What are those strange lights in the sky?
Photos and text by Patrick Cobb

 

Figure 1: A lunar display of a 22° halo, moon dogs, upper tangent arc and a parhelic circle in a cirrus cloud.

      One of the nice tradeoffs about having to endure such long cold winters in interior Alaska is being able to regularly observe optical phenomena such as halos, arcs, pillars and sundogs.  These phenomena sometimes form in cirrus clouds high in the atmosphere, but they can also be generated by gently falling ice crystals, also known as “diamond-dust”, that nucleate out on clear cold humid days near the surface.  Optical phenomena occur when the ice crystals all possess a common shape and orientation, causing the scattered light to become concentrated into particular regions of the sky.  The viewing geometry between the observer and the sun (or moon) also controls whether or not a particular phenomena can be seen.      

Figure 2: Light pillars are a common sight in Fairbanks during winter.  These pillars are originating from the airport runway lights about 1.5 miles from where this photo was taken.  Gently rocking plates behave like miniature mirrors that reflect the light towards the observer.   

      The usual suspects that produce most of the optical phenomena tend to be hexagonally shaped plates and columns.  Plates are responsible for the sun dogs, parhelic circles and circumzenith arcs, and the columns are responsible for the 22° and 46° halos as well as the upper and lower tangent arcs.  It has been shown that both shapes can generate light pillars.      

Figure 3 : Extremely cold surface temperatures generated by strong radiational cooling can cause ice crystals (diamond-dust) to nucleate out of the air.  These crystals tend to be either in the shape of a plate or prism and can create some spectacular halos.  Over time, these tiny crystals may sediment out and actually accumulate on the surface.  Weather observations during these events may report “clear with light snow”. 

      Optical displays can be formulated theoretically using the basic principles of reflection and refraction of light waves through a medium, but in addition to reflection and refraction, diffraction can also cause some rather unusual sights.  Diffraction is the process by which light waves passing around an object, such as a cloud particle, constructively and destructively interfere with one another, forming concentric rings around the central light source.   Sometimes the rings contain colors.  Such displays are known as corona.  The blur around a street light on a foggy night or the bright light of a full moon shining through a thin cloud are common corona forming culprits.  When a cloud contains particles of a similar size, the colors of the corona can be warped and smeared like an oil-slick, causing a phenomenon known as cloud iridescence. 

Figure 4: A circumzenith arc looks like a “smiling” rainbow situated almost directly overhead, hence the word “zenith”.  The solar elevation angle is less than 32°.  On rare occasions, the arc may make a full ellipse “circum” around the sky. 

Figure 5:  An example of cloud iridescence.  It is most often observed in alto-cumulus or lenticular clouds but the most spectacular displays occur in rare nacreous (mother-of-pearl) clouds which form in the stratosphere in the high latitudes.