Sun dog in Sioux Falls, South Dakota (Credit: NWS Sioux Falls/Twitter)

The blast of Arctic air, which has caused a once-in-a-generation deep freeze across the US Midwest, has paralyzed the region’s cities and towns. Hundreds of schools are closed, thousands of flights and trains have been grounded, and most businesses were forced to shut. Authorities are cautioning residents to stay indoors during this unprecedented chilly weather pattern, which is cold enough to freeze boiling water in midair and can result in frostbite in as little as 5 minutes. The polar vortex’s only silver lining? The beautiful sun dogs being observed at many of the affected areas.

Also known as mock suns, or parhelia ("with the sun”), the natural optical phenomenon is one of several kinds of halos caused by the refraction of sunlight by the ice crystals in the atmosphere. Sun dogs typically appear as a pair of subtly-colored patches of light at an angle of about 22 degrees on either side of the Sun. Their hue ranges from red (closest to the Sun) to blue (towards the outer edge). According to experts at the National Weather Service (NWS), sun dogs are most visible when the sun is closest to the horizon – just after sunrise or just prior to sunset – since the atmosphere is thicker close to the ground.

Multiple sun halos captured together (Credit: NOAA.org)

Global News weather specialist Mike Koncan explains the phenomenon further, saying, “Ice crystals form hexagonal prisms and refract the light, creating this effect. The key is the prisms remaining still as they fall. Only crystals with their prism axis roughly perpendicular to the sun’s rays will allow light to pass through two sides.”

In some areas, the extreme weather has also resulted in another atmospheric phenomenon. Known as "22° halo," or simply “halo,” it appears as a large ring with a radius of about 22° around the Sun or Moon and is the best known of all the halos caused by ice crystals. The type of optical illusion that appears depends on whether the ice crystals are horizontally-oriented (sun dogs) or semi-randomly scattered (halo) in the atmosphere. Both are stunning!

A breathtaking circular halo captured outside Western Union, Iowa (Credit: Benjamin Friedrich/Twitter)

Given that the extreme temperatures are expected to linger for a few more days, there will be no shortage of sun dogs and/or halos to lift the spirits of the residents enduring the cold spell. For those of us unable to see them in person, numerous spectacular images are being shared on social media.

Resources: NWSSiouxFalls, Weather.gov, Wikipedia.org