What makes atmospheric effects so cool to me is that, if I was sitting around the universe, bored, and on a whim decided to come up with a moist, rocky planet in a nitrogen-oxygen-based atmosphere around a sun, I'm not sure I could have predicted them. Yet they are some of the most awe-inspiring and eye-filling visual experiences you can have.

Atmospheric effects is the collective term of optical phenomenon observed in the sky and so are also called atmospheric optics, meteorological optics, and aerial spectra. Atmospheric effects include mirages, rays, shadows, and reflective and refractive effects seen in rainbows, aureoles, halos and arcs.

Atmospheric optics is a subset of the atmospheric sciences. As a field it is much more concerned with explanation and observation than history, and very little information is available about its forbearers. Listed below are milestones of optics history that deal specifically with atmospheric effects.

Brief History: Aristotle to Newton

• In book 3 of his Meteorologica (circa 350 BCE), Aristotle discussed many of the phenomena noded here, though he erroneously attributed them all to reflection. For two millennia this work was regarded in the west as the authority in meteorology.
• Following Aristotle closely, Alexander of Aphrodisias took his hand at describing some atmospheric phenomena in 200 BCE. Of particular note is his observation of the dark band that appears between double rainbows and that bears his name today.
• A disciple of Plato, Philippus of Opus (4th century CE), wrote many books. One of the fragments that survive discusses the rainbow as a phenomenon of diffraction.
• In the 9th century CE, the philosopher-of-the-Arabs Jakub Ibn Ishak Al Kindi became enamored with the Greek texts (again, Aristotle) as he was helping to translate them. He wrote a book titled Optics that included some atmospheric effects.
• Shortly after Al Kindi, Ibn Al-Haytham (aka Alhazen and al-Haitham), another Arabic natural philosopher in Basra, wrote 14 books including Kitab al-manazir (The Book of Optics), and Maqala fi qwa' quzah wa al-hala (Treatise on the rainbow and the halo). He also wrote a lost book dealing with the colors of the sunset.
• In the earlier part of the 14th century CE, Theodoric of Freiberg correctly explained rainbows as a consequence of refraction and internal reflection within individual raindrops. However, he incorrectly explained chromatic aberration.
• In the early 17th century CE the Croatian scientist Marko Dominis discussed the rainbow in De Radiis Visus et Lucis (On the Rays of Sight and Light in Lenses and the Rainbow) where he correctly explained the inner arc of the rainbow.
• In the mid 17th century CE, Rene Descartes quantitatively derived the angles at which primary and secondary rainbows are seen with respect to the angle of the sun's elevation.
• In the early 18th century CE, Isaac Newton published Opticks, in which he correctly explained diffraction and chromatic aberration.

Halo Displays

When five or more types of atmospheric effects are visible at once, the results are spectacular and breathtaking. Since such displays almost always involve at least one halo, they are called halo displays, or halo phenomenon. It's worth noting how many of the halo displays create shapes that resemble symbols with deep human spiritual significance:

• 22° halo + parhelia +sun pillars: Greek Cross
  Emperor Constantine I of the Roman Empire is said to have seen such a display in 313 CE near Trier. This sign is supposed to have prompted him to become a Christian.
• Heliac arc: Ichthus (∝)
• Tricker + diffuse arc: Ankh
• 46° halo + circumzenith arc: Crown with a crescent moon
• Wegener + diffuse arcs + invisible Earth: (theoretical, see the node) Lemniscate (∞)

Mark Vornhusen with Germany's AKM shows evidence that there are halo displays described in the Bible (St.John of Jerusalem´s revelation, Daniel's and Ezekiel's visions) and even that many of Hildegard von Bingen's 36 visions of heavenly figures were probably halo displays.

Predicting and Studying

It used to be that atmospheric effects science was a matter of patient observation, documentation, and debate. Nowadays, rather than wait around for good fortune and mother nature to get their respective acts together and make one of these things happen, computer science has aided the study of atmospheric effects by making it primarily a mathematical modeling problem that seeks to confirm hypotheses through comparison to observed facts. Scientists use raytracing programs to model different particles at different angles and dispersions with varying light sources. This software is not that computationally intensive and can run on modern (2004) desktops. In fact, all of the following are available for free download.

• BowSim models rainbows.
  http://www.sundog.clara.co.uk/rainbows/bowsim.htm
• HaloSim models halo effects.
  At http://www.sundog.clara.co.uk/halo/halfeat.htm
• IRIS simulation models glories, fogbows, and aureole.
  http://www.sundog.clara.co.uk/droplets/iris.htm

Other worlds

Our Terran atmospheric effects-especially the ice-based effects-look the way they do partially because of the shape, temperature, and composition of our atmosphere, but mostly because of the molecular structure of water. On other worlds where atmospheric crystals are made of other chemicals, these effects would look different. Altering the modeling software for other crystal shapes, optics can predict the appearance of these extraterrestrial halos and arcs. For example, the octahedral ammonia crystals in the atmospheres of our gas giant planets produce four sundogs instead of two. The Atmospheric Optics site has fine examples of Mars, Jupter, and Saturn.

Where can you see atmospheric effects?

Water-based refraction effects are common all over the world. The other effects are harder to come by. Les Crowley summarizes the perfect formula as follows:

The recipe for an extraordinary display is beguilingly simple. Take a clear sky, cover it with a thin and uniform cirrus haze. Be sure to populate the haze with large and near optically perfect ice crystals of many varieties and precise orientations.

Alternatively in very cold weather, fill a clear blue sky with equally perfect low level diamond dust crystals.

Where would this happen? Some of the most spectacular photographs come from the poles. But as most of us aren't Naomi Uemura, it stands to reason that the closer you get to the poles, the more likely you are to run across these. Russia, Scandinavia, Alaska, Canada and Antarctica seem likely locations if you're trying to stack the deck, but many of the effects are visible at lower latitudes, too. I've seen several since I moved to Seattle. One thing you can do where you are is to simply get in the habit of looking for them. Many of the common halo effects go unnoticed simply because people don't glance up, or recognize what they're looking at.

If you do happen to notice one of these effects, remember that they seldom last longer than an hour. It might be worth stopping what you're doing to appreciate them while they're there. I've found that they are an excellent moment to recontextualize your troubles. A memento belli of sorts.

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_Sources
• _{http://encarta.msn.com/encyclopedia_761571037_4/Meteorology.html#s34}
• _{http://www.sundog.clara.co.uk/atoptics/phenom.htm}
• _{http://www.meteoros.de/indexe.htm}
• _{http://amsglossary.allenpress.com/glossary}
• _{http://homepages.wmich.edu/~korista/atmospheric_optics.pdf}
• _{http://www.nationmaster.com/encyclopedia/Timeline-of-electromagnetism-and-classical-optics}
• _{http://www.sciencedaily.com/releases/2000/03/000314065455.htm}
• _{And one charming email from Les Crowley, inquiring after historical sources:
  > Yes you have a point. I have stated the current position rather than use a historical approach.
  > There are many individual bits of history on the excellent AKM site but a single source does not spring readily to mind, I will think about it while doing some painting!
  > Les}