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A sharp profusion of vivid colors gradually shifting from blue to reddish yellow that literally lit the skies aflame - that is how I would describe the magnificent picture that was first recognized worldwide after the widespread Spokane display in 2006 where it was first dubbed as a "fire rainbow" or "flaming rainbow". The picture is truly a wondrous and magnificent sight that would make you stare in awe and contemplate on the beauty of nature. However, this captivating phenomenon is also a topic of debate as speculations on its origin started to arise. Some say that the phenomenon is not really a rainbow but is in fact a halo. In Australia, some mistook it for cloud iridescence or sun dogs (Thurtell, 2008). Moreover, because only a handful of people were fortunate enough to actually see and photograph this meteorological phenomenon, some claimed that "it is the rarest of all naturally occurring atmospheric phenomena" that "occurs only in cirrus clouds at high altitudes" (Milan, 2007). Later claims of people seeing the phenomenon on a regular basis, and even during a thunderstorm (Milan, 2007; msg. 9; Thurtell, 2008, msg 1) seem to disprove the earlier theories. Hence this paper aims to establish facts about the fire rainbow. It aims to answer the questions of what it truly is, how it is formed, how often it occurs and how it can be differentiated from other atmospheric phenomena of similar appearance.
What is a fire rainbow?
A fire rainbow is an atmospheric phenomenon wherein prisms of color are displayed on the sky when sun rays come into contact with refractile objects like ice crystals in the atmosphere. It looks like any other rainbow with its full spectrum of colors from violet, blue, green, yellow, orange and red arranged from bottom to top, only it appears suspended in the sky, never reaching the earth, and instead of smooth outlines, its upper reddish yellow portion is interspersed with thin wispy clouds making it appear like flames with tendrils of smoke.
The fire rainbow's appearance best fits the description of what optical meteorologists refer to as a circumhorizontal or circumhorizon arc or CHA. The phenomenon was described in 1851 as a rainbow arc tangential or touching the lowest point of the 46° halo and is located about 45° below the sun (Besson, 1902, p. 486). Gray (1916) further described the circumhorizontal arc as relatively flat, composed of a vibrant band of colors with a radius of 22.5°, 30° thicker that the curve of a 46° halo, appears concave to the sun, its midpoint is located in a straight line 46.5° below the sun, and its manifestation coincided when the sun's latitude is between 65° and 63° near noon when the sky is clear and dotted with numerous cirrus and cirro-stratus clouds (p. 245).
The circumhorizontal arc is actually a very large halo that often occur parallel to the horizon on the same side as the sun. It is often too large to be seen in its entirety; hence only a portion of the halo is often seen (Cowley, n.d.). The first observation and description by an ordinary person was noted in Santee, Nebraska on June 23, 1902. Since then sightings were far in between so it was mainly known and described in scientific publications that contain the observations and studies made by meteorologists. In June 3, 2006, a portion of the circumhorizontal arc was widely seen painting the skies of Spokane, Washington and northwestern Idaho for a proportionately long period of time, bringing it to the attention of numerous people. Impressed by this relatively unknown but newsworthy phenomenon, it was then dubbed by clueless journalists as the "fire rainbow".
How is a circumhorizontal arc formed?
Colors become visible in the atmosphere when light traveling in electromagnetic waves of a very wide frequency range, namely radio, microwave, infrared, visible, ultraviolet, x-ray and gamma ray from lowest (long wavelength) to highest (short wavelength), are bent until the narrow visible spectrum from about 700 nanometers (nm) to 400 nm is separated into its individual color wavelengths of red, orange, yellow, green, blue and violet (from longest to shortest wavelengths) and are separately reflected into man's eyes so that he will see the different colors casted in the direction from which they entered his eye.("Color and Vision", 2010). Since various substances of different optical densities, that is, the ability to absorb light, are suspended in the atmosphere, light waves may be reflected, refracted or diffracted.
