Rare Optical Display over Southern Plains
during the Late Afternoon of February 10, 2005
A combination of a late afternoon sun and a thin layer of high-level clouds produced some rare and spectacular optical displays in the late afternoon skies over Oklahoma on February 10, 2005. The displays, consisting of bright spots, arcs, and halos with rainbow-like colors, sent quite a few meteorologists scrambling for their text books on atmospheric optics, simply because some of the phenomena that were seen are so rare that even scientists in the area had never seen them before.
Although some of the colored arcs resembled rainbows, none of them were true rainbows. Rainbows are created by sunlight that is reflected by raindrops, and therefore always appears on the opposite side of the viewer than the sun. In other words, the observer will be between the sun and a rainbow. However, the effects seen this afternoon were created by refraction, or bending, of sunlight through ice crystals that make up the cirrus clouds overspreading the area. The exact shape, structure, and orientation of the ice crystals, and the elevation of the sun in the sky, are what determine which of the various optical effects can be seen.
The two bright spots on either side of the sun were sun dogs. The scientific name is parhelia. They occur when the main axes of the ice crystals in the cloud layer are oriented vertically, or up and down. This results in sunlight being refracted at an angle equal to the elevation angle of the sun. Therefore, the sun dogs appear at the same elevation in the sky as the sun itself.
The sun dogs were within a larger ring of light that encircled the sun. This ring, with the sun at the center, is called a 22-degree halo, and is one of the more common of the many optical phenomena that were observed this afternoon. The name derives from the fact that there is a 22-degree angle between the observer and any 2 points on opposite sides of the halo.
At the top of the 22-degree halo was another relatively bright arc, directly above the sun, which was quite short and bent the opposite way as the halo itself. This is known as a tangent arc.
Beyond the 22-degree halo was another larger halo known as a 46-degree halo. This halo is seen far less often than the 22-degree halo.
Above the 46 degree halo was yet another arc of colored light that intersected the 46-degree halo at its highest point in the sky, but arced in the opposite direction. This is a rare phenomenon known as a circumzenithal arc. Had the arc continued all the way around to form a full circle, the center of the circle would be directly overhead at the highest point in the sky, or the zenith. These arcs usually are short in both length and duration, and are usually faint. But the one seen this afternoon was unusually bright and colorful, and lasted for almost an hour.
More information on these atmospheric phenomena can be found at the Atmospheric Optics website. Figures #1 and #2 show most of the phenomena observed and include descriptive labels.
The following photos were taken at the NWS Norman Forecast office between 4:45 pm and 5:00 pm CST on February 10, 2005 by Mike Foster, Meteorologist-in-Charge. Please select a thumbnail image to view a larger image.
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Figure #1 |
Figure #2 |
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