Sky Show: ‘Sun Dogs’ dazzle Minnesota, ‘Operation Defrost’ begins next week

Call it one of the benefits of an arctic air mass.

Or maybe it's a sign from above that our long harsh winter is about to ease up.

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'Sun Dogs' or 22-degree Parhelia dazzle onlookers in the St. Paul morning sky Thursday. Image: Baill Catlin/MPR News

Whatever the reason, many of you asked about the brilliant sky show this morning as sun dogs decorated the brittle early morning sky. MPR News Weather Fan Jeanette Berger shared this stunning image via Twitter.

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The science of sun dogs

Sun dogs are also called "mock suns" and the technical name is parhelia.

These stunningly beautiful sky shows are the result of interplay between incoming sunlight and ice crystals suspended in the atmosphere.

The University of Illinois has a nice description:

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Image: University of Illinois

The difference between sundogs and halos is the preferential orientation of the ice crystals through which the light passes before reaching our eyes. If the hexagonal crystals are oriented with their flat faces horizontal, a sundog is observed. If the hexagonal crystals are randomly oriented, a halo is observed.

Atmospheric Optics has more on the physics of light and ice crystals in our atmosphere.

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Sundogs, parhelia, are formed by plate crystals high in the cirrus clouds that occur world-wide. In cold climates the plates can also be in ground level as diamond dust.

The plates drift and float gently downwards with their large hexagonal faces almost horizontal. Rays that eventually contribute their glint to a sundog enter a side face and leave through another inclined 60° to the first. The two refractions deviate the ray by 22° or more depending on the ray's initial angle of incidence when it enters the crystal. The condition where the internal ray crossing the crystal is parallel to an adjacent face gives the minimum deviation of about 22°.

Red light is refracted less strongly than blue and the inner, sunward, edges of sundogs are therefore red hued.

Rays passing through plates crystals in other ways form a variety of halos.

When the sun is relatively high, rays cannot pass through the crystal unless they are channeled by being internally reflected from the upper and lower basal (large hexagonal) faces. The skewed angle of incidence also causes the ray deviation to increase and high sun sundogs are farther from the sun.

Plate crystals rarely float exactly horizontal, they wobble and the wobble increases with crystal size. Wobbly plates produce tall sundogs and in the more extreme cases the distinction between a tall sundog and fragments of a 22° halo becomes somewhat arbitrary.

There are many benefits to living in a cold climate, and the beauty of winter sun dogs is one of them.

Operation defrost 

Somebody is about to turn up the thermostat in the weather freezer.

It's going to be a gradual process through the weekend, but by next Wednesday all signs point to a more sustained warming trend. Teens will have to do this weekend, and a Saturday clipper bring another shot of generally light snow from the Twin Cities south to Iowa.

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Image: Weatherspark

This winter's stubborn jet stream pattern shows signs of a subtle shift next week, and a potentiality bigger shift the following week. Temps in the upper 20s, and possible 30s look much more likely by next Wednesday or Thursday. And a string of nights above zero next week? What a concept.

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Image: Weatherspark

In the longer run, the models are still battling over the magnitude of the warm up the week of Feb 17.

Here's the latest run from the National Oceanic and Atmospheric Administration's Global Forecast System, which supports temps in the 30s, and the disappearance of sub-zero air masses from the metro and southern Minnesota. As is often the case in Minnesota in winter, the warm up may come with a price -- increased snowfall potential.

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Image: NOAA via IPS Meteostar

As we say in the weather biz, stay tuned!

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