Reflection occurs when light rays encounter barrier substances or those that do not allow light to pass through. Light rays will simply bounce back at the same angle that it hits the opaque substance and then split based on their wavelengths thereby forming a visible spectrum of colors. Meanwhile, refraction occurs when light rays travel thru two media with different optical densities. As it touches the boundary of the medium with a new density, some of the light gets reflected or goes back to the source while the rest changes direction, speed and wavelength. Light waves passing from lower densities, i.e. the air, to greater densities, i.e. water droplets or ice crystals, they will travel faster, their wavelengths increase and split to become optically visible. The greater the change in density, the faster the speed, the longer the wavelength and the purer colors produced from light. Finally diffraction happens when light waves goes (slightly bends) around the edge of a barrier or a narrow opening. The more acute the angle of the bend, the more pronounced will be the color produced. And the longer the wavelength, the sharper will be the angle of the bend produced. Hence only light rays with wavelengths longer than the barrier they encounter will bend sharp enough to produce well separated prisms of color. ( "Reflection, Refraction and Diffraction", 2010)
Halos are produced when light rays are reflected, refracted or diffracted on ice crystals. And ice crystals n the atmosphere are abundant in cirrus clouds, thin snow, icy fog and blown snow (Dargaud, 2010). As a form of halo, the formation of circumhorizontal arcs follows its basic principles for development. However its appearance and rarity indicate that other specific conditions have to be met this vibrant light show will be displayed for our consumption.
Circumhorizontal arcs require a specific type of ice crystal for its creation. Scientists discovered during a simulation that "hexagonal plate crystals with nearly horizontal end faces" are needed for the formation of the circumhorizontal arcs (Greenier, 1979, p. 643). This means that the required ice crystals should be flat, planar or with two dimensional surfaces, composed of three equal axes at 120° angle in the horizontal plane and a fourth axis of varying length that intersects them in the vertical plane (Crystalline Structures, 2007); that is, hexagonal in shape and lying in a horizontal manner. However, aside from the crystal's shape and orientation, the clarity of a halo is also dependent on the crystal's quality with the best halos formed by clear crystals measuring more than 0.1mm because those that are either too small (<10 microns) or too large, with air bubbles, will cause the light rays to scatter instead of refracting or diffracting on them (Dargaud, 2010).
These six-sided plated crystals are most abundant in high level clouds formed above 20,000 feet (6,000) meters above sea level. At the said height, the temperature is low enough to freeze water and form ice crystals. Clouds at this height generally appear as white and thin, in the form of wispy cirrus or sheet-like cirrostratus clouds. However, the vertically growing white dense cumulonimbus or thunderstorm clouds associated with heavy rain, hail, snow and tornadoes may also develop from near the ground to heights above 39,000 feet hence their upper portion also contain many ice crystals. Furthermore, some ice crystals may also form in mid-level clouds like altocumulus and altostratus between 6,500 to 20,000 feet (2,000 to 6,000 meters) but never in low level clouds below 6,500 feet unless during wintertime when atmospheric temperature becomes cold enough. (Department of Atmospheric Science, 2010)
How light is refracted on the crystal was first described in the Annuaire Metiorologique de France in 1851 by Bravais who suggested that for an ice crystal whose sides are made up of two planes - a vertical side and a horizontal side - at 90° angle to each other, only light rays produced when the sun is at an altitude between 59° and 78° are correctly refracted at the lower base of its vertical side to form circumhorizontal arcs (Besson, 1902, p. 486). Cowley (n.d.) further explained that when the sun is at an altitude greater than 58° the parallel sun rays it emit is at the correct angle to perfectly enter the ice crystal's vertical plane, traverse its 90° angle and exit its horizontal plane at the bottom to bend the light at an angle acute enough to disperse its wavelength into distinctly separate and clearly visible spectrum of colors. He asserted that the light rays emitted by the sun at deeper latitude will enter the crystal at a wider angle that will be reflected instead of refracted by the lower vertical plane thereby preventing the formation of the CHA. Moreover, expect the colorful display of the circumhorizontal arc to be at its brightest when the sun reaches an elevation of 67.9° (GP, 2009) and when the light rays are shining on the thickest portion of the cirrus cloud since the said area is expected to have the greatest abundance of ice crystals that will reflect the spectrum of color.
