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Optical Phenomenon:
The most well-recognized atmospheric optical phenomenon is a rainbow but water drops and ice crystals produce a variety of optical effects; many of which occur regularly but go unnoticed because of our natural tendency not to look into the glare of the sun. Next time you are outdoors under some of the conditions I describe here, take a minute and see if you too start noticing these features with much higher frequency.
 
Rainbow:
Two ingredients are absolutely necessary for rainbow formation: light and water drops. The light can actually be from moonlight as well as sunlight and produces a faint colorless moonbow (because the light from the moon is far dimmer than direct sunlight). The most visible rainbow is the primary bow and has a radius of 42° centered on the anti-solar point (opposite direction from the sun). Often, a secondary bow is visible at 51° thereby giving a double-rainbow. Believe it or not a third rainbow is possible and has been observed. It would be found at 137° from the anti-solar point or 43° away from the sun and would be much dimmer and broader than either the primary or secondary. I have yet to see one in person or see a photograph of one but I hope to photo one myself. Bear in mind that it will be quite difficult to spot because of its dimness, looking toward the sun, and the background may not be dark enough to provide the proper contrast.
   Other named rainbow phenomenon include less frequently visible supernumerary bows (see the third photo above), and Alexander's dark band (clearly evident in the second photo). The diagram (click it for more detail) illustrates how the primary, secondary, and Alexander's dark band are created. The Lynch and Livingston reference (see below) includes detailed explanations of all of these features and much more with excellent diagrams and photos. I highly recommend the book to all.
 
Halos, Perhelia, Glory:
Besides rainbows, the sky is filled with other colorful arches and optical effects. Halos often appear around the sun or moon and can have fringes of washed or metallic colors. The most common halo is 22°^* around the sun as in the leftmost photo of row 2 (WxOpti01b_01). Also common are perhelia, often called sun dogs which are found to the left and right of the sun at 22° (same elevation as the sun). These can be colored or just bright colorless regions on one or both sides of the sun sometimes extending horizontally away from the sun (WxOpti01a_34). Both halos and sun dogs are produced by hexagon shaped ice crystals typical in thin cirrus clouds. The main contributor to whether a halo appears vs. perhelia is the orientation of the ice crystals. When the crystals are randomly oriented a halo results whereas when the crystals tend to lie flat horizontally, perhelia result. Again, refer to the Lynch and Livingston reference (below) for detailed diagrams and discussions of many more halo and arch positions including more rarely observed 9°, 18°, 20°, 24°, 46° halos along with parhelic circle, circumhorizontal, and circumzenithal arcs.
   ^*An easy way to estimate if a halo or sun dog is 22° from the sun is to hold up your arm outstretched and spread your thumb and pinky-finger; while placing your thumb over the sun. The angular measure between your thumb and pinky is 22°. This general rule works extremely well for all people, adults and kids alike since kids have short arms but small hands. Try it sometime and if your thumb covers the sun while your pinky touches the halo or sun dog, then you know it is a 22° halo.
   A glory, pilot's bow, or Brocken bow is now a very common site for anyone who flies over a uniform cloud deck with sunny skies above. As in photo WxOpti03b_08, a glory consists of colored rings centered on the anti-solar point usually within 10°. Mountain climbers were probably the first to see this feature with their shadows in the center and glorious colored rings surrounding their heads. Such spectacles are called Brocken spectre. The rings are produced by light that has traversed the periphery of cloud droplets and scattered back to the eye.
 
Iridescent clouds (irisation):
Perhaps more common than all of the above are coronae and iridescent clouds. Both are due to diffraction by tiny cloud drops. When the drops have very uniform size, the colors tend to be brighter and pure. But, when the drops have varying sizes, the colors are washed and tend toward white. The iridescent cloud photos shown at the top of this page are some of the best I have ever witnessed but isolated patches of iridescent clouds are very common if you just take the time to look toward the sun when thin clouds are present. Blocking the sun with your hand or other object (tree, lamppost, etc.) as well as wearing sunglasses can aid in spotting this phenomenon regularly.
 
Suggested Reading:
   Minnaert, M.J.G., 1954: The Nature of Light and Colour in the Open Air.Dover Publications.
   Lynch, D.K. and W. Livingston, 1995: Color and Light in Nature. Cambridge University Press.
   Bohren, C.F., 1991: What Light Through Yonder Window Breaks. John Wiley and Sons, Inc.
   Greenler, R., 1980: Rainbows, Halos, and Glories. Cambridge University Press.