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Circumhorizon Arcs


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.




July 10, 2008 12:26 AM till 12:54PM

Sun alt 70.6 deg to 73.2 deg to 72.1 deg

More images from 7/10/2008

CHA of July 10th, 2008

Tips and Tricks to Great CHA Photography.

1.) Know your Sky. CHA's form in Cirrus clouds, very often a 22 deg Halo may be visible around the Sun, prior to an event.

Look below the Sun about, 46 deg, at the proper time. Starting approximately an hour before High Noon, dependant upon your latitude; ending about one hour after High Noon. The Sun's altitude determines the amount of time available for a potential event.

2.)Wear polarized Sunglasses to spot CHA's.

3.) Have DSLR camera with 18-55mm Zoom lens or equivalent, with a circular polarized filter. Always have it ready. The filter is a must. It helps capture the polarized rays at their best.


June 11, 2008 11:36 AM till 1:41PM

Sun alt 66 deg to 73.2 deg to 70.8 deg

More images from 6/11/2008

CHA of June 11th, 2008


For 3 days in a row, May, 28 2008, May 29, 2008 and May 30, 2008 Circumhorizon Arcs were visible over Whiting New Jersey.

Below are some images of those CHA's and links for more images.

May 28, 2008 11:38 AM and 2:05 PM

Sun Altitude 65.5 to 71.7 to 66.4 deg

More images from 05-28-2008


May 29, 2008 1:13PM to 1:51PM

Sun Altitude 71.4 to 68.3 deg

More images from 05-29-2008


May 30, 2008 12:35 to 1:05 PM

Sun Altitude 71.5 to 71.9 deg

More images from 05-30-2008


April, 25, 2008 12:50PM

Sun Alt 63.6 deg


A faint CHA was visible that day.

July 22, 2007 1:54 PM till 2:14PM

Sun alt 67.6 deg to 65.2 deg

More images from 7/22/07

CHA 7-22-07

The Best of 2009




CHA Links

Atomspheric Optics

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Astronomy Picture of the Day

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