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.