then it is reflected off the back surface of the droplet, by an effect known as internal reflection. It is refracted once more as it exits through the front surface towards the observer. The different colors each go through this process at slightly different angles so that the combination of angles creates a spectrum of colors. The overall effect is that the incoming light is reflected back and spread out over angles of 40o to 42o (less for saltwater) from the center of the anti-solar point.

Sometimes a second arc is seen outside the first, with the colors in the opposite order, and on rare occasions a third arc is seen, very faint, and reversed back to resemble the color pattern of the first.

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"Moonbow & Victoria Falls" © 2007 photo by Calvin Bradshaw
Moonbows
A moonbow is a rainbow that occurs at night from the light of the moon. Moonbows are very faint and thus not very colorful to the human eye. Long exposure photographs will reveal more intensity, like the photo at the right, which also captured the stars of Orion. The center of the arc is at the anti-lunar point so they are best seen when there is a full moon low in the sky. Sometimes people mistakenly use the term moonbow to refer to a ring around the moon, which is more commonly called a halo.

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"Solar Halo" © 2007 photo by Katherine Rawlins
Halos
A circle of brightness sometime appears about 22o from the Sun or Moon. It is usually too faint to detect much color but the inner part of the circle is red and the outer part blue. Halos are caused when flat hexagonal ice crystals are present at higher altitudes. They refract light that enters and exits the side of the crystals. The fluttering ice crystals are facing random directions in the sky so at any given time a percentage of them are oriented in a way that presents optimal profiles. The profiles add up to a circle of light. A dimmer halo may appear at a radius of 46o.

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"Sundogs" © 2007 photo by Michel Tournay
Sundogs & Moondogs
Sundogs and Moondogs are closely associated with halos (see above). Like halos, they are caused when flat hexagonal ice crystals are present at higher altitudes. If the orientation of the crystals is truely random, the observer will see a halo without the dogs. If a larger percentage of crystals are floating with their flat side horizontal to the observer (wind pressure), a strong concentration of light will occur on the left and right side of the halo. If the overall halo activity is strong, the horizontal bias will also create dogs at the top and bottom of the circle.

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"Glory" © 2007 photo by Nik Szymanek
Glories
A glory is produced by a process similar to rainbows, but with a part of the light that takes a different set of angles of diffraction, reflection, and refraction as it passes through water droplets. The angle of a glory from the light source is much smaller than a rainbow. The colors appear in a spread of about 5o to 20o. Glories are often associated with another optical effect, spectre of the brocken.

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"Sun Pillar" © 2007 photo by Mila Zinkova
Pillars
These verical columns appear when falling ice crystals reflect early morning or late evening sunlight. The crystals often are flat and hexagon shaped. As they flutter a percentage of them are always aligned so as to reflect light to the observer.

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"Fogbow" © 2007 photo by Mila Zinkova
Fogbows
Unlike rainbows, fogbows are rare because the light source has to be low at the back of the observer and the fog mostly in the opposite direction. If the fog is too thick at the back of the observer, the sunlight or moonlight will not be able to penetrate it and cause the effect. The color of the bow is not as intense as a rainbow, in fact it will often be colorless if the water droplets are very small

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"Noctilucent Clouds" © 2007 photo by Eva Seidenfaden
Noctilucent Clouds
These are clouds at unusually high altitudes, as much as 80 to 90 kilometers high, that glow in the night. At that altitude they are lit from below the horizon by direct sunlight, shortly after sunset or before sunrise. They are mostly seen in the summer at latitudes between 50o and 60o north or south of the equator.

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"Spectre of the Brocken" © 2007 photo by Nik Szymanek
Spectre of the Brocken
This effect is named for a peak in the Hartz mountains of Germany where occurrences are common. It is the seemingly enormous shadow of an observer, often witnessed by mountain climbers when their shadow is cast on a cloud or fog so that it is seen hovering over the horizon. The effect was observed and described by Johann Silberschlag in 1780. The apparent magnification is an optical illusion caused when observers compare their shadows on nearby clouds to distant land objects. Another feature of the effect is a halo of colored rings (a glory) around the shadow's head, caused by the water droplets.

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"Heiligenschein" © 2007 photo by Eva Seidenfaden
Heiligenschein
Heiligenschein (German for holy glow) is a bright area around the shadow of the observer's head (the anti-solar point). The name calls to mind a technique used on holy figures in renaissance paintings. Integral to the effect is dew droplets acting as tiny lenses, focusing the sunlight onto the background, and working again as a lens for the backscattered light. Look for the Heiligenschein early in the morning, when the sun is low and your shadow long. Stand so your head's shadow is on wet dewey grass.

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"Crepuscular Rays" © 2007 photo by B.P. Snowder
Crepuscular Rays
Sometimes sunlight appears as several separate beams of light, usually through holes in the clouds or gaps in the mountains. The beams are actually parallel but seem to diverge in the same way that parallel railroad tracks seem to emanate from a vanishing point on the horizon. It is an effect of perspective. The rays are caused by diffraction, reflection and scattering by material in the atmosphere, and by the sharp contrast with the shadowed areas.

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"Green Flash" © 2007 photo by Mila Zinkova

Green Flash
This is seen only on very rare occasions, when conditions are just right. Chances improve with a very clear-sky sunset on an ocean horizon. The Sun reddens as it is sinking below the horizon and suddenly, the last sliver of the Sun's disk turns emerald green just for a second or two, and then it is gone. There are sunrise flashes too but you need to stare at the exact place and moment that the Sun will appear. A prism-like refraction by our atmosphere can create a tiny amount of green on the upper limb of the Sun and a lensing effect as it reaches the horizon can magnify it briefly.


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