Circumhorizon Arcs
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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. They have also been called "Fire Rainbows", but this is a silly term, as they are neither cause by rain or fire. While Circumhorizon Arcs look something like a Rainbow, they are caused by ice not water. The term Circumhorizon Arc is the perfect term, as it is an arc that goes around the horizon. It also goes along with the term for it's alter ego; The Circumzenith Arc, which goes around the Zenith. The Circumzenith Arc can only be seen when the Sun is below 22 degrees in Altitude. |
| The peak days for CHA's, from 40 deg N latitude, are from May 11th till July 31. On those days the Sun is over 68 deg in altitude. CHA's can still be seen from April 10 until August 30 with an steady decrease in viewing time available, frequency and brightness, as the dates move away from the Summer Solstice, and the Sun's altitude decreases in the sky. |
