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Circumhorizon Arc

 

Strange "Rainbow" over the

Spokane/Post Falls area

On the afternoon of June 3rd, 2006 the denizens of the Spokane and Post Falls area were treated to a rare show of atmospheric optics. The skies over the area were adorned with what appeared to be a rainbow, and yet there was no rain. Also unlike a rainbow, these lights did not form a high arch, typical of those associated with nearby rain showers. So what was that in the sky? Many have referred to colorful displays as sun dogs, however that was incorrect as well. Sun dogs are far from this vivid and are much smaller in scope than what was seen that day.

If the colorful feature in the sky was not a rainbow or a sun dog, what could it have been?

Turns out this feature was something far more rare and unusual than a rainbow or sun dog. The feature is what is referred to as a circumhorizon arc.

A circumhorizon arc is one of the rarest halos one can witness at far northern latitudes such as Spokane. First. it requires a very high sun angle of approximately 58° or higher. A sun angle at this latitude can only form during a small portion of the year centered around the summer solstice. Solar elevations of 58° or higher in Spokane are achieved in a small window beginning in May to the first week in August. Even in this window, the sun angle only exceeds this lofty elevation for a mere hour or so in May and August to around three hours near the summer solstice (June 21st).

But a high sun angle is only part of the recipe. The other ingredient is even more restrictive. This ingredient is a cluster of perfectly aligned (i.e. nearly perpendicular to the sunlight) ice crystals. This alignment allows the light to enter the nearly vertical side of the ice crystal and exit the relatively horizontal bottom of the crystal. The crystals refract the light approximately 46°, resulting in a very pure spectrum of colors and a vivid light display. In the case on June 3rd, we had a layer of cirrus clouds provide the needed ice crystals distribution.

For more information about circumhorizon arcs and other atmospheric optical phenomena please direct your favorite search engine to "Atmospheric Optics".

Below are a few of the picture submitted to the National Weather Service. If you have interesting pictures of the phenomena, please submit them to our webmaster

Circumhorizon Arc
Circumhorizon Arc
Circumhorizon Arc
Circumhorizon Arc
Circumhorizon Arc

 

 

 

 


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