Tuesday, September 3, 2013

Circum-Horizon Arc at Joshua Tree

On labor day, 2013, I used my magic for finding locations that I nearly have to myself. My secret this time was to choose desert-baked rocks at Joshua Tree National Park during high temperatures coupled with unusually high humidity. Though the humidity was challenging my personal breaking point, it came with the blessing of great and rare cloud formations. Most notable was a circum-horizon arc that caught me without a camera, so I had to use my cell phone:

Circum-horizon arc in the sky showing green to orange rainbow colors.
In the photo above, I imagine the cloud in the lower right as an obscene gesture taunting me for not having my best camera.

A half hour later, maybe longer, I had my camera and had finished climbing another outcrop of rocks. When I looked back to admire my progress, the arc had returned, now sporting a lot of blue and green color:

Over the next 10 minutes, the arc changed to emphasize the orange part of the rainbow spectrum:

This photo is an attempt to provide a reference point for the location of circum-horizon arcs. The sun is just off the top of the photo, and around it is the more common ice halo that was also visible this day. The circum-horizon arc can be seen in the lower right.

I understand that circum-horizon arcs can be seen only when the sun is high overhead, making this effect a summer phenomenon. That is, I assume at my lattitude (about 32 degress N.) the sun never gets high enough in winter (though the atmospheric conditions are more likely to produce the necessary ice crystals to create this refracted light effect). High summer humidity and thunderstorms, however, puts a lot of moisture high enough where it can freeze and give us these arcs. This is the second circum-horizon arc I've seen. The last one was in early Aug. a couple years ago.


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