A Circumhorizon Arc bursts forth in brightness and color, over the Pine Barrens of New Jersey,during late May, 2008.
For Circumhorizon Arcs (CHA’s) to be visible, a series of perfect conditions must be met. First there must be plate crystals in the atmosphere, and the quality of these crystals must have gemlike perfection. The atmosphere must be stable enough, have the right moisture content, temperture and water particle size for the crystals to grow just right. The best CHA’s have many large plate crystals. Larger crystal take longer to form. But having perfect plate crystals is not enough. These plate crystals now must align horizontally with each other and the ground below. The crystals need to act like a giant prism. If this was not enough, the Sun must be at least 58 degrees in altitude, and CHA’s peak in brightness when the Sun is at 68 degrees in altitude. So it is only when the Sun is highest in the sky, during the time closest to the Summer Solstice, do we get to see these wonderful Arcs. Circumhorizon Arcs only form in Cirrus Clouds, and not many Cirrus Clouds have Circumhorizon Arcs.
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