The World of Atmospheric Optics – Part Two by John Flannery SDAS PDF Print E-mail

In the first installment of this series on atmospheric optics we touched on various aspects of rainbows that one can witness. Now, let us turn our attention to those strange and wonderful halos and spots of light that are grouped in the family of ice-crystal related phenomena.

The classic halo
Most of have probably seen the Sun or Full Moon with a pale ring of light surrounding it roughly two fist-widths out from either edge of the disk. This is the 22-degree halo, the simplest form of ice crystal display. The 22-degree halo is caused by light passing through the faces on alternate sides of long pencil-shaped ice crystals floating in the air.

Sun Halo

They are a surprisingly common phenomenon and the popular saying “Ring around the Moon means rain soon” has a certain element of truth in it. Ice crystals giving rise to halos are what makes up those wispy high-altitude cirrus clouds that we know are often forerunners of a rain area that can accompany a warm front. If you see a halo around the Sun or Moon then statistically, two times of out three, you will get rain in 12 hours. An interesting project is to note any halo displays and monitor when rain showers sweep through your locality.

Moon Halo

Maybe it’s because halo phenomena are at their brightest towards the vicinity of the Sun that we are not often aware of them being there. Few of us have any reason to gaze in the direction of our brilliant daytime star. By hiding the Sun’s disk behind a telephone pole or the corner of a house it makes it easier to glimpse halos however.

Halo phenomena
When the Sun is low down you may notice a brilliant patch of light either side of the solar disk and position almost on the 22-degree halo. These are parhelia, more commonly known as mock suns or sundogs. Caused by sunlight refracted through ice crystals all having a vertical central axis, these can be quite intense and display vivid spectral colours. It is only when the Sun is on the horizon that we get the mock suns exactly on the 22-degree halo.

Sundog

The parhelia move further out as the Sun moves higher in the sky until they disappear altogether when it is above 61-degrees altitude. You can also observe mock moons or parselenes. As with the Moon rainbow however, they are much fainter than their daytime counterpart and thus appear colourless except in a time exposure photograph. We therefore normally only see such phenomena near times of Full Moon. An extraordinary image in recent years however was secured by Carol Lakomiak of dogs and pillars caused solely by the light of Venus. The picture and commentary can be found at:

http://science.nasa.gov/headlines/y2002/06may_pillar.htm

A larger halo is the 46-degree halo but it’s not as common as the 22-degree ring. Rarer still are halos of other radii but they have been noted – and photographed – in more complex displays.

Low-Sun ice crystal phenomena
Another relatively common low-Sun phenomenon is a sun pillar. The pencil-like beam of light rising above the setting Sun appears like the reverse of what you see in postcards from those far-off, sun-soaked isles depicting a path of golden light from the setting Sun threaded across the ocean (in fact, the proper term for this phenomenon of light reflected in water is a “glitter path”.) Most sun pillars are caused by flat crystals floating horizontally and acting like mirrors to reflect the light from the setting Sun. Work by Robert Greenler at the University of Wisconsin has shown that pencil crystals account from those observed at higher altitudes.

Sun Pillar

From an aeroplane or high mountains you may also see a sub-Sun or sun pillar below the Sun. These form in exactly the same way as the standard pillars except are due to plate-shaped crystals below the observer’s perspective. Any light source can give rise to pillars and indeed there are many photographs at Les Cowley’s wonderful atmospheric optics web site http://www.atoptics.co.uk

Associated halo phenomena
The phenomena discussed so far are relatively common, but what about other wonderful objects such as circumscribed halos, circumzenithal arc, and large-scale halo displays?

The circumzenithal arc is just as it name suggests, an arc around the sky’s zenith point. People who have seen one may have thought it was an upside-down rainbow for that is what it looks like. But few have seen it because of its position straight over our heads.

Circumzenithal Arc

The arc forms when the Sun’s altitude is below 32-degrees and cirrus cloud feathers the zenith. Colour saturation in these arcs is often more intense that in a rainbow. At altitudes of around 32 degrees the arc appears as a very short curve, opening out like a flower as the Sun gets lower in the sky. At an altitude of 22 degrees the arc is at its most developed and is brightest. It may then extend as much as one-third the way around an imaginary circle that has the zenith at its centre. You can also get circumhorizontal arcs but these only form at Sun altitudes greater than 58 degrees.

The upper tangent arc is also associated with the 22-degree halo and first appears as a bright spot of light at the top of the halo. The shape is dependant on the Sun’s altitude and the arc is v-shaped when the Sun is near the horizon but flattens out as the Sun climbs higher in the sky.

