A fire rainbow – more scientifically known as a circumhorizontal arc – is one of the rarest occurring optical phenomena on earth. Their rarity is only matched with their beauty, as people lucky enough to see it need to be in the right place and the right time.
What is a fire rainbow, and how exactly do you see one? These amazing fire rainbows need the perfect conditions to be seen, and when they are, they are fantastic. While they’re not technically a weather phenomenon, it requires exact weather conditions in order to appear. If you’re not lucky enough to see one in person, check out these facts and pictures of fire rainbows. Maybe one day you’ll be lucky enough to see one.
Fire Rainbows Are One Of The Rarest Optical Phenomena Known To Man
A fire rainbow is the common name for one of the rarest optical phenomena, scientifically known as a circumhorizontal arc. The beautiful display occurs when light from the sun or moon reflects off ice crystals suspended in the atmosphere. While “fire rainbow” is what it is commonly called, it is a somewhat misleading name as they have nothing to do with fire or rainbows.
They Can Only Occur In Two Types Of Clouds
Fire rainbows can only occur in cirrus or cirrostratus clouds. These cloud types both occur in high altitudes, and are made up of thin, wispy strands. Cirrus clouds can occur anywhere between 16,500 and 45,000 feet, and cirrostratus occur between 18,000 and 21,000 feet.
Interestingly, cirrus clouds form on planets other than Earth, including Mars, Jupiter, Saturn, Uranus, and possibly Neptune. This means fire rainbows may also exist on alien planets.
Light Has To Hit Ice Crystals At The Perfect Angle To Make A Fire Rainbow Appear
A fire rainbow is formed when light enters horizontally-oriented, flat, hexagonal ice crystals vertically and leaves through them horizontally. The 90 degree shift from the light rays’ entrance and exit forces the separation of the spectral colors. The ice halos occur only when the sun is at least 58 degrees above the horizon.
Fire Rainbows Are More Frequent At Certain Latitudes And Longitudes
The frequency of fire rainbows depends entirely on the location and latitude of the observer. While fire rainbows are relatively common throughout the United States, they are rare in northern Europe for several reasons.
Not only do the ice crystal-containing clouds need to be in the right position in the sky, the halo requires the light source must be at an elevation of 58 degrees or higher. In London, England the sun is only high enough for 140 hours per year, between mid-May and late July. In contrast, in Los Angeles the sun is high enough for 670 hours per year, between late March and late September.
Fire Rainbows Take On The Shape Of The Cloud
Fire rainbows are entirely dependent on the light hitting the ice crystal cloud formations, the colors will take on the shape of the cloud themselves. This can create braided fire rainbows, wavy fire rainbows, streaks, rivers, and more. The diversity is what makes for an array of amazing fire rainbows.
Fire Rainbows Are Massive
The circumhorizontal arc is actually just that – an arc. Because of the necessary variables to make them visible to the human eye, we are often left with the ability to only see a fragment of the entire arc. Circumhorizon arcs are so huge their colors often appear to be those of the sky itself rather than an ice crystal halo. In the above shot, the true size of a fire rainbow can be seen when it dwarfs an airplane flying across the sky.
They Look Very Similar To Iridescent Clouds
While they look similar, be sure not to confuse fire rainbows with iridescent clouds. While the two phenomenon can produce a similar effect, iridescence often occur in altocumulus, cirrocumulus, and lenticular clouds. Iridescent clouds look similar to what you might see when oil is spilled in water puddle – a rainbow-like, pastel splash of colors. These occur when the light of the sun or moon goes through semi-transparent clouds where water droplets are present.
The biggest difference between an iridescent cloud and a fire rainbow is the presence of large ice crystals. Iridescent clouds don’t have large ice crystals, while circumhorizontal arcs do.
Fire Rainbows Are Actually Similar To Sunsets
While fire rainbows are incredibly rare, you can see a similar optical phenomenon many days out of the year by watching a sunset. The science that creates fire rainbows is very similar to the science behind the most beautiful sunsets. As a ray of sunlight travels through the atmosphere, the colors are scattered out by air molecules and airborne particles, changing the final color of the beam. Because the shorter wavelength components, such as blue and green, scatter more strongly, these colors are preferentially removed. So at sunrise and sunset when the path through the atmosphere is longer, the blue and green components are removed almost completely. That leaves the longer wavelength orange and red hues we typically see.
When We See Fire Rainbows, Our Brains Are Playing Tricks On Us (Sort Of)
Something to remember, not only with fire rainbows but with all optical phenomena in the sky, is the array of colors in the sky exist at all times. But based on how they’re reflecting in the sky, we don’t necessarily see them. This is why it’s a rare occurrence for the human eye and brain to isolate and identify certain colors, like those seen in a fire rainbow. As senior meteorologist Nick Wiltgen explained:
“In clouds, iridescence is a by-product of sunlight being diffracted by water droplets or ice crystals, causing the various wavelengths of light, which we see as colors, to emerge at different angles. As they reach the observer’s eye, the observer perceives a pattern of various colors as those different wavelengths reach his or her eye from distinct directions, rather than being jumbled together and appearing whitish.”