A sunbow, which is essentially a rainbow caused by sunlight refracting through ice crystals instead of water droplets, is caused by hexagonal plate- and column-shaped ice crystals in the atmosphere.
Understanding the Role of Ice Crystals
The shape and orientation of these ice crystals are crucial to the formation of a sunbow. Different types of crystals interact with sunlight in specific ways:
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Plate Crystals: These crystals are flat and hexagonal. According to the reference, plate crystals generally cause pillars (vertical shafts of light) only when the Sun is within 6 degrees of the horizon.
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Column Crystals: These crystals are shaped like columns or pencils. Column crystals can cause a pillar when the Sun is as high as 20 degrees above the horizon, as indicated in the reference.
How Crystals Create Pillars
The orientation of the ice crystals affects the kind of optical phenomenon that is seen. When the crystals are aligned in a particular way, sunlight passing through them can create:
- Sun Pillars: These appear as vertical beams of light extending above or below the sun. They are a common type of halo phenomenon.
Conditions for Sunbow Formation
For a sunbow to be visible, certain atmospheric conditions must be present:
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High Altitude Ice Crystals: Sunbows are frequently observed with cirrus or cirrostratus clouds. These clouds are composed of ice crystals because they form at high altitudes where temperatures are very low.
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Specific Sun Angle: The angle of the sun relative to the horizon affects the appearance and visibility of the sunbow, as noted with the plate and column crystal examples in the provided reference.
