Occasional Optical Phenomena: Arcs and Bows

Have you ever gazed up at the sky and been lucky enough to feast your eyes on one of these remarkable optical phenomena? If the answer is yes, then you are very lucky indeed! If not, this should give you a small taste of what you’ve been missing. The following were all observed around Nottingham, UK. You don’t have to be in the far flung corners of the globe to witness something rare and exciting!

Halo

The optical phenomenon known as a halo is produced when light interacts with ice crystals that are suspended in the atmosphere. These crystals are typically found suspended in cirrus or cirrostratus clouds, high in the troposphere (approximately 5-10 km). A majority of these halos can be found close to the sun or moon, but they can also appear elsewhere including opposite ends of the sky! The type of halo produced depends on the formation of the ice crystals, from their shape to orientation.

Circumhorizontal Arc

The Circumhorizontal Arc also known as a ‘Fire Rainbow’ is not a rainbow at all but a form of ice halo.

It is a rare phenomenon in Northern Europe, due to the specific requirements of its occurrence. The sun must be located very high in the sky, with an elevation of around 58° or higher. In London, the sun is only high enough for 140 hours between mid-May and late-July, in stark contrast to Los Angeles where it is high enough for 670 hours between late-March and late-September! There must also be cirrus clouds or haze present, containing plate-shaped ice crystals. The arc is then created when light is refracted (bent) through the plate-like crystals, which act like tiny prisms.

Circumscribed Halo

A circumscribed halo will form tangential to another optical phenomenon, the 22° halo. Here only part of the circumscribed halo can be seen. The halo can only form when the sun is higher than approximately 30 degrees above the horizon. Light enters through one of the side faces of a spinning, column-shaped hexagonal ice crystal, and exits through an alternate side face. Resulting in the rather angelic, circumscribed halo.

Double Rainbow

Firstly a primary rainbow is created through light being refracted when it enters a water droplet, reflected inside on the back of the droplet before being refracted again when leaving it. Rainbows caused by sunlight will always be found in the section of sky completely opposite the sun. Occasionally, not all of the energy of the ray of sunlight manages to escape the raindrop after being reflected once. In these cases, the light is subjected to two internal reflections, before exiting the raindrop. This string of events leads to the creation of the acclaimed double rainbow. The rays exit the drop at an angle of 50 degrees with blue light emerging at a greater angle of 53 degrees. Amazingly, this causes the secondary rainbow to display its colour in reverse when compared to the primary!