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Celestial Phenomena
By Linda | March 5, 2008
Have you ever seen an isolated little patch of rainbow just floating in the middle of an otherwise bright and sunny sky? It’s called a sun dog and although I’ve seen this remarkable sight on several occasions, I never thought to find out what caused it or if it had a name. Never that is, until my niece asked me about it earlier this week.
My research taught me that there actually exists a variety of related meteorological phenomena. Each has a slightly different cause and appearance, but all result from the interaction between a light source and a refractive element in the atmosphere (in the form of water or ice). In all of the following examples, ice crystals or water droplets act like prisms—refracting, reflecting, diffracting and dispersing the light that shines on, or passes through them to produce these startling arrays.
Halos: Also known as a nimbus, icebow or Gloriole, a halo is an optical phenomenon that appears near or around the Sun or Moon, and sometimes near other strong light sources such as street lights. There are many types of optical halos, but they are typically caused by ice crystals in cold cirrus clouds located high (3 to 6 miles) up in the upper troposphere. As with the rest of these atmospheric phenomena, light is reflected and refracted by the ice crystals and may split up into colors because of dispersion.
Sun Dogs: Also known as parhelia (beside the sun) or mock suns, sun dogs can appear as part of an “ice bow”. Sun dogs may take on various characteristics depending on the alignment of the ice crystals refracting the sunlight. They can range in appearance from bright white patches of light to colorful spectral displays. White sun dogs are caused by light reflected off of atmospheric ice crystals, while colored sun dogs are caused by light refracted through them. Sun dogs typically appear when the sun is low in the sky and the atmosphere contains ice crystals. Often, two or more sun dogs can be seen on opposite sides of the sun simultaneously.
Moon Dogs: Also known as paraselenae (beside the moon), moon dogs are relatively rare bright circular spots that appears on a lunar halo. Moon dogs appear to the left and right of the moon approximately 22° away. They are caused by the exact same conditions as sun dogs, but are somewhat rarer because in order to be produced the moon must be bright and therefore full or nearly full. While a moon dog may be brightly colored, the lunar halos they form in typically appear colorless to the naked eye because their light is not bright enough to activate the color photoreceptors in the human eye.
Rainbow: An arc or bow-shaped display of spectral colors resulting from the refraction of white light falling on spherical water droplets in the atmosphere. The circular arc of colors in the rainbow is seen on the side of the sky away from the Sun. The bright, primary rainbow shows the spectrum of colors running from red, on the outside of the bow, to blue on the inside. Sometimes a fainter, secondary bow is seen outside the primary bow with the colors reversed from their order in the primary bow.
Icebow: An icebow is similar to a rainbow except that it is formed by the refraction of sunlight through cloud suspended ice crystals as opposed to raindrops or other liquid water suspended in the air. Generally the appearance is as arc sections as opposed to a full bow. Brighter sections usually occur above, below, and lateral to the center (where the sun is visible). These bright areas are the “sun dogs” described above.
Circumhorizon Arc: These are quite rare. Several very specific conditions must exist in tandem for a circumhorizon arc to occur. The sun must be high in the sky with cirrus clouds beneath it. A circumhorizon arc only forms when the clouds contain a specific, hexagonally shaped ice crystal. For light to enter the cirrus cloud crystals at just the right angle to make a circumhorizon arc, the sun must be at an angle of at least 58 degrees above the horizon. That means that this phenomenon can only occur at a very specific latitude
Glory: Also known as an anthelion (opposite the sun), A glory is produced by light backscattered (a combination of diffraction, reflection and refraction) towards its source by a cloud of uniformly-sized water droplets. A glory has multiple colored rings. Most people see only one ring. The glory, however, can show many rings when the cloud is made of uniform water droplets. Sometimes the rings fluctuate wildly in size. This happens when a plane, for instance, skirts a canyon of clouds and its glory shadow comes and goes. This type of optical display is most commonly observed while airborne, with the glory surrounding the airplane’s shadow on clouds (this is often called The Glory of the Pilot).
Topics: Nature, Science Factoids |
