A Light in the Sky

A Light In the Sky

Have you ever wondered how magnificent and beautiful unusual weather phenomena occur? It’s one thing to see it and appreciate it, yet another to understand the scientific processes occurring and to realize true beauty in the world around you. In this paper, I’ll explain the processes behind the Aurora Borealis, 22 degree halos, and Fire Rainbows. All are very rare, yet if you have the chance to experience them in real life, the result is life changing and one that won’t be forgotten.

In the Northern Hemisphere, we refer to the dancing light display in the sky (best seen during the winter months) as the Aurora Borealis or Northern Lights, and in the Southern Hemisphere, it’s referred to as the Aurora Australis or the Southern Lights. The display is the result of energized electrons and gaseous particles colliding with the Earth’s upper atmosphere through acceleration processes in the downwind tail of the magnetosphere and at lower altitudes along auroral field lines (1). The aurora can produce magnificent bands or streamers of red, green, blue, and violet light; with sunspot activity increasing their brilliance (3). Color variations are due to the type of gas that the particles are colliding with; the most common color being a pale, yellowish-green is the result of collision between electrons and oxygen molecules about 60 miles above the Earth’s surface (2). Yet the aurora can produce brilliant shades of red, which is also produced by electron collision with oxygen, however at much higher altitudes; heights of up to 2oo miles. Similarly, nitrogen creates a blue or purplish-red aurora (2). Originally named by astronomer, Pierre Gassendi in 1621, the name aurora borealis comes from the Roman Goddess of dawn, Aurora and the Greek name for the north wind, Boreas. Today the Aurora is viewed by many in awe, however, interestingly enough when the first recorded sighting occurred in New England, 1719, those who witnessed the lights viewed with alarm and fear as they believed it was a sign of upcoming Judgement Day (3). However as the light show became more frequent, general fear decreased as people realized no impeding doom was in store for them. Attached is a picture of the Aurora Borealis as seen from The Alaska Highway (4).

Next our journey of incredible weather phenomena leads us to understanding 22 degree halos found around the sun or moon; they have many names they can be called, however a couple more names include: parhelion (Greek meaning ‘beside the sun’) or sun dogs (because the rings follow the sun like a dog). The halo we see is a result of light refraction in hexagonal ice crystals found at high altitudes in the Earth’s atmosphere; the most common and smallest halo forms at 22 degrees around the sun (or moon), hence the name. As sunlight passes through the ice crystals, the hexagonal shape refracts light, acting as a prism in the sky (5). There is an old weather saying “Ring around the moon means rain soon,” which does actually hold some validity. Atmospheric phenomena such as halos were used as part of weather lore as an empirical means of weather forecasting before meteorology was developed (6). Halos need ice crystals in order to form, and ice crystals are usually present in high altitude cirrus clouds. These clouds arrive days before an advancing cold or warm front, which bring rain. However, not all cirrus clouds are associated with storm systems, and some halos can merely signal an increase in water present in the upper atmosphere (5). There are references to 22 degree halos hundreds of years old in art and literature, many Greek and Roman authors- some dating as far back as Aristotle (384 B.C. – 322 B.C.) who wrote, “two mock suns rose with the sun and followed it all through the day until sunset” (7). Other instances where sun halos have been mentioned include writings from William Shakespeare's “King Henry VI, Part 3” and a painting from Vädersolstavlan depicting halos in Stockholm in 1535 (7) (8). Today, however, halos around the sun or moon are more commonly seen, yet are still awe inspiring and discussed in the scientific world, and also social media. Attached is a picture of a sun halo as seen near the Chilidog Observatory in Monterrey, Mexico (9).

Finally, Fire Rainbows, otherwise technically known as circumhorizontal arcs, are our final stop on our weather learning journey. First, what is a fire rainbow? Despite the name, they aren't associated with wildfires...or fires of any kind, they're actually associated with a series of unusual atmospheric conditions. Fire Rainbows are caused, very much so like 22 degree arcs, by light passing through cirrus clouds- which act as a prism in the sky. However, they only occur when the sun is very high in the sky (at least 58 degrees above the horizon) and, the hexagonal ice crystals in the cirrus clouds must be shaped like thick plates that have their faces parallel to the ground. In addition to the already rare weather conditions, as National Geographic notes, “When light enters through a vertical side face of such an ice crystal and leaves from the bottom face, it refracts, or bends, in the same way that light passes through a prism. If a cirrus’s crystals are aligned just right, the whole cloud lights up in a spectrum of colors (10)”. Fire Rainbows, although rare, are most commonly seen during the middle of summer for those living at mid-altitudes, and in fact, are 5-10 times more likely to form in Los Angeles than in London (11). The circumhorizontal arcs can sometimes be so large that