The Arctic air that descended on the Northeast on Saturday brought dangerously cold sub-zero temperatures and wind chills to the region, including a record-setting wind chill of –108°F (–78°C) on the summit of Mount Washington in New Hampshire. Temperatures got so low that authorities in Massachusetts took the unusual step of keeping the South Station transit hub open overnight so homeless people had a safe place to sleep. Several cities in the Northeast set or tied record low temperatures for the date, while the high winds brought down a tree branch on a car in western Massachusetts killing an infant....