Four basic types of synoptic-scale conditions describe the atmospheric structure and variability observed over the Japan Sea during the 1999/2000 winter season: (1) flow of cold Asian air from the northwest, (2) an outbreak of very cold Siberian air from the north and northeast, (3) passage of a weak cyclone over the southern Japan Sea with a cold air outbreak on the backside of the low, and (4) passage of a moderate cyclone along the northwestern side of the Japan Sea. In winter, the Russian coastal mountains and a surface-air temperature inversion typically block cold surface continental air from the Japan Sea. Instead, the adiabatic warming of coastal mountain lee-side air results in small air-sea temperature differences. Occasional outbreaks of very cold Siberian air eliminate the continental surface-based inversion and stability, allowing very cold air to push out over the Japan Sea for 1–3 days. During these outbreaks, the 0°C surface air isotherm extends well southward of 40°N, the surface heat losses in the center of the Japan Sea can exceed 600 W m⁻², and sheet clouds cover most of the Japan Sea, with individual roll clouds extending from near the Russian coast to Honshu. During December through February, 1991–2002, these strong cold-air outbreak conditions occur 39% of the time and contribute 43% of the net heat loss from the Japan Sea. The average number of strong cold-air events per winter (November–March) season is 13 (ranging from 5 to 19); the 1999/2000 winter season covered in our measurements was normal.