How often does a fire rainbow or CHA occur?
The frequency of its appearance depends on two main factors namely latitude and frequency of sunny days with cirrus clouds. Latitude in astronomy is defined as "the angular distance of a celestial body from an ecliptic" while geographically it is described as the "angular distance north or south from the earth's equator" (latitude, 2010) measured in degrees from 0 at the equator and 90 degrees at the north and south poles. Because the sun needs to be in a position higher than 58°to form the circumhorizontal arc, Cowley (n.d.) asserted the importance of latitude in this event because the sun shines highest and longest near the equator and the time the sun stays at its summit shortens as you go further away from the equator. Hence, you cannot expect to see the CHA at the north and south poles. Nor can it be visualized in places with latitudes of +55° north and -55° south unless you go up the mountains (GP, 2009). Hence you it would be a miracle for you to see a circumhorizontal arc even once if you live in the continent of Antarctica; northern Europe including Scotland, Norway, Sweden, Finland, Estonia, Latvinia, Lithuania, Denmark; some parts of Russia namely Archangel, Krasnoyarsk, Leningrad, Moscow and Sverdlovsk; parts of the United Kingdom including Edinburg and Glasgow;
Addis Ababa in Ethiopia; Greenland; Alaska; and parts of Canada including the northwest territories, Yukon and Nunavut. (Look up... n.d.) In the same thread, countries belonging to the middle latitude, that is, between + 23º26'22" and 66º33'39" North and - 23º26'22" and 66º33'39" South will have fewer sightings of the CHA because of the shorter time the sun stays at its zenith it these countries. This is especially true for nations at the 40º-55º latitudes like New Zealand, Austria, Bulgaria, Punta Arenas in Chile, Denmark, Italy, parts of Japan including Sapporo, some parts of China and Mongolia, parts of the United States like Connecticut, Idaho, Illinois, parts of Indiana, Iowa, Maine, Massachusetts, Michigan, Minnesota, Montana, Nebraska, New Hampshire, New York, North Dakota, Oregon, South Dakota, Vermont, Washington, Wisconsin, and Wyoming; and mid Europe, including Ireland, England, Belgium, Netherlands, France, Germany, Czech Rep., Poland, Beelarius, Ukraine, Spain, Switzerland, Yugoslavia. (Look up... n.d.) The best chance to see a CHA in the aforementioned countries where it is uncommon will be during the summer solstice or the beginning of summer on June 20 or 21 of every year which marks the longest day of the year (Solstice, 2000); hence the longest time we would expect the sun to stay at its zenith. In most countries belonging to the Unites States of America, Asia, Australia, and the Carribean, it is relatively common to catch a glimpse of the circumhorizontal arc especially during the summer season if only one is inclined to gaze at the sky every once in a while.
The next factor that influences the incidence of the CHA forming is the frequency of sunny days with cirrus clouds. This factor is highly unpredictable because of the properties of weather. However, the cloud climatology study done by Wylie et al. (1994) between June 1989 to May 1993 showed that cirrus clouds are more abundant during the summer season compared to winter and that more than half of all cirrus clouds worldwide is found in the Inter-Tropical Convergence Zone (p. 1972), which is the area always near the equator and moves from about 5° to as much as 40-45° latitude north or south where the northern and southern wind come together. With its diurnal pattern of precipitation, clouds are seen late in the morning and early in the afternoon. (Rosenberg, n.d.) Hence, countries near the equator like those in Asia and Africa are the best places to catch a glimpse of a circumhorizontal arc.