As the arc flattens and lengthens, it curves downward at the ends and may meet the lower tangent arc curving upwards to give rise to the circumscribed halo (now, more correctly called the “circumscribing halo”.)

Complex halos
Can all these phenomena described appear at the same time? Yes they can, and such complexes have been witnessed a number of times. They probably occur more often than we realise but perceptive observers are often not present to witness and record the displays properly.

Robert Greenler’s “Rainbows, Halos, and Glories” describes and models displays witnessed in the last few centuries including one seen by the astronomer Hevelius on February 20th, 1661 from Gdansk. The Sun was about 26 degrees above the horizon and Hevelius has sketched the 22- and 46-degree halos along with the upper tangent arc, sundogs on both halos, the parhelic circle, and the anthelion. The drawing has become known as the “Seven Suns of Hevelius” and is a valuable record of a rare complex halo display.

The St. Petersburg display of June 18th, 1790 was seen by Tobias Lowitz and included some new phenomena that lead to some fierce debate in meteorological circles. Lowitz drew two short arcs extending from sundogs towards the 22-degree halo. Later to be known as Lowitz arcs, they have been confirmed as genuine phenomena in modern times through photographic records of complex halo displays.

Over the last two hundred years even rarer marvels have appeared associated with complex halo displays. Who knows if we have accounted for them all yet. It is only by being aware of the various types of halo effects that can occur that we can observe them carefully – maybe leading to the discovery of some these new and rare phenomena.

Further reading:
“Rainbows, Halos, and Glories” by Robert Greenler Cambridge University Press (1980). ISBN 0521236053. Already mentioned in the “Rainbows” article, this book by Greenler is also an invaluable resource on halo phenomena by a leading researcher of the subject.

“The Snowflake: Winter’s Secret Beauty” by Kenneth Libbrecht. Colin Baxter Photography Ltd (2004). ISBN 1841072532. Not strictly a book on halo phenomena but a beautiful addition to your library. The photographs of the complex forms of snowflakes are stunning.

“Atmospheric Halos” by Walter Tape. American Geophysical Union (1994). ISBN 0875908349. Stunning photographs from the Antarctic complement Tape’s book on halo phenomena.

“Atmospheric Halos and the Search for Angle X” by Walter Tape. American Geophysical Union (2005). ISBN 087590727X. A broad treatment of halo phenomena from early observations through to modern research is accompanied by amazing images of complex halos.

“Introduction to meteorological optics” by R.A.R. Tricker. Harlequin Mills & Boon (1970). ISBN 0263702634. This book is the classic mathematical treatise on the subject of atmospheric optics. From almost page one you are wading through formulae which will put off all but the most dedicated halo chaser!

“Light and Colour in the Outdoors” by Marcel Minnaert. Springer-Verlag New York Inc. (1995). ISBN 0387944133. This is the updated version of Minnaert’s ground-breaking work on all aspects of atmospheric optics and the interplay of light and colour in the atmosphere. Earlier editions of the work go under the title “The Nature of Light and Colour in the Open Air”.

“Out of the Blue: A 24-hour Sky-watcher’s Guide” by John Naylor. Cambridge University Press (2002). ISBN 0521809258. This book covers all aspects of atmospheric optics and astronomical phenomena observable with the unaided eye. I highly recommend purchasing this book as a good introduction to the subject.

“Colour and Light in Nature” by David Lynch and William Livingston. Cambridge University Press (revised 2001). ISBN 0521775043. Now in its second edition, this book is a nice companion to Minnaert’s classic. The authors cover a broad range of optical phenomena with many complimentary photographs.

Book search:
Two very good book search web sites I use are www.bookfinder.com and used.addall.com These sites search multiple book-sellers on the internet and you may pick up a number of the tomes mentioned here far cheaper than via a regular Amazon search.

Web sites:
An interesting site on the causes of rainbows is at http://www.eo.ucar.edu/rainbows/

Les Cowley hosts probably the best resource on the internet on all aspects of atmospheric phenomena at http://www.atoptics.co.uk/

Pekka Parviainen is world famous for his stunning images of atmospheric phenomena at http://www.polarimage.fi/

The Meteorological Work Group (AKM) have an equally awe-inspiring site of atmospheric phenomena images at http://www.meteoros.de/indexe.htm

A blog on atmospheric phenomena observations can be found at http://haloreports.blogspot.com/

Check out http://www.spaceweather.com/ for a regular update on sky phenomena visible with the unaided eye
 

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