Summer weather in northern Japan since 1982 appears to exhibit a distinct five-year cycle, with the pressure difference between Wakkanai and Sendai (PDWS) as an index of this climatic variability. The temporal variation in the June—August mean PDWS in the period 1982 to 2001 progresses through four cycles, each with conspicuous similarities, such as a peak PDWS in the second year of each cycle. The temperature also progresses in similar five-year cycles, varying inversely to the trend in the PDWS. The pattern of mean surface-pressure anomalies in the second year of each cycle is typical of the Yamase, a cold northeasterly wind, and is consistent with the cool summer in northern Japan. The following year of each cycle is characterized by a prevailing subtropical high-pressure cell, consistent with the hot summer. The correlations between the seasonal variations in the PDWS, and sea-surface temperature indicate that in the summer following an El Niño event, the PDWS tends to become positive and a cool summer ensues. An area of strong positive correlation between the PDWS and the 500 hPa geopotential height, can be identified extending from Southeast Asia to east of the Philippines, appearing to be closely related to the convective activity around the western tropical Pacific. The progression of the difference in sea-surface temperature between the South China Sea, and east of the Philippines, exhibits similar temporal variations to the PDWS, and may play an important role in the sudden change from cool summer to hot summer in northern Japan.