In this paper, the author has tried to clarify the spatial distribution of extreme monthly temperature minima (T min-ex) in Hokkaido, the northern part of Japan and the synoptic pattern on days when over 25% of AMeDAS observatories in Hokkaido recorded Tmin (Lower Temperature Days in Hokkaido=LTD). The lowest January temperature in Hokkaido was observed in Etanbetsu near Asahikawa (-37.1°C). The lowest June temperature in this district was observed in Kawayu (-3.7°C) in the eastern part of Hokkaido. In July, 42% of the AMeDAS observatories in Hokkaido recorded temperatures below 0°C. In winter, colder regions are located in the inland areas: Nayoro Basin, Furano Basin and Rikubetsu region. In summer, colder temperatures tend to be found in the eastern part of Hokkaido. The reasons were shown to be as follows: In winter, the synoptic characteristics of LTD are such that cold air moves southward from the North Pole, after a cyclone at 500hPa geopotential height has passed over Sakhalin. Wind velocity on LTD in Wakkanai is 5.8ms-1 less than the mean. In summer season, the Okhotsk High was enhanced. The temperature at 850hPa in Nemuro was reduced due to the effect of the Okhotsk High. The condition of colder temperature in this area is different from the one in the Tohoku district, and it is clear that Hokkaido district is an independent district in view of the meso and synoptic scale climatology.
