Halos and Glories: Tricks of Light

Light reflecting through water droplets or ice crystals in the atmosphere can produce a variety of fascinating optical phenomena. Most are visible during the day but a few can be seen at night. You need to know when and where to find them but they are all worth looking for.

Perhaps the most common optical phenomena are solar and lunar halos. These halos form when light is separated into its various colours by the prism effect of the six sided ice crystals found in cirrostratus clouds. These clouds are very high up in the atmosphere, usually around twenty thousand to fifty thousand feet high. Light is bent by the ice at an angle of twenty two degrees, and so when they converge at the observer, he sees a ring of twenty two degree diameter around the Sun or Moon. The halo is pale, faintly coloured ring, usually with red on the inside and blue on the outside.

Light reflecting off water or ice can produce various rings and arcs around the Sun or Moon. They are all classified as halo phenomena. On rare occasions, a double halo can be seen, sometimes with faint spokes.

If the cirrus clouds are intermittent, partial halos are produced, and are called parhelia (a Greek word for "with the Sun.") They are more commonly called sun dogs.. It is likely the name sun dog came from the long horizontal ray of white light that sticks out like a dog's tail. Sun dogs are most often seen when the sun is low in the sky shining through loose cirrus clouds.

A mock sun is the same effect as sun dogs. If you have ever looked up into the sky and been shocked to see what appears to be two or three suns, these are mock suns. These occur when the hexagonal ice crystals are aligned so that the longest axis is in the vertical position. They cause a reflection of the Sun which appears usually as two brightly coloured luminous spots at about twenty two degree angles on either side of the Sun.

Sun pillars are formed around sunrise or sunset when there are enough ice crystal surfaces to reflect a beam of sunlight upward. These appear like a spotlight, shining upward from the rising or setting sun.

A subsun is visible only from an aricraft. It appears in ice clouds below the observer at an altitude below the sun and looks like a long ellipse. The ellipse becomes more circular as the Sun rises higher above the horizon. On rare occasions, the subsun can be so bright that it produces a halo of its own!

One of the more rare optical phenomena is a solar or lunar corona. Don't confuse this with the actual corona of the Sun! The optical corona occurs when there are water droplets rather than ice crystals in the atmosphere. The corona appears as a bright disk pf white or pinkish light in a continuous disk around the Sun. Sometimes the outer edge of the corona has a reddish tint. The smaller the water droplets, the larger the corona will be.

Another optical phenomena which is commonly seen from an aircraft is a sun glory. Glories appears as concentric rings of colour surrounding the shadow of an aircraft cast by the Sun on a cloud of water droplets. Sometimes you can see a glory on the ground if the circumstances are just right. If you stand facing a bank of fog with the Sun behind you, the glory may appear around the head of your shadow. Glories were originally thought to only surround the heads of saints, but we know now what causes them, and that even sinners can have a glory!

Constellations

The Solar System