Climate of Hokkaido district

In winter (December-January-February), the Siberian High develops over the Eurasian Continent and the Aleutian Low develops over the northern North Pacific. Prevailing northwesterly winds cause the advection of cold air from Siberia to Hokkaido and bring heavy snowfall to the Sea of Japan side of Hokkaido (upstream of mountainous land) and sunny weather to the Pacific side (downstream of mountainous land). Temperatures as low as -20°C are frequently observed in inland areas. When sea ice reaches the Sea of Okhotsk side of Hokkaido, the area is very cold.

In spring (March-April-May), migratory cyclones and anticyclones that alternately move eastward prevail across Japan. Temperature increases (decreases) in front (back) of cyclonic systems due to warm southerly (cold northerly) flow. Temperature in Hokkaido rises gradually with large short-term variations. The sunshine duration is the longest in the second half of spring due to the predominance of anticyclonic systems.

When the North Pacific High extends northwestward around northern Japan, it brings warm and sunny conditions to Hokkaido. Temperatures as high as 30°C are frequently observed in inland areas. The Foehn phenomenon also sometimes causes temperatures of 30°C or above on the Sea of Okhotsk side. Sea mist forms on the Pacific side when moist air transported by southerly winds is cooled by the ocean's cold Oyashio Current. Precipitation amounts in Hokkaido increase in August due mainly to the activity of fronts, though the Baiu front that forms from June to July hardly affects the region's weather.

In autumn (September-October-November), temperatures in Hokkaido fall gradually and cyclonic/anticyclonic systems move eastward across Japan alternately. The frequencies of cold northwesterly flows across Japan and snowfall on the Sea of Japan side of Hokkaido show an increasing tendency.