Japan's weather changes greatly throughout the four seasons. At the root of these changes are monsoons and differences in temperature between the continent and ocean, together with large-scale atmospheric circulation.
Basic Knowledge
The dominant air masses and pressure patterns differ by season.
Winter (12〜2月): The Siberian High develops, creating the “high in the west, low in the east” pressure pattern. As northwesterly monsoon winds cross the Sea of Japan, they gain water vapor and bring heavy snow to the Sea of Japan side. The Pacific side has dry, clear weather.
Spring (3〜5月): Migratory highs and mid-latitude cyclones pass alternately, so the weather changes periodically. Yellow dust and pollen also arrive.
Rainy season (6〜7月): The stationary front (Baiu front) remains near Honshu. The Okhotsk air mass in the north and the Ogasawara air mass in the south have nearly equal strength.
Summer (7〜8月): The Pacific High expands, creating a “high in the south, low in the north” pattern. The influence of the Ogasawara air mass brings hot, humid weather. In a foehn phenomenon, air that has crossed a mountain becomes dry and warmer.
Autumn (9〜10月): The autumn rain front and migratory highs occur. This is a period when many typhoons move northward.
Monsoon (A prevailing wind whose direction reverses with the season)
Siberian High (A cold high-pressure system that develops over the continent in winter. Source of northwesterly monsoon winds)
Pacific High (Ogasawara High) (A warm, humid high-pressure system that expands in summer. Southeasterly monsoon)
High in the west, low in the east (Winter pressure pattern. Isobars form vertical stripes)
Foehn phenomenon (A phenomenon in which air becomes dry and hot after crossing a mountain)
Okhotsk air mass (Develops during the rainy season. Cool and humid, forming the northern side of the Baiu front)
Further Study (Background and Significance)
The main cause of monsoons is the difference in specific heat between the continent and ocean. In summer, the continent is strongly heated, producing rising air (low pressure), and wind blows in from the ocean (summer monsoon). In winter, the continent cools rapidly and a high-pressure system develops, so wind blows from the continent toward the ocean (winter monsoon).
Heavy snow on the Sea of Japan side is closely related to the topography of the Japanese Archipelago. As northwesterly monsoon winds pass over the warm Tsushima Current in the Sea of Japan, they receive large amounts of water vapor and heat from the sea surface and collide with the backbone mountains, forming snow clouds. The Pacific side, beyond the mountains, becomes dry.
Mechanism of the foehn phenomenon: As humid air rises along a mountain slope, it cools at the saturated adiabatic lapse rate (about 0.5℃/100m). After losing moisture as rain at the mountaintop, it warms while descending at the dry adiabatic lapse rate (about 1℃/100m). This difference causes the temperature to rise on the leeward side.
- Winter = high in the west, low in the east = northwesterly monsoon = heavy snow on the Sea of Japan side and clear weather on the Pacific side
- Summer = high in the south, low in the north = southeasterly monsoon = hot, humid weather
- Rainy season = stationary front = Okhotsk air mass vs. Ogasawara air mass
- Spring and autumn = migratory highs and lows pass alternately
- Foehn = temperature rises and air dries after crossing a mountain
- Fundamental cause of monsoons = difference in specific heat between the continent and ocean
- Cause of heavy snow on the Sea of Japan side = northwesterly monsoon winds passing over the Tsushima Current
Points to Note (Common Confusions)
① The winter pattern is “high in the west, low in the east,” while the summer pattern is “high in the south, low in the north.” ② In winter, the Sea of Japan side has snow and the Pacific side has dry, clear weather. It is not the reverse. ③ The foehn phenomenon occurs on the side beyond the mountain. ④ The Baiu front is a type of stationary front.
Practice
- What is the typical pressure pattern near Japan in winter called?
- Name the high-pressure system that expands in summer.
- Explain why the temperature rises on the leeward side of a mountain during a foehn phenomenon.