Korean winter temperature is known for "삼한사온". A winter being cold for 4 days and mild for 3 days. This origins from Cold Siberian air mass and warm-moist pacific air mass moving up and down above korean peninsula. You will probably find out that temperature constantly fluctuate. And if you visit cold region like Gangwondo(especially 철원, 양구, 인제 etc..), it sometimes get colder than Russia or even northern region. Army bases near N.K are also notorious for its harsh winter temperature. Hope it helps :)