Cold wave - Kanpa

A phenomenon that mainly occurs in winter, in which air cooled in the poles or high latitudes flows out over a wide area in mid- and low-latitude regions. The onset of the cold air outflow is abrupt, dropping temperatures by 10 to 20 degrees in a single day, resulting in extremely low temperatures or severe cold. In polar and temperate regions, it is accompanied by blizzards and heavy snowfall, and has a major impact on society, including daily life, transportation, and energy consumption. Small-scale cold air outbreaks weaken within a day or two, but large-scale cold waves are called cold waves because they last for several weeks, as strong cold air repeatedly blows out in waves, or the cold air spreads over a wide area like a wave. Large-scale cold waves are closely related to the large-scale wave phenomenon of the westerly winds above. Generally, when the temperature difference between high and low latitudes increases, the westerly winds in the upper atmosphere become stronger, but once they exceed a certain limit, they tend to meander north and south, carrying cold air south and warm air north, thereby reducing the temperature difference between the north and south. Therefore, when a cold wave hits a mid-latitude region, unseasonably warm weather appears somewhere in the poles or high latitudes. As the westerly winds meander, cold air in high latitudes flows out toward the area where the westerly winds are biased toward low latitudes, i.e., toward the low-pressure trough, while warm air in low latitudes moves north toward the high-pressure ridge where the westerly winds are biased north. However, the eastern coasts of Eurasia and North America are prone to cold waves because of the large low-pressure troughs that develop due to the distribution of land and sea and the influence of large mountainous terrain such as the Himalayas and the Rockies. Large-scale troughs also sometimes develop in Europe, and when large-scale troughs appear in these three regions, major urban areas around the world are hit by cold waves at the same time. When the meandering of the westerly winds in the Far East and Pacific Ocean regions in winter increases, the Siberian high and the Aleutian low develop, strengthening the so-called high pressure in the west and low pressure in the east, causing Japan to be hit by strong cold waves.

[Masayuki Noto]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

おもに冬季の現象で、極または高緯度地方で冷却された空気が、中緯度や低緯度地方の広い範囲に流れ出すこと。寒気流出の始まりは急激で、1日に10℃から20℃も下降し、著しい低温あるいは厳しい寒さをもたらす。寒帯や温帯地方では暴風雪や大雪などを伴い、日常生活や交通・運輸、エネルギー消費など社会に与える影響が大きい。小規模な寒気の吹き出しは1日か2日で弱まるが、大規模な寒波は数週間にわたって、波状的に繰り返し強い寒気を吹き出すことから、あるいは波のように広範囲に寒気が広がっていくことから寒波とよばれる。大規模な寒波は上空の偏西風の大規模な波動現象と関係が深い。一般に、高緯度と低緯度の大気の温度差が大きくなると上空の偏西風が強まるが、ある限度を越えると、偏西風は南北に大きく蛇行し、寒冷な空気を南方へ、温暖な空気を北方に運ぶことによって、南北の温度差を小さくしようとする性質がある。したがって、中緯度地方のどこかが寒波にみまわれているときは、極または高緯度地方のどこかでは季節はずれの温暖な天候が現れることになる。偏西風の蛇行に伴って、高緯度地方の寒気は偏西風の流れが低緯度に偏ったところ、すなわち気圧の谷に向かって流れ出し、低緯度地方の暖気は偏西風の流れが北に偏った気圧の尾根に向かって北上するが、ユーラシア大陸や北米大陸の東岸の地域は、海陸の分布やヒマラヤやロッキーなどの大きな山岳地形の影響で大規模な気圧の谷が発達し、寒波にみまわれやすい地域である。このほか、ヨーロッパでも大規模な気圧の谷が発達することがあるが、これら3地域に大規模な気圧の谷が現れると、世界の主要な大都市地帯が同時に寒波に襲われることになる。冬期に極東・太平洋地域で偏西風の蛇行が大きくなると、シベリア高気圧とアリューシャン低気圧が発達し、いわゆる西高東低の気圧配置が強まって、日本は強い寒波にみまわれる。

［能登正之］

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