How can it be differentiated from other similar atmospheric phenomena?
Contrary to what its moniker implies, the fire rainbow is actually an ice halo, hence it is neither a rainbow nor does it have any association with fire or heat. Though both are formed by the bending of light against a refractile object suspended in the atmosphere, a halo is different from a rainbow. Rainbows are visible arcs of colored lights encompassing the full spectrum of hues red, orange, yellow, green, blue, indigo and violet from its outer to its inner edge and formed by both the refraction and reflection of light rays on water droplets in the atmosphere from rain, mists, spray and dew. (Rainbow, 2010) It occurs most frequently after raining. In contrast, halos are either complete or incomplete rings of multicolored radiance characterized by an inner red, an outer purple and a spectrum of other hues in between encircling their light source, mainly the sun or moon. They are produced by the bending of light rays on ice crystals suspended in the atmosphere. Hence they often occur during summer in very high in the atmosphere where the temperature is cold enough to produce ice crystals or during winter when ground temperature is low enough to form the crystals (Dargaud, 2010). Though there are rare types, in general halos are seen more frequently than rainbows.
Furthermore, the circumhorizontal arc may be mistaken for cloud iridescence and for other forms of halo like the circumzenithal arc, infralateral arc and sun dog. Appearance-wise, the circumhorizontal arc is most similar to a circumzenithal and an infralateral arcs. All of them produce vibrant multicolored bands of light of light. In terms of rarity, cirumzenithal arc is relatively common whereas circumhorizontal arcs are less common and the infralateral arcs are rare. Circumzenithal arc (CZA) is produced by the same type of crystal required for the circumhorizontal arc. However, instead of the light rays entering the bottom of the vertical side, they will be refracted at the top of the vertical side (Besson, 1902, p.486). Also in contrast to the CHA that requires the sun to be at its zenith, circumzenithal arcs are formed only when the sun is less than 32.3° and becomes brighter as the sun descends lower. Moreover, instead of the CHA's straighter or slightly convex form, the CZA forms a complete arc in the form of an "upside-down rainbow" high in the sky. (Cowley, n.d.) Meanwhile, the infralateral or lower lateral tangent arc curves upwards at its ends and may be found at the lower lateral side of the sun whereas the CHA is relative straight and wide and is always parallel to the horizon directly below the sun.
It is easier to differentiate Sun dog and cloud iridescence from CHA based on appearance. Both the sun dog and cloud iridescence are relatively common. Similar to the CHA, the sun dog, also known as parhelia or mock sun, is also an ice halo. They are also formed by hexagonal plates only the ice crystals are larger (> 30 micrometers) and their flat sides are positioned horizontally. They appear as extremely bright or dull patch of colored light at the same level and about 22 to the right or left of the sun. They have a characteristic reddish tinge on its sunward edge. (Cowley, n.d.)
Finally, cloud iridescence is formed by the diffraction of light rays on water droplets suspended in the atmosphere hence they commonly appear in rapidly forming altocumulus and cirrocumulus (rain) clouds and not in plain cirrus clouds. Furthermore, unlike the CHA which follows the characteristic sequence of colors in a rainbow, the iridescence's delicate colors are randomly arranged in bands and they rarely form rings unless the cloud contains uniformly sized droplets.
In conclusion, fire rainbows are circumhorizontal arcs, a form of halo formed when light rays refract horizontally oriented hexagonal plates of ice suspended mainly in cirrus clouds. The incidence of their appearance mainly depends on the latitude and frequency of sunny days with cirrus clouds. They are most often seen in countries nearer the equator and will not be formed in the north and south poles. They are often mistaken for other forms of ice halo and the more common cloud iridescence. Their characteristic appearance and the factors that are necessary for their display like the altitude of the sun, presence of cirrus clouds, and their location relative to the sun and the atmosphere will help differentiate the CHA from the other atmospheric phenomena.