The fire rainbow or circumhorizontal arc is a captivating optical phenomenon where cirrus clouds appear rainbow-colored due to the diffraction of light by hexagonal ice crystals. The event actually has nothing to do with fire, although cirrus clouds sometimes take flame-like shapes so the circumhorizontal arc appears like a multicolored fire in the sky.
- A fire rainbow only appears when the Sun or Moon is at least 58 degrees high in the sky.
- Cirrus clouds display a horizontal spectrum of colors, with red on top and violet on the bottom.
- The phenomenon is relatively rare because the Sun or Moon has to be at the right altitude and the hexagonal ice crystals in the cirrus clouds must be parallel to the ground.
What Is a Fire Rainbow?
A fire rainbow appears as colored cirrus clouds, often with other uncolored clouds in the sky. Because of the position of the Sun (or Moon) in the sky, the colors form a horizontal band that is parallel to the horizon, where the “rainbow” is red on top and violet on the bottom. The distance between the arc and the Sun or Moon is twice as far as the usual 22-degree halo.
How a Fire Rainbow Works
A fire rainbow only forms when the Sun is in the right position in the sky and there are cirrus or cirrostratus clouds containing ice crystals with the requisite shape and orientation. The Sun must be at an elevation of at least 58 degrees above the horizon, and the sky must contain cirrus clouds with hexagonal plate-shaped ice crystals aligned with the flat faces parallel to the ground. When sunlight enters the ice crystals, it refracts or bends and then exits the crystals at a precise angle. This bending of light causes the different colors to separate, producing the striking array of colors seen in a fire rainbow.
Fire rainbows also occur at night with moonlight. The requirements are the same: the Moon must be high and the ice crystals in the cirrus clouds need to be parallel to the horizon.
Is a Fire Rainbow Rare?
Circumhorizontal arcs are considered rare because they depend on specific atmospheric conditions and solar positions. Really, how often you see them depends on where you live. The phenomenon occurs most often at mid-latitudes during the summer months when the sun is high in the sky. In the United States, for instance, fire rainbows are more common in the southern states than in the northern regions. Fire rainbows are less common closer to the poles because the sun’s elevation rarely reaches the necessary 58 degrees.
How to See a Fire Rainbow
Look for a fire rainbow whenever you see cirrus clouds and the Sun or Moon is high in the sky. Wearing polarizing sunglasses aids visibility, especially when the circumhorizontal arc is dim. Although truly rare, fire rainbows at night are easily spotted.
Similar Optical Phenomena
There are several other optical phenomena related to fire rainbows that result from the interaction of sunlight with atmospheric particles:
- Cloud Iridescence: Cloud iridescence occurs when sunlight interacts with small water droplets or ice crystals in clouds, causing the light to diffract or scatter in multiple directions. This scattering of light produces a pastel-colored, shimmering effect often seen on the edges of clouds. Unlike a fire rainbow, cloud iridescence does not depend on the Sun or Moon being at a particular position.
- Sundogs: Also known as parhelia, sundogs are bright spots or bands that appear on either side of the Sun, typically when it is low on the horizon. They are caused by the refraction of sunlight through hexagonal ice crystals in the atmosphere. Sundogs are red on the side closest to the Sun and blue on the opposite side. They don’t display a whole rainbow of colors and only appear along the arc 22° away from the Sun (usually to the sides, but sometimes above and below the Sun).
- Circumzenithal arc: This optical phenomenon appears as an upside-down rainbow (violet to red), forming a circle around the zenith (the point directly overhead). It occurs when sunlight refracts through plate-shaped ice crystals in the atmosphere.
- Halo: A halo is a circular ring of light surrounding the Sun or Moon, caused by the refraction, reflection, and dispersion of light through ice crystals in the atmosphere.
- Infralateral Arc: The infralateral arc is a specific type of halo that looks a lot like a fire rainbow. Like the fire rainbow, the infralateral arc appears when the Sun or Moon are at high elevation. The difference is that a fire rainbow is parallel to the horizon, while the infralateral arc curves upward at the ends. This type of arc occurs when light passes through horizontally-oriented rod-shaped hexagonal ice crystals.
Make an Artificial Circumhorizontal Arc
You don’t have to wait for the Sun and clouds to cooperate to see a circumhorizontal arc. All you need is a glass of water to produce the effect. The glass of water behaves like an upright hexagonal plate ice crystal.
- Fill a cylindrical glass almost completely full of water.
- Place the glass of water at the edge of a table.
- Illuminate the glass from below and to the side.
The glass refracts the light into the water. The second refraction at the top of water projects a hyperbola onto the wall. If no wall is nearby, use a sheet of white paper to capture the artificial fire rainbow.
References
- Lahiri, Avijit (2016). “Electromagnetic Theory and Optics”. Basic Optics: Principles and Concepts. Elsevier. ISBN 978-0-12-805357-7. doi:10.1016/B978-0-12-805357-7.00001-0
- McDowell, R. S. (1979). “ Frequency analysis of the circumzenithal arc: Evidence for the oscillation of ice-crystal plates in the upper atmosphere”. J. Opt. Soc. Am. 69 (8): 1119–1122. doi:10.1364/JOSA.69.001119
- Selmke, Markus; Selmke, Sarah (2017). “Artificial circumzenithal and circumhorizontal arcs”. American Journal of Physics. 85 (8): 575–581. doi:10.1119/1.4984802