Rainbows, Moon Halos, Fog Bows and Sun Pillars

 

Rainbow.
Lake Otto, Alaska

Rainbows, moon halos, fogbows and light pillars are striking sights and have been the basis of stories, myths, legends and religious symbols for millennia.  Sun pillars were symbols of divine guidance, moon halos were predictors of rain, and the great rainbow was a covenant between God and the ancient Hebrews or the actions of the goddess Iris to the Greeks.  Science has a different take on these optical phenomena. Water drops or ice crystals in the atmosphere refract or reflect light to make these impressive displays in the sky.

Rainbows appear in the sky opposite the sun.  Rain droplets refract light like a prism and break it into its constituent colors; red, orange yellow, green, blue and violet, the visible spectrum.  The refracted light is bent back toward the observer and makes a ring in the sky.  Sometimes part of this ring is below the horizon and only a part, a bow, is visible.  The angle of the arc of a rainbow is 42^o from the line between the sun and the observer, so a complete rainbow covers 84^o of the sky.  The spectrum visible in a rainbow has red on the outside of the arc and violet on the inside.     

Double rainbow. 
Tongariro National Park, New Zealand. 

Sometimes a secondary set of refractions produce a secondary rainbow outside the primary rainbow.  This is called a a double rainbow.  The second rainbow is at 50^o from the line between the sun and the observer.  The colors of the spectrum are less intense than the primary rainbow and their order is reversed with violet on the outside and red on the inside.  Double rainbows are considered omens of good luck. 

Moon halo.
Rowan County, North Carolina. 

A Moon halo is a ring around the moon caused by refraction of light through hexagonal ice crystals at high altitude.  A moon halo is 22^o from the line between the observer and the moon and is often white, but sometimes the spectrum is faintly visible. Moon halos are most common in winter, but they can appear in any season of the year. 

Fog bow.
Oregon Inlet, North Carolina. 

Fog bows are similar to rainbows.  They are opposite the sun and form at a 42^o angle. As their name suggest they form in fog rather than rain and the refraction is from fog droplets that are smaller than rain droplets.  Fog bows are usually white but red on the outside and violet on the inside of the arc can sometimes be seen.  

Sun pillar.
Hatteras, North Carolina. 

Sun pillars occasionally appear when the sun is close to the horizon.  A sun pillar looks like a beam of light shot straight up into the sky.  Sun pillars are usually red or orange, the same color as the rising or setting sun.  Sun pillars are produced by horizontally oriented hexagonal ice crystals reflecting sunlight, so it appears as a column of light. 

In Shakespeare's Hamlet, the melancholy prince says to his friend, “There are more things in heaven and earth, Horatio, than dreamt of in your philosophy”.  While the physical properties of refraction and reflection account for rainbows, moon halos, fog bows and sun pillars, I think their very existence falls under Hamlet’s admonition.