Complex sentence: "Cold Sunday" was a meteorological event which took place on January 17,1982, when unprecedentedly cold air swept down from Canada and plunged temperatures across much of the United States far below existing all-time record lows.
Split into simpler sentences: Cold Sunday was a specific meteorological event which took place on January 17,1982. On that date, unprecedentedly cold air swept down from Canada and plunged temperatures across much of the United States far below existing all-time record lows.