The phenomenon of upside-down rainbow is different from the ordinary rainbow principle. The rainbow that people often see is a seven-color spectral arc produced by raindrops or fog droplets refracting sunlight. The formation condition of magic solar arc is clear sky and few clouds. At the same time, sunlight can be refracted when it shines on tiny ice crystals in clouds 6100-7625m away from the ground. In addition, the surface of ice crystals in clouds must be curved.
Zenith arc is an astronomical wonder formed by sunlight refraction. It is formed by tiny ice crystals in clouds 20,000 to 25,000 feet above the ground (about 6,096 to 7,620 meters) being refracted by sunlight at a certain angle. The surface of ice crystals must be curved, and the particles are smaller than salt particles. When the sun is under the sky and sunlight touches the flat surface of ice crystals, the light bends in each crystal, reflecting the seven-color spectrum unique to the rainbow, including red, orange, yellow, green, blue, indigo and blue-purple.
Circumzenithal arc is usually formed in cirrus clouds and cold fog about five miles, which has nothing to do with rainfall. Circumzenithal arc is more common than rainbow in the extremely cold northern region, but it is very rare in temperate climate. Particles are smaller than salt particles.
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